Document:

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                                                                   Exhibit 10.52

                               HEADS OF AGREEMENT:
                  IMMUNOGEN /GENENTECH COLLABORATIVE AGREEMENT

A.   EFFECTIVE DATE: The effective date of this Heads of Agreement ("HEADS OF
AGREEMENT") shall be as of May 2, 2000 (the "EFFECTIVE DATE").

B.   PARTIES: The parties to this Heads of Agreement are Genentech, Inc., a
Delaware corporation with offices located at 1 DNA Way, South San Francisco, CA
94080 ("GENENTECH") and ImmunoGen, Inc., a Massachusetts corporation with
offices located at 333 Providence Highway, Norwood, MA 02062 ("IMMUNOGEN").
Genentech and ImmunoGen are each also referred to herein singly as a "PARTY" and
collectively as the "PARTIES".

C.   PREAMBLE:

     1.   ImmunoGen owns or controls certain proprietary technology (including
          patent rights and know-how) relating to the conjugation of maytansine
          derivatives such as DM1 to binding proteins;

     2.   Genentech owns or controls certain proprietary technology (including
          patent rights and know-how) relating to antibodies and other binding
          proteins;

     3.   As of the Effective Date, the Parties have entered into an exclusive
          license agreement pertaining to the use of ImmunoGen's proprietary
          toxin conjugation technology with a particular antibody and related
          binding proteins (the "GENENTECH LICENSE");

     4.   Concurrently with such Genentech License, the Parties have also agreed
          upon the terms and conditions for a broader arrangement relating to
          the conjugation of a larger array of antibodies and binding proteins
          to maytansine derivatives such as DM1; and

     5.   The Parties desire to enter into this Heads of Agreement to evidence
          their agreement to the terms and conditions, set forth hereinbelow,
          relating to such broader arrangement.

D.   AGREEMENT: The Parties intend shortly to enter into a detailed agreement
     (the "AGREEMENT") relating to the above-referenced broader arrangement,
     under which Genentech may select and test a number of antibodies and other
     binding proteins together with ImmunoGen's conjugation technology, and may
     acquire exclusive option rights and/or exclusive license rights from
     ImmunoGen. The terms and conditions of the Agreement shall be consistent
     with the terms and conditions of this Heads of Agreement, and the Parties
     agree to enter into the Agreement as soon as practicable, with its final
     terms to be subject to the approval of each Party's senior management.
     Pursuant to this Heads of Agreement, Genentech shall be entitled to
     exercise its non-exclusive research license rights and its rights to take
     Exclusive Target Options and/or Exclusive Licenses, Genentech shall pay
     ImmunoGen the signing fee noted below, and Genentech and ImmunoGen shall

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     establish and commence the functioning of the Joint Process Development
     Committee, all as provided hereinbelow.

E.   BINDING TERMS AND CONDITIONS: In consideration of the mutual covenants
contained herein, the Parties hereby agree to the following terms and
conditions, which shall be binding upon the Parties with respect to this Heads
of Agreement and the Agreement to be entered into:

AB-MAY LICENSED

PRODUCT             Any and all antibodies and other binding proteins owned,
                    controlled or made available to Genentech by any third party
                    "(THIRD PARTY") under a material transfer agreement
                    (including any fragments, derivatives, or single-chain
                    versions of any of the foregoing), are referred to herein
                    collectively as "ABS". Any maytansine or maytansine
                    derivative owned or controlled by ImmunoGen, including P-3
                    and DM1 (and any fragments or derivatives of any of the
                    foregoing), are referred to herein collectively as "MAY".
                    The term "AB-MAY LICENSED PRODUCT" means, collectively, any
                    Abs conjugated with any MAY, provided that each individual
                    Ab-MAY Licensed Product is distinguished by the Target (as
                    defined in the next paragraph) bound by the particular Ab
                    incorporated into such Ab-MAY Licensed Product. ImmunoGen
                    shall grant Genentech the option and license rights set
                    forth herein with respect to the development, manufacture,
                    use and sale of Licensed Products in the Field in the
                    Territory.

TARGET              The term "TARGET" means any particular antigen (whether a
                    protein, carbohydrate, etc.) that is bound by a particular
                    Ab used to create an Ab-MAY Licensed Product, and a Target
                    encompasses all epitopes of a particular antigen, so that a
                    license (or option) to Genentech to make Ab-MAY Licensed
                    Product to a given Target is a license to make any Ab-MAY
                    conjugates that are directed against any and all epitopes of
                    that Target.

FIELD               "FIELD" means the use of Ab-MAY Licensed Product for any
                    human indication.

TERRITORY           The scope of Genentech's option and license rights is every
                    country and territory in the world (the "TERRITORY").

GNE NON-EXCLUSIVE
RESEARCH LICENSE    ImmunoGen hereby grants Genentech a non-exclusive
                    research license, under the patent rights and know-how owned
                    or controlled by ImmunoGen, for Genentech (with or without
                    Third Parties) to conduct safety, toxicity and other
                    pre-clinical studies in vitro and in

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                       CONFIDENTIAL TREATMENT REQUESTED.
CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                    vivo in any non-human species with any Ab-MAY Licensed
                    Product to any Target that is not already covered by an
                    Exclusive Target Option (as defined below), an Exclusive
                    License (as defined below) to Genentech, or an exclusive
                    license from ImmunoGen to any Third Party for an
                    antibody-MAY conjugate to that such Target. [*] Genentech
                    (with or without Third Parties) will have full
                    responsibility, at its sole cost and discretion, to conduct
                    any and all pre-clinical work on any Ab-MAY Licensed
                    Products under the research license set forth herein, under
                    any Exclusive Target Option, and/or under any Exclusive
                    License. The non-exclusive research license hereunder, and
                    Genentech's Exclusive Target Options and Exclusive Licenses,
                    shall permit Genentech (with or without Third Parties) to
                    conjugate any antibodies for use as experimental controls in
                    connection with the research, development and
                    commercialization of Ab-MAY Licensed Products.

GNE EXCLUSIVE
TARGET OPTIONS      [*] grant to Genentech of an exclusive [*] option to
                    take an exclusive license to all patent rights and knowhow
                    owned or controlled by ImmunoGen and necessary or useful for
                    (i) the manufacture of any Ab-MAY Licensed Product in which
                    the Ab binds to the Target [*] or (ii) the conduct by
                    Genentech (with or without Third Parties) of any
                    pre-clinical research using any Ab-MAY Licensed Product in
                    which the Ab binds such Target (the "EXCLUSIVE TARGET
                    OPTION"). [*].

                    EXCLUSIVITY OF EXCLUSIVE TARGET OPTIONS: [*] for so long as
                    such Exclusive Target Option is in effect, ImmunoGen may not
                    undertake (with or without Third Parties) any development,
                    manufacture or commercialization of, or pursue discussions
                    with a Third Party regarding, or grant any Third Party any
                    license or other rights with respect to, any antibody-MAY
                    conjugates in which the antibody or binding protein
                    (including any fragments, derivatives, or single-chain
                    versions of any of the foregoing) binds to the Target
                    covered by Genentech's Exclusive Target Option.

AVAILABILITY OF     LIMIT ON NUMBER OF EXCLUSIVE TARGET OPTIONS: For so long as
EXCLUSIVE TARGET    Genentech has in effect [*] Exclusive Target Options (for
OPTIONS             Ab-MAY Licensed Products to [*] separate Targets), or when a
                    given Exclusive Target Option expires unexercised, or is
                    exercised for an Exclusive License, or is terminated early
                    by Genentech (all as provided below), at any such time
                    Genentech may request from ImmunoGen additional options up
                    to the limit of [*] such Exclusive Target Options in effect
                    at any one time, under the

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                       CONFIDENTIAL TREATMENT REQUESTED.
CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                    procedures set forth above for selecting, confirming
                    availability of and automatically granting such Exclusive
                    Target Options.

                    EXERCISE OF EXCLUSIVE TARGET OPTIONS: Genentech in its sole
                    discretion may exercise any Exclusive Target Option to take
                    an Exclusive License to the Target covered by such Exclusive
                    Target Option, at any time prior to the expiration of such
                    Exclusive Target Option, as provided in the next paragraph.
                    Any exercise of an Exclusive Target Option by Genentech must
                    be evidenced by timely written notice of exercise to
                    ImmunoGen and timely payment of the Exclusive License Opt-in
                    Fee (as provided below). If Genentech exercises its
                    Exclusive Target Option prior to expiration, at such time or
                    thereafter, subject to the limit of [*] such Exclusive
                    Target Options in effect at any one time, Genentech may
                    select and be granted another Exclusive Target Option to
                    replace the one exercised.

                    EXPIRATION OF EXCLUSIVE TARGET OPTIONS: If an Exclusive
                    Target Option is not exercised or renewed (as provided in
                    the next paragraph), such Exclusive Target Option shall
                    expire on the date that is [*] after the date of automatic
                    grant of such Exclusive Target Option as part of ImmunoGen's
                    confirmation in writing to Genentech of the availability of
                    the requested Target, as provided in "SELECTION AND
                    CONFIRMATION OF EXCLUSIVE TARGET OPTIONS", above. [*] In
                    connection with any expiration of an Exclusive Target
                    Option, with respect to such expired Target Genentech
                    automatically shall lose the exclusivity set forth above in
                    "EXCLUSIVITY OF EXCLUSIVE TARGET Options" but shall retain
                    the following non-exclusive rights. In the event of such
                    expiration (or early termination, as provided below)
                    Genentech automatically shall be granted a fully paid up
                    non-exclusive license under the patent rights and knowhow
                    owned or controlled by ImmunoGen as necessary or desirable
                    for (i) the manufacture (with or without Third Parties) of
                    any Ab-MAY Licensed Product in which the Ab binds the
                    expired Target, and (ii) the conduct by Genentech (with or
                    without Third Parties) of pre-clinical research using any
                    such Ab-MAY Licensed Product, in each case until such time
                    as ImmunoGen notifies Genentech in writing that ImmunoGen
                    has (if at all) granted an exclusive Third Party License
                    that prohibits any further non-exclusive license to
                    Genentech hereunder.

                    EARLY TERMINATION OR RENEWAL OF EXCLUSIVE TARGET OPTIONS: As
                    to a given Exclusive Target Option, at any time after [*]
                    from the date of automatic grant of any Exclusive Target
                    Option as provided above, but prior to the date of
                    expiration of such Exclusive Target

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                       CONFIDENTIAL TREATMENT REQUESTED.
CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                    Option as provided above, Genentech may notify ImmunoGen
                    that Genentech is terminating such Exclusive Target Option
                    prior to its expiration, in which event at such time
                    Genentech automatically shall have the non-exclusive license
                    rights set forth in the preceding paragraph (subject to
                    continued availability as provided above), and at such time
                    or thereafter Genentech may select and be granted another
                    Exclusive Target Option to replace the one terminated,
                    subject to the limit of [*] Exclusive Target Options in
                    effect at any one time. In addition, prior to expiration of
                    any Exclusive Target Option, Genentech may elect to renew
                    such Exclusive Target Option for an additional [*] period
                    from the date of such renewal; PROVIDED, HOWEVER, that such
                    renewal may be accomplished only if prior to the expiration
                    of such Exclusive Target Option, Genentech provides written
                    notice to ImmunoGen of Genentech's election to take a
                    second, consecutive Exclusive Target Option for such Target,
                    which shall continue on and after the date of expiration of
                    the first Exclusive Target Option for such Target.

SIGNING OF          SIGNING FEE: In consideration for the rights granted to
AGREEMENT           Genentech under this Heads of Agreement and under the
                    Agreement (including rights to utilize any technology
                    arising from prior work of ImmunoGen), Genentech will pay
                    ImmunoGen a non- refundable $3 million signing fee on
                    execution and delivery of the Heads of Agreement by both
                    Parties.

EXCLUSIVE TARGET    OPTION ELECTION TERM: Genentech may elect to take
OPTION TERM         Exclusive Target Options as provided herein until the
                    [*] anniversary of the effective date of the Agreement (the
                    "OPTION ELECTION TERM"), subject to extension as provided in
                    the next paragraph. In any event (including any extension of
                    the Option Election Term) Genentech shall be entitled to
                    elect to take Exclusive License(s) until the expiration of
                    the last-to-expire Exclusive Target Option.

EXCLUSIVE TARGET
OPTION TERM         EXTENSION OF OPTION ELECTION TERM: Upon payment of a
EXTENSION           non- refundable extension fee of [*] to ImmunoGen prior to
                    the expiration of the Option Election Term, Genentech in its
                    discretion may extend the Option Election Term for an
                    additional [*] years.

EXCLUSIVE LICENSES  EXCLUSIVE LICENSES: For (a) any Target that is covered by an
                    unexpired and unexercised Exclusive Target Option, at any
                    time prior to the expiration of the [*] option period (as
                    may be renewed as provided above), and (b) any other Target
                    not then covered by an Exclusive Target Option, provided
                    that such uncovered Target

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                       CONFIDENTIAL TREATMENT REQUESTED.
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                    is selected by Genentech by written notice to ImmunoGen and
                    confirmed by ImmunoGen to be available as provided in the
                    next paragraph, Genentech in its discretion may elect, on a
                    Target-by-Target basis, to take an exclusive license (with
                    rights to sublicense) in the Territory under all patent
                    rights and know-how owned or controlled by ImmunoGen which
                    are necessary or useful for the development, manufacture,
                    use or sale of any Ab-MAY Licensed Products in the Field in
                    the Territory (an "EXCLUSIVE LICENSE"). The Agreement shall
                    contain a form of Exclusive License agreement with
                    provisions substantially identical to the Genentech License.

                    GRANTING AND AVAILABILITY OF EXCLUSIVE LICENSES: With
                    respect to any Target covered by an unexpired and
                    unexercised Exclusive Target Option, the grant of such
                    Exclusive License shall be deemed to occur automatically on
                    Genentech's exercise of its Exclusive Target Option as
                    provided above in "EXERCISE OF EXCLUSIVE TARGET OPTIONS",
                    and the Parties shall enter into an Exclusive License
                    agreement as provided above. Genentech may also notify
                    ImmunoGen in writing of Genentech's selection of a Target
                    not then covered by an Exclusive Target Option, in which
                    event ImmunoGen may only deny a request by GNE to take an
                    Exclusive License to such Target if the circumstances set
                    forth above in "NON-AVAILABILITY OF EXCLUSIVE TARGET
                    OPTIONS" apply with respect to such Target. In such event
                    ImmunoGen shall notify Genentech promptly in writing (and in
                    any event within [*] business days after Genentech's notice
                    to ImmunoGen hereunder) of the non-availability of the
                    Exclusive License to such Target and the reason(s) therefor.
                    Otherwise the grant of the Exclusive License to the
                    uncovered Target shall be deemed to occur automatically on
                    ImmunoGen's written confirmation of its availability, and
                    the Parties shall enter into an Exclusive License agreement
                    as provided above. In connection with any Genentech exercise
                    of an unexpired and unexercised Exclusive Target Option, or
                    promptly after the written confirmation by ImmunoGen of the
                    availability of an uncovered Target selected by Genentech as
                    provided herein, Genentech shall pay ImmunoGen the Exclusive
                    License Opt-in Fee (as provided below), as well as the
                    future milestone payments and royalties as outlined below,
                    if achieved and owed in accordance with the terms of such
                    Exclusive License.

                    GENENTECH PROPERTY AND JOINTLY OWNED PROPERTY: Under no
                    circumstances (including a decision by Genentech not to take
                    an Exclusive License) shall anything set forth herein or in
                    the Agreement grant ImmunoGen any rights with respect to any

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                    technology, data, intellectual property, or know-how of
                    Genentech (including any technology, data, intellectual
                    property or know-how developed solely by Genentech). As to
                    any inventions, know-how or other proprietary information
                    jointly developed by the Parties (under the JPDC or
                    otherwise) and jointly owned by the Parties, Genentech and
                    ImmunoGen shall have the same rights with respect thereto as
                    set forth in the Genentech License as to any such jointly
                    owned technology thereunder.

DEVELOPMENT AND     For any and all Ab-MAY Licensed Products covered by an
COMMERCIALIZATION   Exclusive License to Genentech, Genentech (with or without
                    Third Parties) will have sole responsibility, at its sole
                    cost and discretion, for all research and development of
                    Ab-MAY Licensed Products, including any pre-clinical work,
                    any IND-enabling work, selection of any drug candidate(s),
                    any clinical trials (including Phase I, II, III and IV
                    clinical trials), and any other development of such Ab-MAY
                    Licensed Products. GNE shall also have sole responsibility,
                    at its sole cost and discretion, for all regulatory filings
                    relating to any development, manufacture or
                    commercialization (including sale) of such Ab-MAY Licensed
                    Products. Except as otherwise provided herein with respect
                    to jointly owned inventions, know-how or other proprietary
                    information of the Parties, all data, results, know-how,
                    inventions, regulatory filings and information relating to
                    the research, development, manufacture, use, sale or
                    commercialization of any Ab-MAY Licensed Product shall be
                    held and owned solely and exclusively by Genentech.
                    ImmunoGen will transfer to Genentech all licensed technology
                    necessary or useful for such purposes.

MANUFACTURING       For any and all Ab-MAY Licensed Products covered by an
                    Exclusive License to Genentech, Genentech (with or without
                    Third Parties) will have sole responsibility, at its sole
                    cost and discretion, for the manufacture of any and all Abs,
                    MAY compound(s), and any Ab-MAY Licensed Products for all
                    pre-clinical, clinical, and commercial use of such
                    materials, including all process development and scale-up
                    work. ImmunoGen will transfer to Genentech all licensed
                    technology necessary or useful for such purposes. For
                    purposes of Genentech's non-exclusive research license, any
                    Exclusive Target Option and any Exclusive License, ImmunoGen
                    agrees to supply Genentech (in accordance with an ordering
                    process to be agreed upon) with P3, DM1, other MAY
                    compounds, and conjugated Ab-MAY Licensed Products (and/or
                    conjugated control antibodies) as requested for pre-clinical
                    testing, with Genentech to reimburse ImmunoGen's fully
                    burdened manufacturing costs for any such pre-clinical
                    materials.

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JOINT PROCESS DEVELOPMENT
COMMITTEE           JPDC: Upon execution of the Genentech License, the Parties
                    have formed a "Joint Process Development Committee"
                    ("JPDC"). The same JPDC shall be responsible for
                    coordinating and monitoring of process development efforts
                    relating to MAY production and/or Ab-MAY conjugation in
                    connection with the development, manufacture and
                    commercialization of any Ab-Licensed Products hereunder. The
                    meetings and functioning of the JPDC shall be as provided in
                    the Genentech License.

                    JOINTLY OWNED CONFIDENTIAL INFORMATION: All data, know-how
                    and other proprietary information developed jointly by the
                    Parties through the JPDC will be considered joint
                    confidential information of Genentech and ImmunoGen and will
                    not be disclosed to any Third Party without prior written
                    consent of both Parties. All such jointly owned confidential
                    information shall be included within the scope of any
                    Exclusive License to Genentech hereunder, and Genentech and
                    ImmunoGen shall have the same rights with respect thereto as
                    set forth in the Genentech License as to any such jointly
                    owned confidential information thereunder.

SALES / MARKETING   For any and all Ab-MAY Licensed Products covered by an
                    Exclusive License to Genentech, Genentech (with or without
                    Third Parties) will have sole responsibility, at its sole
                    cost and discretion, for all promotion, sales, marketing,
                    export, import and distribution of any and all Ab-MAY
                    Licensed Products covered by an Exclusive License.

MILESTONES          Under any Exclusive License, Genentech will pay the
                    following milestones to ImmunoGen upon successful completion
                    of the following goals achieved by any Ab-MAY Licensed
                    Products covered by such Exclusive License to Genentech
                    (provided that under each such Exclusive License taken, each
                    milestone shall be paid only once, for the first Ab-MAY
                    Licensed Product thereunder to achieve such milestone):

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                       CONFIDENTIAL TREATMENT REQUESTED.
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                    Milestone reached    Cash
                    -----------------    ----

                    [*].

[*]

PAYMENTS            [*]

                    [*]

                    [*]

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                       CONFIDENTIAL TREATMENT REQUESTED.
CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

ROYALTIES           ROYALTIES: Under any Exclusive License, except if the [*]
                    set forth below apply, Genentech will pay to ImmunoGen a
                    royalty on worldwide net sales of each Ab-MAY Licensed
                    Product covered by such Exclusive License equal to the
                    following [*] figures for such Product in [*]:

                    [*]

                    [*]

F.   MISCELLANEOUS: This Heads of Agreement shall be deemed to incorporate by
reference the following provisions of the Genentech License as if set out in
full herein, such that such provisions shall also apply with respect to this
Heads of Agreement: Section 5 (confidential information), Section 6 (patent
rights), Section 8 (representations and warranties), Section 9 (indemnification
and liability) and Section 10 (miscellaneous). The Agreement (including the form
of Exclusive License agreement attached thereto) shall contain provisions
substantially identical to the foregoing provisions of the Genentech License.
This Heads of Agreement shall terminate automatically upon the execution and
delivery of the Agreement by the Parties.

     IN WITNESS WHEREOF, the Parties have caused this Heads of Agreement to be
executed as of the Effective Date by their duly authorized representatives in
two (2) originals.

GENENTECH, INC.                             IMMUNOGEN, INC.

By: _________________                       By: _________________
Its: ________________                       Its: ________________

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                       CONFIDENTIAL TREATMENT REQUESTED.
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                               DATED: MAY 4, 2000

                     BRITISH BIOTECH PHARMACEUTICALS LIMITED

                                       AND

                                 IMMUNOGEN, INC

                         DEVELOPMENT, COMMERCIALIZATION
                              AND LICENSE AGREEMENT

                                 RE: huN901-DM1

                                         British Biotech Pharmaceuticals Limited
                                                                 Watlington Road
                                                                  Oxford OX4 6LY
                                                               Tel: 01865 748747
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                       CONFIDENTIAL TREATMENT REQUESTED.
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                                INDEX OF CONTENTS

<TABLE>
<S>                                                                             <C>
1.  Definitions..............................................................

A   COMMENCEMENT PROVISIONS

2.  Grant and acceptance of rights...........................................
3.  Sub-licenses.............................................................
4.  Other Countries..........................................................
5.  Provision of Technical Information and assistance........................
6.  Term.....................................................................
7.  Formation of Collaboration Committee.....................................

B   ANCILLARY PROVISIONS

8.  [*]......................................................................
9.  [*]......................................................................
9A  [*]......................................................................

C   COMPENSATION PROVISIONS

10. Compensation.............................................................
11. Royalties in respect of sales of Licensed Compound and/or Product........
12. Accounting and Payment...................................................

D   CLINICAL DEVELOPMENT PROVISIONS

12A Preparation and filing of IND............................................
13. Development Plan.........................................................
14. Pre-Clinical Studies and Pharmacokinetic Studies.........................
15. Agreed Clinical Studies..................................................
16. Clinical Development for other indications
17. Investigators' meetings..................................................
18. Adverse Event Reporting..................................................

E   MANUFACTURING AND SUPPLY PROVISIONS

19. Product Specification....................................................
20. Transfer of Manufacturing Information....................................
21. Development obligations of ImmunoGen.....................................
22. Supply for Clinical Development purposes.................................
23. Non-conforming clinical supplies.........................................
24. Commercial Manufacture and Supply of Product.............................
25. Contract Manufacturing...................................................
26. Retention of records.....................................................
27. ImmunoGen's inability to supply..........................................
28. Regulatory compliance and reporting......................................
29. Audit rights of BB.......................................................
</TABLE>

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<TABLE>
<S>                                                                             <C>
F   COMMERCIALIZATION PROVISIONS

30. BB's Diligence Obligations...............................................
31. Obtaining Regulatory Approval in the Territory...........................
32. Regulatory Approvals outside the Territory...............................
33. Marketing and sale of Product............................................
34. Trade marks..............................................................

G   PATENT PROVISIONS

35. Patent Prosecution and Maintenance.......................................
36. Inventions of BB.........................................................
37. Infringement and Invalidity Proceedings..................................
38. Third Party Infringement Proceedings.....................................
39. Miscellaneous Provisions.................................................
40. Effect of Interim or Final Restraining Orders or Injunctions on Agreement
41. Further Third Party Licenses.............................................
42. Compulsory Licenses......................................................
43. Direct Affiliate Licenses................................................

H   TERMINATION PROVISIONS

44. BB's Rights to Terminate.................................................
45. ImmunoGen's Rights to Terminate..........................................
46. General rights of termination by either party............................
47. Effect of Termination....................................................

I   PUBLICITY AND PUBLICATIONS PROVISIONS

48. Publicity................................................................
49. Publications.............................................................

J   DISPUTE PROVISIONS

50. Governing Law and Jurisdiction...........................................
51. Dispute Resolution.......................................................
52. Legal Proceedings........................................................

K   CONFIDENTIALITY, NON-SOLICITATION AND STANDSTILL PROVISIONS

53. Confidentiality..........................................................
54. Additional provision of information by ImmunoGen.........................
55. Non-solicitation of Staff................................................
56. Standstill Provisions....................................................

L   WARRANTY AND INDEMNITY PROVISIONS

57. Representations and Warranties of each Party.............................
58. Representations and Warranties of ImmunoGen..............................
59. Non-compete..............................................................
</TABLE>

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<TABLE>
<S>                                                                             <C>
60. No Inconsistent Agreements...............................................
61. Representation by Legal Counsel..........................................
62. Indemnification..........................................................

M   GENERAL PROVISIONS

63. Bankruptcy protection....................................................
64. Notices..................................................................
65. Force Majeure............................................................
66. Assignments..............................................................
67. Severability.............................................................
68. Waiver...................................................................
69. VAT......................................................................
70. Costs of Preparation.....................................................
71. Government Consent.......................................................
72. Independent Discoveries by BB............................................
73. Independent Relationship.................................................
74. Counterparts.............................................................
75. Recording................................................................
76. Further Actions..........................................................
77. Entire Agreement.........................................................

Schedule 1    Part 1 - Chemical Structure of huN901..........................
              Part 2 - Chemical Structure of Licensed Compound...............
Schedule 2    Patents........................................................
Schedule 3    Development Plan...............................................
Schedule 4    Principal terms of Commercial Supply Agreement.................
Schedule 5    List of Third Party Licenses...................................
Schedule 6    Registration in the RoW based on an EU/USA Development Plan....
Schedule 7    CMC Development activities for huN901-DM1......................
Schedule 8    Calculation of COGS............................................
Schedule 9    Schedule of costs for bioanalytical work to be conducted by
              ImmunoGen under Clause 14.2....................................
Schedule 10   Further Third Party licenses...................................
</TABLE>

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THIS AGREEMENT is made the 4th day of May, 2000

BETWEEN

BRITISH BIOTECH PHARMACEUTICALS LIMITED an English company (registered number
1985479) whose registered office is at Watlington Road, Oxford OX4 6LY, England
("BB")

and

IMMUNOGEN, INC. a corporation organized and existing under the laws of the
Commonwealth of Massachusetts, USA and having its principal place of business at
333 Providence Highway, Norwood, Massachusetts, 02062 USA ("ImmunoGen")

WHEREAS

(A)      ImmunoGen owns and controls proprietary rights, titles and interest in
         the Patents, the Technical Information and the Manufacturing
         Information relating to a conjugated antibody known as huN901-DM1 and
         the components thereof.

 (B)     ImmunoGen has conducted a number of pre-clinical studies evaluating
         huN901-DM1 as an anti-cancer agent in the treatment of small cell lung
         cancer and the results of such studies have shown huN901-DM1 to be
         effective in treating models of small cell lung cancer at non-toxic
         doses.

(C)      The parties wish to enter into a development and commercialization
         collaboration whereby, inter alia:

         (1)      BB shall be responsible for the clinical development of
                  huN901-DM1 in the treatment of small cell lung cancer in the
                  Territory and the USA together with the right to develop
                  huN901-DM1 for other indications within the Field in the
                  Territory;

         (2)      ImmunoGen shall retain responsibility for all pre-clinical
                  activities and the manufacture and supply of huN901-DM1 for
                  clinical development, registration and commercialization
                  purposes both inside and outside the Territory;

         (3)      BB shall be responsible for filing and obtaining Regulatory
                  Approvals in the Territory and ImmunoGen shall be responsible
                  for filing and obtaining Regulatory Approvals outside the
                  Territory; and

         (4)      BB shall have the right to market and sell products containing
                  huN901-DM1 in the Territory and ImmunoGen shall have the right
                  to market and sell products containing huN901-DM1 outside the
                  Territory.

         subject to and in accordance with the terms and conditions set out
         below.

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NOW THEREFORE IT IS AGREED AS FOLLOWS:

1        DEFINITIONS

1.1      "Accounting Period" means any period of three (3) months ending on 31st
         January, 30th April, 31st July or 31st October (each an "Accounting
         Date") in any year. The first Accounting Period in a particular country
         in the Territory shall commence on the day of the first Launch in such
         country and shall terminate on the next Accounting Date at least three
         (3) months thereafter.

1.2      "Affiliate" means any Entity which (directly or indirectly) owns, is
         owned by or is under common ownership with a party to this Agreement or
         any Entity actually controlled by, controlling or under common control
         with a party to this Agreement. For the purposes of this definition
         "ownership" or "control" shall mean the possession (directly or
         indirectly) of more than fifty per cent (50%) of voting stock and/or
         the ability to direct the business affairs of another Entity.

1.3      "Agreed Clinical Study" means any study or studies in humans more
         particularly detailed in the Development Plan that will be undertaken
         by BB in order to obtain Regulatory Approval to market and sell Product
         in the Territory or the USA and to assist in obtaining Regulatory
         Approvals in other parts of the world for use in the treatment of the
         Primary Indication.

1.4      "BB Improvements" means any inventions, discoveries, improvements or
         enhancements relating to Licensed Compound or Product, whether
         patented, patentable or non-patentable, conceived or first reduced to
         practice by BB during the term of this Agreement by or on behalf of BB
         but independent of any patents, patent applications, technical
         information or know-how (including ImmunoGen Improvements) of ImmunoGen
         and in respect of which BB is free to disclose and grant licenses to
         ImmunoGen.

1.5      "BLA" means a Biologics License Application or a New Drug Application
         ("NDA") (whichever is applicable to Product) or equivalent filed with
         the FDA, a Marketing Authorization Application filed with the EMEA or a
         Marketing Authorization Application or a Product License Application or
         equivalent filed in any one or more of the countries within the
         Territory.

1.6      "Clinical Development" means all activities subsequent to the Effective
         Date relating to human clinical trials specifically required to support
         Regulatory Approvals to market and sell Product in the USA or the
         Territory. Clinical Development specifically excludes Manufacture, CMC
         Development and any and all activities relating to Pre-Clinical
         Studies, which shall be the responsibility of ImmunoGen.

1.7      "Clinical Study" means any study or studies in humans that will be
         undertaken in order to obtain or to assist in obtaining Regulatory
         Approval (i) to market and sell Product in the Territory or any part
         thereof for use in any part of the Field or (ii) to market and sell
         Product in any country outside the Territory.

1.8      "CMC Development" means all 'chemical, manufacturing and controls'
         development activities relating to Product designed to ensure the
         registration of the Product both inside and outside the Territory,
         including without limitation, Manufacture, scale-up, quality assurance,
         quality control, Product characterization and stability.

1.9      "COGS" means the costs of producing the Product as defined in Schedule
         8.

                                       6
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1.10     "Collaboration Committee" means the committee to be established
         pursuant to Clause 7.

1.11     "Commercialization Study" means any or all of the following:

         1.11.1   studies required to obtain local opinion leader support ahead
                  of marketing outside of the Territory;

         1.11.2   studies required to further study the use of the drug in
                  normal practice (for example Phase IV studies) outside the
                  Territory; or

         1.11.3   studies required to support pricing and reimbursement outside
                  the Territory.

1.12     "Confidential Information" means that information defined as
         confidential in Clause 53.1.

1.13     "Development Plan" means the plan, as amended from time to time, which
         sets forth:

         1.13.1   BB's strategies, plans, activities and estimated time
                  schedules with regard to Clinical Development in the Primary
                  Indication in the USA and the Territory; and

         1.13.2   BB's strategies, plans, activities and estimated time
                  schedules for obtaining Regulatory Approvals for Product in
                  the Territory;

                  a draft of which is attached as Schedule 3 and which shall be
                  agreed by the parties pursuant to Clause 13.3 and amended
                  pursuant to Clauses 13.4 or 13.5.

1.14     "DM1" means that maytansinoid drug whose more specific chemical name is
         N2'-deacetyl-N2'-(3-mercapto-1-oxopropyl)-maytansine.

1.15     "Effective Date" means 4th May 2000.

1.16     "EMEA" means the European Agency for the Evaluation of Medicinal
         Products or its successor in title.

1.17     "Entity" means, and includes, any person, firm or company or group of
         persons or unincorporated body.

1.18     "FDA" means the United States Food and Drug Administration or its
         successor in title.

1.19     "Field" means the treatment of any and all human therapeutic
         indications (including, without limitation, the Primary Indication) by
         use of Licensed Compound and/or Product either alone or in combination.

1.20     "Good Manufacturing Practice" or "GMP" means current good manufacturing
         practices for the methods to be used in, and the facilities and
         controls to be used for the Manufacture and/or holding of Licensed
         Compound or Product, all as set forth from time to time by the FDA and
         relevant regulatory authorities in such other countries within the
         Territory. With respect to the EU Territory, "GMP" means the standards
         set out in The Rules Governing Medicinal Products in the European
         Community, Volume IV and Annex to the EC Guide, Manufacture of
         Investigational Medicinal Products (111/3004/91-EN) as amended from
         time to time.

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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

1.21     "Gross Profits" means [*]

1.22     "huN901" means a humanized antibody which binds to the same epitope as
         N901. One such humanized antibody has the cDNA sequence outlined in
         Schedule 1, Part 1.

1.23     "huN901-DM1" means huN901 conjugated to DM1 using proprietary
         technology developed by ImmunoGen.

1.24     "ICH Guidelines" means the applicable guidelines recommended by the
         International Conference on Harmonization with regard to, amongst other
         things, the conduct of laboratory, clinical or manufacturing activities
         in respect of pharmaceutical products, as amended from time to time.

1.25     "ImmunoGen Improvements" means any inventions, discoveries,
         improvements or enhancements relating to Licensed Compound or Product,
         whether patented, patentable or non-patentable, conceived or first
         reduced to practice by ImmunoGen during the term of this Agreement by
         or on behalf of ImmunoGen, but independent of any patents, patent
         applications, technical information, know-how (including BB
         Improvements) of BB, and in respect of which ImmunoGen is free to
         disclose and grant licenses to BB.

1.26     "IND" means an Investigational New Drug Application filed with the FDA
         or any other regulatory approval that may be required to be issued in
         order to commence a Clinical Study in any one or more of the countries
         within the Territory.

1.27     "Independent Third Party" means any Entity other than ImmunoGen, BB or
         their respective Affiliates or any Sub-licensee.

1.28     "Joint Improvements" means any inventions, discoveries, improvements or
         enhancements relating to Licensed Compound or Product, whether
         patentable or non-patentable, conceived or first reduced to practice
         jointly by both BB and ImmunoGen or by either party with the use of any
         patents, patent applications, technical information or know-how or
         improvements of the other party.

1.29     "Launch" means, in respect of each country within the Territory, the
         first commercial sale of Product by BB or Sub-licensees within such
         country for the treatment of a particular disease indication within the
         Field following Regulatory Approval.

1.30     "Licensed Compound" means huN901-DM1. huN901-DM1 is illustrated by the
         structure more particularly set out in Schedule 1, Part 2.

1.31     "Licensed IP" means the Patents, Technical Information, Manufacturing
         Information, Third Party Licenses and ImmunoGen's interest in any Joint
         Improvements.

1.32     "Major EU Markets" means France, Germany, Italy, Spain and the UK.

1.33     "Manufacture" means all such steps and processes to be undertaken to
         produce Product in appropriate pharmaceutical form including, without
         limitation to the generality of the foregoing, formulation of Licensed
         Compound or Product with appropriate excipients for oral use, its
         tabletting or encapsulation, its filling into ampoules or vials for
         intravenous, subcutaneous or similar administration, all in-process and
         final quality control testing, assessment and release, the labeling,
         insertion of inserts and packaging, all in compliance with the
         applicable specifications and GMP.

                                       8
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1.34     "Manufacturing Information" means all information and know-how in the
         possession and at the free disposal of ImmunoGen in relation to the
         methods and processes for the Manufacture of huN901-DM1 or Product,
         including all in-process and final release test methods together with
         information and know-how relating to the co-formulation of Licensed
         Compound or Product with another product or pharmaceutical agent, to
         the extent applicable.

1.35     "Market Exclusivity" means, in respect of any country in the Territory
         where a Patent subsists, that BB can enforce its rights under the
         Patent licenses granted to it pursuant to this Agreement to prevent an
         Independent Third Party from commercializing a generic compound
         containing Licensed Compound or Product (whether alone or in
         combination with other therapeutically active compounds).

1.36     "N901" means any antibody or fragment thereof which has the six
         complementarity-determining region sequences as described in Schedule
         1, Part 1.

1.37     "Net Sales" means the total gross amounts invoiced in respect of all
         sales of Product by or on behalf of BB to an Independent Third Party,
         and exclusive of inter-company transfers or inter-company sales, less:

         1.37.1   normal and customary trade, cash and quantity discounts,
                  allowances and credits granted or allowed;

         1.37.2   credits or allowances actually granted for damaged goods,
                  returns or rejections of Product and retroactive price
                  reductions;

         1.37.3   sales taxes, duties or other taxes with respect to such sales
                  (including duties or other governmental charges levied on,
                  absorbed or otherwise imposed on the sale of Product
                  including, without limitation, value added taxes or similar
                  taxes or other governmental charges otherwise measured by the
                  billing amount, when included in billing but excluding income
                  or other taxes levied with respect to gross receipts) actually
                  collected by BB or Sub-licensees;

         1.37.4   insurance, postage, customs duties and transportation costs
                  incurred in shipping Product to any Independent Third Party by
                  BB or Sub-licensees to the extent separately itemized and
                  included in the invoiced amount;

         1.37.5   charge back payments and rebates granted to managed health
                  care organizations or to federal, state and local governments,
                  their agencies, and purchasers and reimbursers or to trade
                  customers, including but not limited to, wholesalers and chain
                  and pharmacy buying groups;

         1.37.6   rebates (or equivalents thereof) granted to or charged by
                  national, state or local governmental authorities in countries
                  other than the United States.

1.38     "Patents" means the patents and patent applications set out in Schedule
         2 and any other patents or patent applications that ImmunoGen has been
         granted or has filed in the Territory or any part thereof at the
         Effective Date and any further patents or patent applications that
         ImmunoGen may be granted or may file in the Territory or any part
         thereof with regard to the composition of matter, methods of
         administration, processes and/or intermediates for preparing,
         formulation or use of Licensed Compound or Product in the Field
         (whether alone or in combination with another

                                       9
<PAGE>   10
         product or pharmaceutical agent) including patent applications made in
         respect of ImmunoGen Improvements thereto, and any continuations,
         continuations-in-part, patents of addition, revisions, divisions,
         substitutions, registrations, confirmations, any other patent
         applications claiming priority from any of those patents or patent
         applications, re-examinations, reissues, renewals or extensions
         (including supplementary protection certificates or their equivalent)
         of any such patents and international equivalents thereof.

1.39     "Pre-Clinical Development" means all activities relating to the conduct
         of Pre-Clinical Studies specifically required to support Regulatory
         Approvals to market and sell Product in the USA or the Territory.

1.40     "Pre-Clinical Study" means any one of those studies, other than a
         Clinical Study, carried out, in vivo or in vitro, by or on behalf of
         ImmunoGen on Licensed Compound and/or Product including, without
         limiting the generality of the foregoing, drug absorption,
         distribution, metabolism and excretion ("ADME"), toxicology,
         pharmacokinetics, pathology, pharmaceutical formulation, drug
         metabolism, stability and pharmacology studies which are required or
         advisable to be carried out in order to obtain the grant of an IND to
         develop Licensed Compound and/or Product in any one or more countries
         of the Territory and/or required for Regulatory Approval purposes.

1.41     "Primary Indication" means small cell lung cancer.

1.42     "Product" means Licensed Compound in any pharmaceutical form or dosage
         in which Licensed Compound is developed, manufactured, packaged,
         marketed, used or sold in final form by prescription, over-the-counter
         or any other method in accordance with the terms of this Agreement.

1.43     "Regulatory Approval" means the technical, medical and scientific
         licenses, registrations, authorizations or approvals (including,
         without limitation, approvals of BLA's, supplements, amendments,
         pre-and post-approvals, pricing and third party reimbursement
         approvals; marketing authorizations based upon such approvals,
         including any prerequisite manufacturing approvals or authorizations
         related thereto and labeling approval(s)), of any national, regional,
         state or local regulatory agency, department, bureau, commission,
         council or other governmental entity, necessary for the development,
         manufacture, distribution, marketing, promotion, offer for sale, use,
         import, export or sale of Licensed Compound and/or Product by BB in the
         Territory or any part thereof or by ImmunoGen outside the Territory or
         any part thereof.

1.44     "Regulatory Authority" means the EMEA or its equivalent in each part of
         the Territory or the FDA in North America or its equivalent in other
         countries outside the Territory.

1.45     "Specifications" means the specifications for Licensed Compound and
         Product and any raw materials employed in the Manufacture thereof to be
         established pursuant to Clause 19.

1.46     "Sub-licensee" means any Entity, not being an Affiliate of BB, which is
         authorized directly by BB through express license or consent to
         develop, make, have made, import, export, use, keep (whether for
         disposal or otherwise), distribute, market, promote, offer for sale and
         sell Licensed Compound and/or Product in the Territory or any part
         thereof for use in the Field in accordance with the provisions of this
         Agreement.

1.47     "Technical Information" means all information, know-how and/or
         expertise in relation to the research, development or use of huN901-DM1
         in the Field, whether alone or in combination with another product or
         pharmaceutical agent, (excluding Manufacturing Information) including,

                                       10
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         without limiting the generality of the foregoing, (1) results of
         Pre-Clinical Studies and Clinical Studies, (2) discoveries, practices,
         methods, knowledge, processes, ideas, skill, experience, know-how,
         technology, trade secrets, purification and isolation techniques,
         instructions, formulae, data, assays, drawings and designs, (3)
         chemical, pharmacoeconomic, toxicological, pharmacological, analytical,
         safety, quality control and testing data, and (4) all applications,
         registrations, licenses, authorizations, approvals and correspondence
         submitted to or received from any regulatory authorities in the
         Territory relating to huN901-DM1 (including, without limitation,
         minutes and meeting notes relating to any communications with any
         regulatory authority in the Territory relating to huN901-DM1) in the
         possession and at the free disposal of ImmunoGen, BB or Sub-licensees.

1.48     "Territory" means:

         1.48.1   all countries for the time being of the European Union ("EU")
                  and the European Economic Area ("EEA") as constituted at the
                  Effective Date. For the avoidance of doubt, new member
                  countries of the EU and/or the EEA will automatically be
                  included in the definition of Territory provided that
                  ImmunoGen has not otherwise licensed the Licensed IP to
                  another party in that country between the Effective Date and
                  the date the country joins either the EU or the EEA; and

         1.48.2   Japan.

1.49     "Third Party Licenses" means the licenses to third party intellectual
         property rights, including without limitation, patents, patent
         applications, trade secrets, technical information and/or know-how
         covering or related to N901, huN901, DM1, huN901-DM1, Licensed
         Compound, Product or the manufacture thereof, under which ImmunoGen or
         its Affiliates have a right to grant a sublicense to BB. A list of
         Third Party Licenses is attached as Schedule 5.

1.50     The singular includes the plural and vice versa, words denoting any
         gender include all genders.

1.51     Where the context so admits or requires, references to "ImmunoGen" and
         "BB" and "Sub-licensee(s)" shall include their respective employees,
         officers, directors and agents.

1.52     Headings to Clauses in this Agreement are included for convenience only
         and shall not affect the construction and interpretation of this
         Agreement.

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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                        PART A - COMMENCEMENT PROVISIONS

2        GRANT AND ACCEPTANCE OF RIGHTS

2.1      GRANT to BB. Subject to the terms and conditions of this Agreement,
         ImmunoGen grants to BB and its Affiliates an exclusive license,
         exclusive even as to ImmunoGen, under the Licensed IP to:

         2.1.1    develop Licensed Compound for the Primary Indication in the
                  USA and the Territory;

         2.1.2    develop Licensed Compound for such other indications as may be
                  agreed pursuant to Clause 16 in the Field in the USA and the
                  Territory;

         2.1.3    file applications for, and obtain, Regulatory Approvals
                  relating to Licensed Compound in the Field in the Territory;
                  and

         2.1.4    import, export, keep (whether for disposal or otherwise),
                  market, distribute, use, promote, offer for sale and sell
                  Licensed Compound or Product in the Territory;

         together with the right to grant sub-licenses pursuant to Clauses 3.1
         and 3.2.

         BB accepts the exclusive rights granted by ImmunoGen pursuant to Clause
         2.1 subject to the terms and conditions of this Agreement.

2.2      GRANT TO ImmunoGen. Subject to the terms and conditions of this
         Agreement BB grants to ImmunoGen and its Affiliates an exclusive
         license (exclusive even as to BB), under BB Improvements and BB's
         interest in Joint Improvements, to:

         2.2.1    develop Licensed Compound for such other indications as may be
                  agreed pursuant to Clause 16 in the Field outside of the
                  Territory;

         2.2.2    file applications for, and obtain, Regulatory Approvals
                  relating to Licensed Compound in the Field outside the
                  Territory; and

         2.2.3    import, export, keep (whether for disposal or otherwise),
                  market, distribute, use, promote, offer for sale and sell
                  Licensed Compound or Product outside the Territory.

                  together with the right to grant such licenses pursuant to
                  Clauses 3.1 and 3.2.

2.3      THIRD PARTY TECHNOLOGY. The parties acknowledge that the licenses
         granted to BB in this Agreement include sub-licenses under Third Party
         Licenses. [*] BB agrees to abide by the terms and conditions of such
         Third Party Licenses applicable to BB as ImmunoGen's sublicensee.
         ImmunoGen shall use reasonable commercial efforts to abide by the terms
         and conditions of all Third Party Licenses to maintain the Third Party
         Licenses for BB as ImmunoGen's sublicensee. ImmunoGen agrees not to
         terminate or assign, nor by act or omission permit the termination or
         assignment of, any of the Third Party Licenses, nor to amend or by act
         or omission permit the amendment of any Third Party Licenses to the
         extent such an amendment would adversely affect BB's rights under this
         Agreement, without the prior written consent of BB, which consent will
         not be unreasonably withheld. Within [*] after entering into any
         amendment of a Third Party License, ImmunoGen shall notify BB and
         provide BB with a copy of the amendment. To the extent that ImmunoGen
         enters into any Further Third Party Licenses (as such term is defined
         in Clause 41) the provisions of Clause 41 shall apply.

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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

2.4      REGISTRATION OF LICENSE. BB and any Sub-licensee shall be entitled to
         be registered as a licensee of any Patents in respect of the rights
         granted pursuant to Clause 2.1 in the Field in any part of the
         Territory where such registration is possible and ImmunoGen shall do
         all acts and sign and swear such documents as may be reasonably
         required by BB to procure such registration and all fees and expenses
         reasonably incurred by ImmunoGen associated with such registration
         shall be borne by BB.

3        SUB-LICENSES

3.1      RIGHT TO GRANT. Either party shall have the right to grant sub-licenses
         under the license granted by Clause 2.1 and 2.2, respectively, and to
         employ Affiliates and Independent Third Parties in connection with the
         performance of its rights and obligations under this Agreement, subject
         to any limitations imposed upon either party's ability to sub license
         by any Third Party License. BB's right to grant sub-licenses under this
         Clause shall be exercisable provided that BB shall remain responsible
         as primary obligor to ImmunoGen for the performance of its Affiliates
         and/or Sub-licensees and provided, further, that BB shall remain
         obligated to ensure payment of all compensation and royalty obligations
         as set forth in Section C. BB confirms that it shall not grant a
         Sub-licensee any greater rights than is reasonably required to enable
         such Sub-licensee to fulfill its contractual obligations to BB.

3.2      TERMS OF SUB-LICENSE. The terms and conditions of each sub-license
         granted pursuant to Clause 3.1 shall be communicated to the other party
         in writing within [*] of the grant thereof (excluding financial terms
         or information relating to products which are not covered by this
         Agreement) and in order to be validly granted hereunder shall contain
         the following provisions:

         3.2.1    that the sub-license shall be personal to the Sub-licensee and
                  shall not be assignable or licensable;

         3.2.2    covenants by the Sub-licensee to observe and perform
                  conditions at least equivalent to those contained in this
                  Agreement in respect of that part of the Territory to which
                  the sub-license applies insofar as the same are applicable
                  and, in addition, such sub-license shall also contain
                  provisions for ipso facto termination in whole or in part in
                  the event of, and contemporaneously with, the termination of
                  this Agreement or termination of this Agreement in respect of
                  that part of the Territory to which the sub-license applies.

         Any sub-license purported to be granted by either party and not
         containing the above provisions shall be deemed invalid and of no
         effect against the other party until such time as such party remedies
         the discrepancies.

4        OTHER COUNTRIES

4.1      RESTRICTIONS ON FUTURE ImmunoGen LICENSEES. Subject to Clause 4.2
         below, ImmunoGen retains the right to grant licenses for the sale,
         marketing and distribution of the Product in all countries outside the
         Territory; provided, however, that ImmunoGen agrees that for each such
         license entered into after the Effective Date, it will impose on each
         such licensee, to the extent permitted by applicable law, a covenant
         prohibiting the licensee from:

         4.1.1    seeking approval, directly or indirectly, from the relevant
                  Regulatory Authorities, to label or re-label the Product in a
                  manner that would permit it to be marketed or sold inside the
                  Territory,

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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

         4.1.2    selling or exporting the Product to any Independent Third
                  Party for use or resale inside the Territory,

         4.1.3    selling the Product to any Independent Third Party that
                  ImmunoGen has reason to believe intends to resell or export
                  the Product inside the Territory.

4.2      COVENANT. ImmunoGen covenants with BB, [*]:

         4.2.1    [*];

         4.2.2    [*];

         4.2.3    [*];

         4.2.4    [*];

         4.2.5    [*].

5        PROVISION OF TECHNICAL INFORMATION AND ASSISTANCE

5.1      TIMETABLE FOR PROVISION. Within [*] of the Effective Date, ImmunoGen
         shall make available to BB, and shall provide BB with a list of, all
         Technical Information in its possession or control which it has not
         previously disclosed to BB. BB shall, at its cost, be entitled to
         conduct a review of such Technical Information and shall be entitled to
         require ImmunoGen to provide BB with copies of such documentation as it
         may request. Within [*] following receipt of such request, ImmunoGen
         shall, at its cost, provide BB with copies of all such requested
         documentation.

5.2      FURTHER TECHNICAL INFORMATION. Any further Technical Information
         developed or acquired (whether by license, assignment or otherwise) by
         ImmunoGen or which otherwise comes into ImmunoGen's possession during
         the term of this Agreement shall be made available to BB at no cost as
         soon as reasonably practicable after such development or acquisition by
         ImmunoGen, to the extent ImmunoGen has the right to make such Technical
         Information available to BB.

5.3      ASSISTANCE. ImmunoGen shall,[*] provide reasonable technical assistance
         to enable BB to utilize such Technical Information.

6        TERM

6.1      This Agreement shall commence on the Effective Date and, unless
         otherwise terminated, on a country by country basis, shall expire:

         6.1.1    [*];

         6.1.2    [*].

6.2      Upon the expiry of this Agreement by passage of time on a country by
         country basis (as determined pursuant to Clause 6.1), BB shall not be
         precluded from using any Technical Information or Manufacturing
         Information without payment of royalties and shall be automatically
         granted a fully paid-up, perpetual, transferable, royalty free,
         non-exclusive license

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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

         to the Licensed IP (including, for the avoidance of doubt, ImmunoGen
         Improvements) to develop, apply for Regulatory Approvals, import,
         export, keep (whether for disposal or otherwise), market, distribute,
         use, promote, offer for sale and sell Licensed Compound and/ or Product
         in the Field in the Territory.

7        FORMATION OF COLLABORATION COMMITTEE

7.1      ESTABLISHMENT. To facilitate the development collaboration between the
         parties contemplated by this Agreement, within [*] of the Effective
         Date, the parties shall establish the Collaboration Committee
         containing, for example, senior scientists, clinical research,
         manufacturing, regulatory and/or marketing/sales members from both
         parties.

7.2      APPOINTMENT OF MEMBERS. The Collaboration Committee shall consist of
         six (6) members, three (3) of whom shall be appointed by BB and three
         (3) of whom shall be appointed by ImmunoGen. Each party shall designate
         its members to the Collaboration Committee and shall notify the other
         party in writing if it substitutes or replaces any of its members,
         whether on a permanent basis or due to the unavailability of a
         particular member to attend a particular meeting.

7.3      MEETINGS. The Collaboration Committee shall meet at least four (4)
         times per year unless otherwise agreed by the parties. Meetings may be
         held by telephone or video conference or in person and members may
         participate in any of the foregoing ways, it being recognized that it
         is the current intention of the parties that the Collaboration
         Committee should meet in person at least twice a year. To the extent
         that it is necessary for the Collaboration Committee to meet in person,
         unless otherwise agreed, such meetings shall alternate between Oxford,
         UK and Boston, Massachusetts, USA. All costs relating to participation
         by each member in the activities of the Collaboration Committee shall
         be borne by the party appointing such member. Furthermore, if the
         parties regard it as being necessary, the Collaboration Committee shall
         be allowed to constitute sub-committees to deal with specific issues
         that may arise and in such event the provisions of this Clause shall
         govern the conduct and voting at such sub-committee meetings.

7.4      LANGUAGE. All meetings of the Collaboration Committee shall be held in
         the English language and all documentation submitted to, or generated
         by, the Collaboration Committee shall be in the English language.

7.5      QUORUM; VOTING. The presence of at least two (2) members, one (1) of
         whom shall have been selected by each party, shall constitute a quorum
         for the purpose of consideration and action by the Collaboration
         Committee. Each party shall have only one (1) vote, irrespective of the
         number of attendees at such meeting.

7.6      DEADLOCK. The Collaboration Committee shall strive to make decisions by
         unanimous consensus. In the event that consensus cannot be reached on
         matters assigned to the Collaboration Committee, then, save as
         otherwise provided in this Agreement:

         7.6.1    [*];

         7.6.2    [*].

7.7      CHAIRMAN AND SECRETARY: RESPONSIBILITIES. The Collaboration Committee
         shall appoint a Chairman and a Secretary from the members, it being
         agreed that the Chairman shall be a member appointed by BB and the
         Secretary shall be a member appointed by ImmunoGen. The Chairman shall
         be responsible for calling meetings and preparing and circulating
         agendas. The

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         Secretary shall be required to prepare the minutes of the Collaboration
         Committee meetings and to circulate these to the members within [*]
         following the conclusion of the meeting. All Collaboration Committee
         minutes shall, following review by the parties and agreement of their
         contents, be countersigned on behalf of each party as evidence of
         acceptance of the correctness of such minutes.

7.8      PARTICIPATION OF NON-MEMBERS. Additional non-voting representatives or
         consultants may be invited by either ImmunoGen or BB to attend and
         participate in Collaboration Committee meetings (for example, to
         evaluate and advise on business or scientific issues) subject to
         compliance with the confidentiality provisions of Clause 53. Each party
         shall be responsible for the costs and expenses incurred in connection
         with the attendance and participation in Collaboration Committee
         meetings of representatives or consultants invited by such party.

7.9      COMMITTEE RESPONSIBILITIES. The Collaboration Committee shall be
         responsible, without limitation, for monitoring and coordinating
         certain of the parties activities regarding the development and
         commercialization of Licensed Compound and/or Product as set out below:

         7.9.1    [*];

         7.9.2    [*];

         7.9.3    [*];

         7.9.4    [*];

         7.9.5    [*];

         7.9.6    [*];

         7.9.7    [*];

         7.9.8    [*];

         7.9.9    [*];

         7.9.10   [*];

         7.9.11   [*]

         7.9.12   [*]

         7.9.13   [*]

         7.9.14   [*];

         7.9.15   [*];

         7.9.16   [*];

         7.9.17   [*]

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         7.9.18   [*].

7.10     ADDITIONAL RESPONSIBILITY: THIRD PARTY REQUESTS. In addition to its
         other responsibilities, the Collaboration Committee shall also be
         responsible for considering any requests from Independent Third Parties
         relating to potential investigator-held IND studies, compassionate use
         of Licensed Compound or extension studies, and to the extent that the
         Collaboration Committee unanimously agrees to the commencement of such
         activity it shall agree how the costs relating thereto shall be borne
         by the parties;

7.11     PERIOD OF EXISTENCE. The Collaboration Committee shall remain in
         existence until the termination of the Agreement in all countries
         within the Territory or until disbanded by agreement of the parties.

7.12     Save as specifically provided in this Agreement, the Collaboration
         Committee shall not be entitled or have the power to vary or amend the
         terms of this Agreement or commit either party to any financial
         expenditure not contemplated in this Agreement.

                      PART B - ADDITIONAL GRANT PROVISIONS

8        [*]

8.1      [*]

9        [*]

9.1      In the event that the [*]:

         9.1.1    [*];

         9.1.2    [*];

         9.1.3    [*];

         9.1.4    [*];

         [*]

9A       [*]

9A.1     If during the term of this Agreement, [*]

9A.2     In the event that the parties are unable to reach agreement on terms
         following [*]days good faith discussions, such negotiations shall
         cease, unless otherwise agreed.

                        PART C - COMPENSATION PROVISIONS

10       COMPENSATION

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10.1     INITIAL PAYMENT. In consideration of the rights granted to it under
         this Agreement, BB shall pay in cash to ImmunoGen the sum of one
         million five hundred thousand United States dollars (US$1,500,000)
         within ten (10) business days following the Effective Date.

10.2     PAYMENT BY ImmunoGen. In consideration for the performance of those
         clinical trials which BB has agreed to conduct under this Agreement,
         ImmunoGen agrees to pay to BB the sum of [*] days of the grant to
         ImmunoGen of a BLA in the USA.

10.3     MODE OF PAYMENT. Payment of the consideration referred to in Clause
         10.1 or 10.2 shall be paid by check or bank wire transfer in
         immediately available funds to such bank account as is designated in
         writing by the receiving party.

11       ROYALTIES IN RESPECT OF SALES OF LICENSED COMPOUND AND/OR PRODUCT

11.1     As further consideration for the rights granted to BB under this
         Agreement, BB shall pay to ImmunoGen a royalty in respect of Gross
         Profit for Licensed Compound and/or Product in the Territory made by BB
         or Sub-licensees, for the term of this Agreement as provided for in
         Clause 6, on the following basis:

         11.1.1   BB shall pay a royalty of[*] of Gross Profit to ImmunoGen in
                  respect of sales of Product in each country in the Territory
                  where [*].

         11.1.2   BB shall pay a royalty of [*] of Gross Profit to ImmunoGen in
                  respect of sales of Product in each country in the Territory
                  where:

                  11.1.2.1  [*]

                  11.1.2.2  [*].

11.2     INTERNAL TRANSFERS. No royalties shall be due upon the sale or other
         transfer of Licensed Compound or Product among BB or Sub-licensees, but
         in such cases the royalty shall be due and calculated upon BB's or its
         Sub-licensees' sale to the first Independent Third Party.

11.3     SAMPLES AND DONATIONS. No royalties shall accrue on the disposition of
         Licensed Compound or Product by BB or Sub-licensees as samples
         (promotion or otherwise) or as donations (to non-profit institutions or
         government agencies for a non-commercial purpose) or for Clinical
         Studies, it being agreed that BB or Sub-licensees shall not in
         aggregate dispose of more than [*] of annual sales of Licensed Compound
         and Product by way of sample or donation, it being further expressly
         agreed that donations of Licensed Compound and Product [*].

11.4     PAYMENT OF ROYALTIES; CURRENCY AND EXCHANGE. Payments to be made by BB
         to ImmunoGen under this Agreement, and in accordance with Clause 12.4
         and 12.5, shall be paid by check or by bank wire transfer in
         immediately available funds to such bank account as is designated in
         writing by ImmunoGen from time to time. Royalties shall be deemed
         payable from the country in which they are earned in local currency and
         subject to foreign exchange regulations then prevailing. Royalty
         payments shall be made in United States dollars to the extent that free
         conversion to United States dollars is permitted. The rate of exchange
         to be used in any such conversion from the currency in the country
         where such sales are made shall be the average rate of exchange
         applicable for the relevant Accounting Period as determined by the Bank
         of New York. If, due to restrictions or prohibitions imposed by any
         national or international authority, payments cannot be made as
         aforesaid, the parties shall consult with a view to finding a prompt

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         and acceptable solution, and BB will deal with such monies as ImmunoGen
         may lawfully direct at no additional out-of-pocket expense to BB.
         Notwithstanding the foregoing, if royalties in any country cannot be
         remitted to ImmunoGen for any reason within six (6) months after the
         end of the relevant Accounting Period during which they are earned,
         then BB shall be obligated to deposit the royalties in a bank account
         in such country in the name of ImmunoGen.

12       ACCOUNTING AND PAYMENT

12.1     AUDIT OF RECORDS. BB shall, and shall procure that any Sub-licensee
         shall, keep at its usual place of business complete and proper records
         and books of account showing the quantity, description and sales of
         Licensed Compound and/or Product sold hereunder and in respect of each
         country within the Territory. Upon forty-five (45) days prior written
         notice from ImmunoGen, BB shall permit an independent certified public
         accounting firm of nationally recognized standing selected by ImmunoGen
         and approved by BB (such approval not to be unreasonably withheld or
         delayed) at ImmunoGen's expense, to examine pertinent books and records
         of BB and Sub-licensees at their respective usual places of business as
         may be reasonably necessary to verify the accuracy of the royalty
         reports hereunder and BB shall procure the cooperation of Sub-licensees
         in this regard. BB may designate competitively sensitive information
         which such auditor may see and review but which it may not disclose to
         ImmunoGen; provided however, that such designation shall not encompass
         or restrict the auditor's conclusions. The accounting firm shall be
         entitled to report to ImmunoGen on the scope of its audit, any
         reservations that it may have resulting from such audit, any other
         material information which it determines in its reasonable opinion
         should be brought to ImmunoGen's attention and to disclose to ImmunoGen
         whether it is of the opinion that the royalty reports are correct or
         incorrect and the specific details concerning any discrepancies. No
         other information shall be provided to ImmunoGen. ImmunoGen shall
         obtain a confidentiality agreement (in form and substance reasonably
         acceptable to BB) requiring such auditor to keep any information
         gathered from the inspection of such records and books confidential,
         for the sole purpose of verifying the accuracy of the payments made by
         BB under this Agreement. Such review shall be conducted no more
         frequently than once per calendar year and shall be scheduled during
         ordinary business hours at such a time as is reasonably acceptable to
         BB.

12.2     RETENTION OF RECORDS. Such books and records referred in Clause 12.1
         shall be retained by BB or Sub-licensees for two (2) years from the
         date of their origin; provided that, if a review is requested during
         the third year, each such book and record subject to review shall be
         retained for one (1) year beyond the completion of the review. Any
         adjustment required as a result of such review to the sums payable by
         BB to ImmunoGen shall be made in the next payment by BB to ImmunoGen.
         Furthermore, in the event of a dispute between the parties relating to
         such books or accounts, BB shall, and shall procure that Sub-licensees
         shall, retain such books and accounts until such dispute is settled or
         a final unappealable decision has been reached by the competent
         judicial authorities.

12.3     BINDING CALCULATION. Upon the expiration of the two (2) year period set
         forth above, the calculation of royalties payable under this Agreement
         with respect to such year shall be binding and conclusive upon the
         parties, and BB and Sub-licensees shall be released from any liability
         or accountability with respect to royalties for such calendar year.

12.4     PROVISION OF ROYALTY STATEMENT. Within forty-five (45) days of the end
         of each Accounting Period, BB shall submit or cause to be submitted to
         ImmunoGen a written statement in respect of BB and Sub-licensees
         recording the quantity and description and of Licensed Compound and/or
         Product invoiced hereunder in each country in the Territory during the
         relevant

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         Accounting Period to which the report relates together with a
         calculation of Gross Profit. In the event that no invoiced sales of
         Licensed Compound and/or Product have been made during any such
         Accounting Period, BB shall submit a nil statement.

12.5     SIMULTANEOUS PAYMENT. Simultaneously with the submission of such
         written statement, BB shall pay to ImmunoGen a sum equal to the royalty
         due for such Accounting Period, calculated in accordance with this
         Agreement (reconciled for any previous overpayments or underpayments).

12.6     WITHHOLDING TAXES. If at any time, any jurisdiction within the
         Territory requires the withholding of income taxes or other taxes
         imposed upon payments set forth in this Agreement, BB shall make such
         withholding payments as required and subtract such withholding payments
         from the payments set forth in this Clause. BB shall provide ImmunoGen
         with documentation of such withholding and payment in a manner that is
         satisfactory for purposes of UK tax laws. Any withholdings paid when
         due hereunder shall be for the account of ImmunoGen and shall not be
         included in any calculation of Gross Profit. To the extent that
         payments of withholding taxes made by BB pursuant to this Clause are
         based upon financial information to be provided to BB by ImmunoGen and,
         to the extent that such information is incorrect or incomplete,
         ImmunoGen shall be liable for any fine, assessment or penalty, or any
         deficiency, imposed by any taxing authority in the Territory for any
         deficiency in the amount of any such withholding or the failure to make
         such withholding payment. If BB is required to pay any such deficiency,
         or any such fine, assessment or penalty for any such deficiency,
         ImmunoGen shall promptly reimburse BB for such payments, which shall
         not be included in the calculation of Gross Profit. BB shall and shall
         procure that any Sub-licensees shall, at its cost, give ImmunoGen such
         reasonable assistance, which shall include the provision of appropriate
         certificates of such deductions made together with other supporting
         documentation as may be required by the relevant tax authority, to
         enable ImmunoGen to claim exemption from such withholding or other tax
         imposed or obtain a repayment thereof or reduction thereof and shall
         upon request provide such additional documentation from time to time as
         is reasonably required to confirm the payment of tax.

                    PART D - CLINICAL DEVELOPMENT PROVISIONS

12A      PREPARATION AND FILING OF IND APPLICATION

12A.1    While it is agreed that BB shall be responsible for the preparation and
         submission of the application for an IND in its name, it is
         acknowledged that ImmunoGen is in possession of, and developed, certain
         Technical Information that may form a substantive proportion of the IND
         application. In recognition of the foregoing, as soon as reasonably
         practicable following the Effective Date, the parties shall meet to
         commence the preparation of the IND application and ImmunoGen shall
         provide BB with all necessary information, advice, assistance,
         documentation and data as may be reasonably requested by BB for and in
         connection with the preparation of such IND application.

12A.2    If requested by BB, ImmunoGen shall attend a pre-IND submission meeting
         with the relevant Regulatory Authorities and answer such questions as
         the Regulatory Authorities may have relating to the data generated by
         ImmunoGen and to advise and assist BB as may be reasonably required. To
         the extent that a Regulatory Authority requires any amendments to the
         IND which require the assistance of ImmunoGen, ImmunoGen shall provide
         such assistance. In addition, if

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         any further assistance is required following submission, and prior to
         approval of the IND approval, ImmunoGen provide such assistance to BB
         in accordance with any timetable set by BB for responding to such
         Regulatory Authority's requests.

12A.3    If, following the pre-IND meeting, BB reasonably believes that the
         Development Plan requires amendment(s) to reflect the views expressed
         or comments made by the Regulatory Authority, the Collaboration
         Committee shall meet to agree any amendments to be made to such Plan.

13       DEVELOPMENT PLAN

13.1     CONTENTS OF PLAN. The Development Plan will set out, amongst other
         things, the details of those Agreed Clinical Studies that BB is
         obligated to undertake, a copy of which is attached at Schedule 3. It
         is expressly agreed that BB may discharge its responsibilities under
         the Development Plan through its Affiliates, Sub-licensees or
         Independent Third Parties under contract.

13.2     OBJECTIVE OF PLAN. The initial objective of the Development Plan shall
         be the generation of a body of data (in addition to the Technical
         Information) that shall be sufficient to support the filing of
         applications for Regulatory Approvals to enable (a) BB to commence the
         sale of Product in respect of the treatment of the Primary Indication
         in the Territory; and (b) ImmunoGen to commence the sale of Product in
         respect of the treatment of the Primary Indication in the USA.

13.3     AGREEMENT OF PLAN. The Development Plan shall be agreed by the parties
         within [*] of the Effective Date (subject always to any amendments
         required pursuant to Clause 12A.3). For this purpose, representatives
         of the parties shall meet as frequently as required following the
         Effective Date in order to agree the Development Plan. Following
         agreement of the Development Plan by the parties, such Plan shall be
         submitted to the FDA for review and approval at a pre-IND meeting. Any
         amendments required by the FDA shall be incorporated into the
         Development Plan. The Development Plan shall be designed on the
         assumption that BB shall devote all reasonably necessary resources
         (including financial and manpower resources) in the conduct of the
         Agreed Clinical Studies in order to meet the timetable stipulated in
         the Development Plan.

13.4     ANALYSIS OF DATA; REVISIONS TO PLAN. Following agreement of the
         Development Plan, the Collaboration Committee shall regularly review
         such Plan in the light of clinical data arising from the reporting of
         each of the Agreed Clinical Studies and to changing regulatory
         requirements and consider the effect (if any) that such data or
         regulatory changes may have on the Development Plan and agree any
         appropriate revisions to be made thereto.

13.5     AMENDMENTS TO PLAN. In agreeing the Development Plan the parties
         recognize that:

         13.5.1   certain reasonable variations (both shortening and
                  lengthening) of the Development Plan timetable may be required
                  in which case the parties shall be able to make such
                  amendments as may be necessary and/or appropriate to the
                  Development Plan, which revised plan shall thereafter become
                  the Development Plan;

         13.5.2   the Development Plan (or parts thereof) may be unattainable if
                  unforeseen material technical problems beyond BB's control
                  arise (which shall not include technical problems arising as a
                  result of insufficient resource being devoted to the
                  Development Plan by

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                  either party or Sub-licensees) and that significant alteration
                  or amendment may have to be made to the Development Plan (or
                  parts thereof) as a result of such technical problems. Should
                  this occur, any amendment that may be necessary to the
                  Development Plan shall be agreed between the parties and such
                  amended Development Plan shall thereafter be the Development
                  Plan.

13.6     COORDINATION OF PLAN. The Development Plan shall be coordinated and
         supervised by the Collaboration Committee. In order to enable the
         Collaboration Committee to carry out its duties of coordination and
         supervision with regard to the Development Plan, BB shall, following
         agreement of the Development Plan, be required to report to each
         meeting of the Collaboration Committee which shall include the progress
         made with the Development Plan since the Effective Date or the last
         meeting of the Collaboration Committee (as the case may be).

13.7     ImmunoGen DEVELOPMENT PLAN.

         13.7.1   Within [*] of the Effective Date the Collaboration Committee
                  shall meet to agree a development plan prepared by ImmunoGen
                  relating to ImmunoGen's strategies, plans, activities and
                  estimated time schedules for obtaining Regulatory Approval in
                  the USA.

         13.7.2   In due course, following the grant of Regulatory Approval in
                  the USA, the Collaboration Committee shall meet to agree a
                  development plan prepared by ImmunoGen relating to ImmunoGen's
                  plans, activities and estimated time schedules for obtaining
                  Regulatory Approval in such other of ImmunoGen's retained
                  territories.

         Once such plans have been agreed, the provisions of Clauses 13.4, 13.5
         and 13.6 (as adjusted, to the extent necessary, to substitute ImmunoGen
         for BB) shall apply in respect of such plans.

14       PRE-CLINICAL STUDIES AND PHARMACOKINETIC STUDIES

14.1     RESPONSIBILITY. ImmunoGen shall be responsible, at its cost, for all
         necessary Pre-Clinical Development required to be conducted in respect
         of the development of Licensed Compound or Product to enable Regulatory
         Approval to be obtained both inside and outside the Territory. The
         Collaboration Committee shall be responsible for determining what
         Pre-Clinical Studies need to be conducted with regard to development of
         the Licensed Compound and in this regard within [*] of the Effective
         Date, the Collaboration Committee shall meet to agree upon a formal
         Pre-Clinical Development plan. Once such plan has been agreed, the
         provisions of Clauses 13.4, 13.5 and 13.6 (as adjusted, to the extent
         necessary, to substitute ImmunoGen for BB and references to Development
         Plan for Pre-Clinical Development plan) shall apply in respect of such
         plan.

14.2     It is further agreed that ImmunoGen shall carry out, on BB's behalf,
         all necessary bioanalysis in support of the phase I Clinical
         Development programme, including, without limitation, plasma huN901 and
         huN901-DM1 (conjugate) pharmacokinetics (PK) by ELISA and human
         anti-human antibody (HAHA) and human anti-DM1 antibody (HADA)
         responses. Plasma huN901-DM1 bioactivity may also be measured in plasma
         samples, as agreed by the Collaboration Committee. Such studies are
         required to support applications for Regulatory Approvals in the
         Territory and the USA and ImmunoGen shall provide BB with written
         reports relating thereto in a form suitable for use in applying for
         Regulatory Approvals. BB agrees to reimburse ImmunoGen for its direct
         costs in carrying out any of the above agreed studies in accordance
         with the provisions of Schedule 9. In addition, ImmunoGen shall provide
         all necessary documentation and assistance to support Regulatory
         Approvals in the Territory in respect of any such studies so conducted.

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15       AGREED CLINICAL STUDIES

15.1     RESPONSIBILITY. BB shall be responsible for and shall bear all its
         costs associated with conducting, and preparing clinical reports on,
         the Agreed Clinical Studies. The Agreed Clinical Studies shall be
         conducted in accordance with all applicable laws and regulations in
         force from time to time and in accordance with the Development Plan.

15.2     DATA SHARING. In recognition of the fact that ImmunoGen shall be
         responsible for filing applications for Regulatory Approval with the
         appropriate regulatory authorities in respect of the Primary Indication
         outside the Territory, BB agrees that, following the completion of the
         Agreed Clinical Studies and generation of clinical reports, it shall
         make available to ImmunoGen all necessary data relating to the Agreed
         Clinical Studies with respect thereto to enable ImmunoGen to file
         applications for Regulatory Approvals outside the Territory.

16       CLINICAL DEVELOPMENT FOR OTHER INDICATIONS

16.1     OTHER INDICATIONS. With regard to the pre-clinical and clinical
         development of Licensed Compound for all indications [*] other than the
         Primary Indication, the parties shall be responsible for such
         activities on equivalent terms as provided in this Agreement. Prior to
         the commencement of clinical development of Licensed Compound for other
         indications, to the extent that either party wishes to recommend such
         development, such party shall make a proposal for development to the
         Collaboration Committee outlining the following matters:

         16.1.1   the scientific rationale for development;

         16.1.2   the commercial viability for Licensed Compound in the
                  treatment of such indication; and

         16.1.3   the proposed development plan;

         it being agreed that in the event that the Collaboration Committee is
         not able to reach unanimous consensus on development, then development
         of the Licensed Compound for such indication shall not proceed.

17       INVESTIGATORS' MEETINGS

17.1     RIGHT TO ATTEND. In respect of any Clinical Studies conducted by one
         party, the other party shall, at its own cost, have the right (but not
         the obligation) to ensure that one of its senior clinical staff is
         present as an observer at any major investigators' meeting taking place
         subject to the prior consent of the investigators attending the
         meeting. The party conducting the meeting will use its reasonable
         efforts to obtain such consent prior to any such meeting.

17.2     LIMITATION OF RIGHT. It is agreed by the parties that to the extent
         that a representative is in attendance at a major investigators'
         meetings in respect of the other party's Clinical Studies, such
         representative shall only be entitled to be present to the extent that
         Licensed Compound or Product are being discussed and not other
         unrelated independent products, compounds or other confidential
         information proprietary to that other party.

18       ADVERSE EVENT REPORTING

18.1     AGREEMENT OF REPORTING PROCEDURE. Within[*]of the Effective Date, the
         parties shall meet to agree upon standard operating procedures for the
         investigation and reporting of adverse experiences concerning Licensed
         Compound or Product. Such written procedures shall be approved by the
         parties within[*] of the Effective Date and for the avoidance of doubt
         shall

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         also relate to BB's Sub-licensees or ImmunoGen's licensees outside the
         Territory. Giving due consideration to each parties current standards
         pursuant to which they conduct adverse event reporting, the following
         matters shall be taken into account when agreeing the standard
         operating procedures:

         18.1.1   BB shall be responsible for the investigation and reporting of
                  adverse experiences in respect of the Agreed Clinical Studies;

         18.1.2   in respect of Clinical Studies conducted for other indications
                  within the Field, BB shall be responsible for the
                  investigation and reporting of adverse experiences in respect
                  any Clinical Studies it conducts in the Territory and
                  ImmunoGen shall be responsible for the investigation and
                  reporting of adverse experiences in respect of any Clinical
                  Studies it conducts outside the Territory;

         18.1.3   each party shall notify the other party at least in accordance
                  with the time limits laid down in the ICH Guidelines for the
                  time being in force of the receipt by it (or such other more
                  stringent standards that the parties may agree) of a report of
                  a "serious adverse experience" or "unexpected adverse
                  experience" (as defined below). The notification shall be made
                  in the format mutually agreed from time to time and shall be
                  sent by express courier or facsimile transmission or, if none
                  of these methods is available, notification shall be made by
                  telephone with written copy to follow by the fastest available
                  means;

         18.1.4   the relevant party shall promptly investigate a serious
                  adverse experience and shall submit follow-up reports (copies
                  of which shall be provided to other party) at least in
                  accordance with the ICH Guidelines' time limits (or such other
                  more stringent time limits as the parties may agree) of the
                  receipt of new information;

         18.1.5   each party shall notify the other party of any other adverse
                  experiences, not being serious adverse experiences or
                  unexpected adverse experiences;

         18.1.6   in respect of those Clinical Studies for which it is
                  responsible, BB shall be responsible for reporting all
                  relevant adverse experiences relating to Licensed Compound or
                  Product to the relevant regulatory authorities in the
                  Territory in accordance with the legal requirements prevailing
                  from time to time in the Territory with respect to such
                  reporting. ImmunoGen shall, where reasonably necessary, assist
                  BB in such reporting; and

         18.1.7   in respect of those Clinical Studies for which it is
                  responsible, ImmunoGen shall be responsible for reporting all
                  relevant adverse experiences relating to Licensed Compound or
                  Product to the relevant regulatory authorities in the
                  Territory in accordance with the legal requirements prevailing
                  from time to time outside the Territory with respect to such
                  reporting. BB shall, where reasonably necessary, assist
                  ImmunoGen in such reporting.

18.2     DEFINITIONS. For the purposes of this Clause:

         "serious adverse experience" shall mean any adverse event associated
         with the use of Product in humans (whether or not considered
         drug-related) that gives rise at any dose to one or more of the
         following: death, threat to life, new or prolonged in-patient
         hospitalization, permanent, persistent or significant disability or
         incapacitation, overdose, congenital abnormality, a congenital anomaly
         or birth defect or any other serious event or laboratory abnormality
         which is thought by the reporting physician to be serious or associated
         with relevant clinical signs or symptoms; and

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         "unexpected adverse experience" shall mean any adverse event that is
         not consistent in nature, severity, or frequency with information
         contained in the current clinical investigation brochure for Product
         not yet marketed or in the approved prescribing information for
         marketed Product.

                  PART E - MANUFACTURING AND SUPPLY PROVISIONS

19       PRODUCT SPECIFICATION

19.1     AGREEMENT AND AMENDMENT. Within [*] of the Effective Date, the parties
         shall meet to review and agree upon initial Specifications for Licensed
         Compound and/or Product. Such initial Specifications shall be agreed
         upon through good faith discussions by the parties within [*] of the
         Effective Date, and shall set forth reasonably attainable
         specifications based upon data accumulated from historical batches of
         Licensed Compound, including without limitation safety and
         manufacturing data. Such agreed Specifications shall be designed to
         meet all applicable regulatory requirements and shall then be used as
         the standard pursuant to which ImmunoGen shall fulfill its
         Manufacturing and supply obligations under this Agreement. The
         Specifications may be amended from time to time by the parties as
         required to reflect the development of Product or as required to obtain
         Regulatory Approval for the Product.

19.2     AMENDMENT FOR REGULATORY PURPOSES. Following Regulatory Approval for
         Product in the Territory, BB shall have the right to require ImmunoGen
         to modify (i) the Specifications and (ii) the facilities, equipment and
         quality control procedures applicable to Product, in each case to the
         extent required by Regulatory Authorities in the Territory.

20       TRANSFER OF MANUFACTURING INFORMATION

20.1     TIMETABLE FOR PROVISION. Within [*] of the Effective Date, ImmunoGen
         shall make available to BB, and shall provide BB with a list of, all
         Manufacturing Information. BB shall, at its cost, be entitled to
         conduct a review of such Manufacturing Information and shall be
         entitled to require ImmunoGen to provide BB with copies of such
         documentation as it may request. Within [*] following receipt of such
         request, ImmunoGen shall, at its cost, provide BB with copies of all
         such requested documentation.

21       DEVELOPMENT OBLIGATIONS OF IMMUNOGEN

21.1     DEVELOPMENT OF PRODUCT. ImmunoGen agrees that it shall, [*] be
         responsible for the continued development of the Licensed Compound into
         the final Product, in a form acceptable to the Regulatory Authorities
         in the USA and the Territory, which will be used by both BB and
         ImmunoGen in the Clinical Development and commercialization of Licensed
         Compound for the treatment of the Primary Indication. ImmunoGen shall
         complete all reasonable steps required by the Regulatory Authorities to
         obtain Regulatory Approval for Product sale in such dosage form. BB
         shall have the right to consult with ImmunoGen with respect to such
         development activities. In the event that ImmunoGen does not develop
         the Licensed Compound or Product in a manner consistent with and within
         the time frame set forth in the Development Plan, the parties shall
         meet and discuss a reasonable amendment to the time frame. If the
         parties are unable to agree upon such amendment within [*] BB shall
         have the right, [*] to either take responsibility for such development
         or appoint an Independent Third Party reasonably acceptable to
         ImmunoGen to complete such activities.

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21.2     CMC DEVELOPMENT REQUIREMENTS. IMMUNOGEN AND BB WILL CONSULT, FROM TIME
         to time, on the CMC Development that will be necessary to accumulate
         data required for submitting a BLA for the Product in the Territory.
         ImmunoGen shall be responsible for providing to BB in writing, all data
         and documents necessary to complete the chemistry, manufacturing and
         controls sections of applications for Regulatory Approval for Licensed
         Compound in accordance with the timetable provided for in the
         Development Plan. Such data and documents shall be of a form and
         substance as might reasonably be expected to be sufficient to support
         an application for Regulatory Approval. By way of example only, and
         without limiting the generality of the foregoing, the parties agree
         that such data and documents shall include, but not be limited to, the
         method of manufacture, in-process and release specifications, completed
         or ongoing stability program data, validated analytical procedures for
         release and in support of the stability program, and development of all
         appropriate packaging and labeling. Without limitation, certain of
         ImmunoGen's CMC Development obligations are set out in Schedule 7 of
         this Agreement. Furthermore, within[*] of the Effective Date the
         Collaboration Committee shall meet and agree upon a formal CMC
         Development Plan. Once such plan has been agreed, the provisions of
         Clauses 13.4, 13.5 and 13.6 (as adjusted, to the extent necessary, to
         substitute ImmunoGen for BB and references to Development Plan for CMC
         Development plan) shall apply in respect of such plan.

21.3     DATA DEFICIENCIES. In the event that BB reasonably determines that the
         data and documents provided by ImmunoGen pursuant to Clause 21.2 are
         not of a form and substance as might reasonably be expected to be
         sufficient for either the filing of an application for Regulatory
         Approval or Regulatory Approval of Licensed Compound, then BB shall
         submit to ImmunoGen in writing a description of the deficiencies in
         such data or documents. Subject to Clause 21.4, ImmunoGen shall,
         promptly, and at its own cost and expense, take all steps reasonably
         necessary, including conducting additional analyses or manufacturing
         additional Licensed Compound, to generate or otherwise obtain
         sufficient data or documents necessary to cure the deficiency.

21.4     In the event that ImmunoGen disputes the determination in Clause 21.3
         that the data or documents are deficient, then ImmunoGen shall submit
         in writing to BB, within [*]of receipt of BB's notice of deficiencies
         provided pursuant to Clause 21.3, above, ImmunoGen's bases for
         disputing BB's determination. Within [*] of ImmunoGen's notice, the
         parties shall either resolve the dispute or agree upon a procedure for
         resolving the dispute. The procedure shall include either (a) the
         selection of a third-party expert experienced in such matters who shall
         review submissions from both parties and make a determination that
         shall be binding upon the parties or (b) a process for seeking a
         determination from the applicable regulatory authority, which
         determination shall be binding upon the parties. Notwithstanding the
         foregoing, in the event that an applicable regulatory authority rejects
         BB's application as deficient with respect any of the matters for which
         ImmunoGen is responsible pursuant to Clause 21.2, above, then ImmunoGen
         shall not be entitled to dispute whether the data and documents are
         deficient.

22       SUPPLY FOR CLINICAL DEVELOPMENT PURPOSES

22.1     ESTIMATES. ImmunoGen and BB shall cooperate, through the Collaboration
         Committee, in estimating the volume of Licensed Compound required from
         time to time by BB or Sub-licensees during Clinical Development.

22.2     SUPPLY OBLIGATIONS. During the Clinical Development phase, ImmunoGen
         shall supply all of BB's required quantities of Licensed Compound for
         Clinical Studies within a reasonable period of receipt of an order from
         BB provided that such order is in accordance with the estimated volumes
         agreed pursuant to Clause 22.1 for Licensed Compound for such Clinical
         Study and

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         further ImmunoGen shall use reasonable commercial efforts to ensure
         continuity of supply of Licensed Compound during the Clinical
         Development phase. All such supplies of Licensed Compound during the
         Clinical Development phase shall be in conformity with the
         Specifications. Following the grant of Regulatory Approval for Product
         for the treatment of a particular indication within the Field, the
         supply provisions set out in Clause 24 shall apply; provided always
         that to the extent that Clinical Development of Licensed Compound is
         ongoing for other indications within the Field the provisions of this
         Clause shall apply to supply therefor.

22.3     CHANGE TO PRODUCTION PROCEDURE ETC. The parties recognize that in order
         to optimize the production procedure for commercial purposes, changes
         and modifications are expected to be required during the Clinical
         Development phase. It is further acknowledged that it is the parties'
         intention that the manufacturing production process for commercial
         purposes should be formalized [*]. If ImmunoGen decides that any change
         or modification of the production procedure of the Licensed Compound is
         necessary during the term for any reason or if any changes to
         manufacturing facilities, equipment (except to the extent that changes
         are made as a result of routine maintenance) or quality control
         procedures are proposed, ImmunoGen shall promptly notify BB in writing
         of the need for such change or modification. To the extent that any
         change or modification is material, ImmunoGen shall not implement such
         change or modification to the actual production procedure without BB's
         prior written consent, which shall not be unreasonably withheld and
         shall be deemed given unless ImmunoGen is otherwise notified in writing
         by BB within [*] of ImmunoGen's corresponding notice. It is agreed that
         it shall be reasonable for BB to withhold its consent to any changes
         which may materially impact on BB's ability to apply for Regulatory
         Approvals in the Territory or which would result in a material
         interruption in the supply of Licensed Compound or Product. To the
         extent that any material changes are made to the manufacturing process,
         ImmunoGen shall carry out, [*], all necessary pre-clinical work to
         minimize the need for clinical comparability studies to be conducted,
         provided always that in the event that the Regulatory Authorities
         require any such clinical comparability studies to be so conducted,
         these shall be conducted [*]t. Furthermore, in the event that any
         changes result [*].

22.4     PRICE AND PAYMENT. Those amounts of Licensed Compound required by BB to
         carry out Clinical Studies in the Territory and the USA during the
         Clinical Development phase shall be supplied by ImmunoGen to BB at [*]
         (as defined below). Payment in respect of the supply of any Licensed
         Compound shall be made within [*] of receipt of the appropriate invoice
         by BB and shall be paid in the currency in which the invoice provides.
         For the purposes of this Clause, the [*] shall mean [*]. To the extent
         that any [*]

         [*]

22.5     IMPORT APPROVALS. BB shall be responsible for ensuring that the
         appropriate approval has been obtained for, and to manage, the
         importation of Licensed Compound for such Clinical Study within that
         part of the Territory into which such Licensed Compound is to be
         imported. ImmunoGen shall supply all necessary documentation reasonably
         requested by BB to obtain consent to import Licensed Compound required
         during the Clinical Development phase and provide all reasonable
         assistance required to enable Licensed Compound to be imported into
         that part of the Territory in which it is required.

22.6     LABELING. The parties shall, through the Collaboration Committee,
         cooperate to ensure that Licensed Compound for any Pre-Clinical Study
         or Clinical Study is appropriately labeled.

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22.7     PROVISION OF RECORDS. Each shipment of Licensed Compound shall be
         accompanied by certified copies of its batch records, environmental
         controls records and a certificate of analysis issued and signed by the
         person responsible for the Quality Control Unit (or nominated deputy)of
         ImmunoGen confirming or detailing, amongst other things, (i) the
         company issuing such certificate; (ii) the product and batch; (iii) a
         list of all tests conducted on the Licensed Compound together with the
         acceptance limits for each test; and (iv) that such shipment complies
         with the Specification. The Collaboration Committee shall agree any
         other requirements for the certificate of analysis as soon as
         reasonably practicable.

22.8     SHIPMENTS. ImmunoGen shall promptly notify BB of the shipments of
         Licensed Compound during the Clinical Development phase. Such shipments
         shall be sent at [*] to such place of delivery as BB shall specify to
         meet any agreed delivery dates, which expense shall [*].

22.9     SAFETY DATA To the extent that any new safety data is generated by
         ImmunoGen it shall promptly notify BB and the Collaboration Committee
         shall agree upon the appropriate actions to be taken and the
         preparation and circulation of appropriate safety data sheets to
         relevant parties.

23       NON-CONFORMING CLINICAL SUPPLIES

23.1     NONCONFORMING CLINICAL SUPPLIES. Within [*] after the delivery of
         Licensed Compound and the accompanying certificate of analysis to BB,
         BB shall submit to ImmunoGen in writing any claim that such Licensed
         Compound does not conform with the Specifications, accompanied by a
         report of BB's analysis (which analysis shall be conducted in good
         faith) and a sample of the Licensed Compound at issue, explaining in
         reasonable detail the basis on which the allegedly nonconforming
         Licensed Compound does not meet the Specifications. BB shall not be
         obligated to pay for such nonconforming shipment of Licensed Compound.
         Only those tests listed in the Specifications may be used to
         demonstrate nonconformance of Licensed Compound.

23.2     REPLACEMENT; CANCELLATION; SETTLEMENT. ImmunoGen shall conduct its own
         analysis of the sample in good faith within [*] after the receipt by
         ImmunoGen of the report and sample from BB, and provide the results to
         BB. If after ImmunoGen's own analysis of the sample ImmunoGen agrees
         with the claim of nonconformity, BB shall promptly inform ImmunoGen if
         BB wishes to have ImmunoGen replace the nonconforming Licensed Compound
         with conforming Licensed Compound. If BB wishes to receive such
         replacement Licensed Compound, ImmunoGen shall provide such replacement
         as soon as reasonably practicable thereafter, in which case BB shall be
         obligated to pay only for such replacement Licensed Compound. BB shall
         not be obligated to pay for the nonconforming Licensed Compound, and
         ImmunoGen shall: (i) credit BB for the amount paid by BB for the
         nonconforming Licensed Compound if BB has already paid for such
         nonconforming Licensed Compound or (ii) cancel its invoice to BB for
         such nonconforming Licensed Compound if BB has not yet paid for such
         nonconforming Licensed Compound, and BB shall not be obligated to pay
         such canceled invoiced amount. If, after its own analysis, ImmunoGen
         does not agree with the claim of nonconformity or determines that BB is
         responsible for the nonconformity, the Parties shall in good faith
         discuss and agree upon a settlement of the issue, and BB shall not be
         obligated to pay for such alleged nonconforming Licensed Compound until
         such settlement is reached.

23.3     RETURN OR DESTRUCTION OF NON-CONFORMING STOCK. After ImmunoGen has
         agreed that the Licensed Compound shipment is nonconforming, and if
         ImmunoGen is responsible for the nonconformity, BB shall return or
         destroy it at ImmunoGen's request and cost in the most cost effective
         and environmentally safe and appropriate manner available, consistent
         with all relevant laws and regulations.

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24       COMMERCIAL MANUFACTURE AND SUPPLY OF PRODUCT

24.1     PRODUCT FOR COMMERCIAL SALE. ImmunoGen shall retain responsibility for
         the commercial Manufacture and supply of Licensed Compound and/or
         Product to BB until the termination or expiration of this Agreement,
         which Manufacture and supply shall be in accordance with all Regulatory
         Approvals granted, and all laws and regulations in force, in each
         country within the Territory. ImmunoGen shall use commercially
         reasonable efforts to manufacture for and supply to BB all of BB's
         requirements for Product in accordance with the terms of any agreement
         to be agreed between the parties taking in accordance with the terms
         attached to this Agreement as Schedule 4 ("Supply Agreement"). The
         price to BB for commercial supply of Product shall be [*].

24.2     Subject to Clause 27 below, during the term of this Agreement, BB shall
         purchase all its requirements for Product from ImmunoGen. Upon BB's
         request, during the[*] period preceding the expiration of the term of
         this Agreement, the parties agree to negotiate in good faith for an
         extension of supply by ImmunoGen and purchase of the Product by BB.

25       CONTRACT MANUFACTURING

25.1     ImmunoGen may contract with one or more contract manufacturers to
         perform any or all of its Manufacture obligations under this Agreement
         and the Supply Agreement, provided that (i) ImmunoGen incorporates all
         relevant provisions of this Agreement into any arrangements it enters
         into with contract manufacturers after the Effective Date (each a
         "Contract Manufacturer Agreement"), and (ii) ImmunoGen provides BB with
         a true and accurate copy of each such Contract Manufacturer Agreement.

25.2     ImmunoGen agrees to use its commercially reasonable efforts to include
         in each Contract Manufacturer Agreement the following provisions: (i) a
         prohibition against sublicensing by such contract manufacturer of
         Licensed IP licensed to such Contract Manufacturer by ImmunoGen; (ii) a
         prohibition against the sale by such contract manufacturer to any
         Independent Third Party of (A) Licensed Compound for use in any product
         to be sold or distributed in the Territory or (B) Product for resale
         (other than by ImmunoGen to BB, or by BB itself) in the Territory, and
         (iii) a right for ImmunoGen to terminate such Contract Manufacturer
         Agreement in the event of a breach of the terms set forth in either of
         (i) or (ii) above.

25.3     ALTERNATE SUPPLY. ImmunoGen agrees to use commercially reasonable
         efforts to cause at least one further source of supply of Licensed
         Compound (in addition to its primary source of supply) to become and
         remain pre-qualified as soon as practicable after the first Regulatory
         Approval is granted in the Territory and during the remainder of the
         term of this Agreement.

26       RETENTION OF RECORDS

26.1     RECORDS. ImmunoGen shall maintain, and shall cause its Affiliates,
         sub-contractors or other agents to maintain, all records necessary to
         comply with applicable laws, rules and regulations relating to the
         manufacture and storage of Licensed Compound and the Product (in bulk
         or finished form). All such records shall be maintained for such period
         as may be required by law, rule or regulation; provided, however, that
         all records relating to the manufacture, stability and quality control
         of each batch or partial batch of the Product shall be retained at
         least until the first anniversary of the end of the approved shelf life
         for all Product from such batch or partial batch; and provided further
         that neither party shall destroy such records without first notifying
         the other

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         party and giving the other party an opportunity to take control of such
         records if the party being notified believes that applicable law or its
         own written corporate policy requires such records to be maintained.

27       IMMUNOGEN'S INABILITY TO SUPPLY

27.1     INABILITY TO SUPPLY. If ImmunoGen fails for any reason, either during
         the term of this Agreement or following its expiry, other than for a
         force majeure event described in Clause 65, to deliver Licensed
         Compound and/or Product to BB, then BB shall have the right to make or
         have made all of its requirements of Licensed Compound and/or Product
         and all such licenses as may be required by BB to conduct such
         manufacture shall automatically be deemed to be granted to BB by
         ImmunoGen; provided that BB shall not have the right to manufacture if,
         [*] days after BB first notifies ImmunoGen of its intent to exercise
         its rights under this Clause, ImmunoGen provides BB with a plan to cure
         the inability to supply within the next [*] and then does so within
         such [*] period. In addition to the other remedies provided for with
         respect to any failure to supply the Product, the Supply Agreement
         shall set forth a mechanism by which ImmunoGen will transfer to BB,
         upon request, such Manufacturing Information, technology and know-how
         (not already in BB's possession) so as to permit BB to manufacture
         Product and/or Licensed Compound, and ImmunoGen agrees to cooperate
         with BB to facilitate the transition. Notwithstanding the foregoing, if
         ImmunoGen provides BB with notice of its ability to once again supply
         BB with its requirements of Licensed Compound and/or Product, the
         parties shall meet to discuss whether BB requires ImmunoGen to
         recommence all or any part of supply of Licensed Compound or Product to
         it.

28       REGULATORY COMPLIANCE AND REPORTING

28.1     GOVERNMENT INSPECTION. ImmunoGen agrees to advise BB by telephone and
         facsimile immediately of any proposed or announced visit or inspection,
         and as soon as possible but in any case within [*] of any unannounced
         visit or inspection, by any Regulatory Authority of any facilities used
         by or on behalf ImmunoGen in the performance of its obligations under
         this Agreement, including the processes or procedures used at such
         facilities in the manufacture of Licensed Compound or Product.
         ImmunoGen shall provide BB with a reasonable description of each such
         visit or inspection promptly (but in no event later than [*] calendar
         [*]) thereafter, and with copies of any letters, reports or other
         documents (including Form 483's) issued by any such authorities that
         relate to Licensed Compound, Product, or such facilities, processes or
         procedures. BB may review ImmunoGen's responses to any such reports and
         communications, and if practicable, and, insofar as timely received,
         BB's reasonable views and requests shall be taken into account prior to
         submission of such reports and communications to the relevant
         Regulatory Authority.

28.2     NOTIFICATION AND RECALL. If any Regulatory Authority issues or requests
         a recall or takes similar action in connection with Licensed Compound
         or the Product, or if either party determines that an event, incident
         or circumstance has occurred which may result in the need for a recall
         or market withdrawal, the party notified of or wishing to call such
         recall or similar action shall, within [*], advise the other party of
         notification or its determination by telephone or facsimile, after
         which the parties shall promptly discuss and work together to effect an
         appropriate course of action; provided, however, that either party may
         initiate a recall or market withdrawal thereafter if it deems such
         action necessary or appropriate. ImmunoGen shall be responsible for
         notification to applicable Regulatory Authorities outside the Territory
         and compliance with applicable laws outside the Territory in conducting
         such recall. BB shall be responsible for notification to the

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         applicable Regulatory Authorities with respect to countries in the
         Territory and compliance with applicable laws in the Territory in
         conducting such recall.

28.3     RECALL EXPENSE. If a recall results from the breach of a party's
         warranties or obligations under this Agreement, the breaching party
         shall bear the full expense of both parties incurred in any such
         recall. Such expenses of recall shall include, without limitation, the
         expenses of notification and destruction or return of the recalled
         Product and the sum paid for the recalled Product. In the event,
         however, that a recall is partially caused by both parties' actions or
         omissions, then each party shall be responsible for its proportionate
         share of the recall expenses based on its proportionate share of
         causation.

28.4     REGULATORY COMPLIANCE. ImmunoGen shall ensure that any Licensed
         Compound and/or Product packaging (together with any inserts or
         material relating to the sale of Licensed Compound and/or Product)
         which is Manufactured by or on its behalf and supplied to BB complies
         with all relevant regulatory requirements in the Territory or any part
         thereof. Furthermore, to the extent permitted by relevant laws and
         regulations, BB shall ensure that Product sold in the Territory shall
         contain a clear and prominent statement that Licensed Compound and/or
         Product is sold under a license from ImmunoGen.

29       AUDIT RIGHTS OF BB

29.1     AUDIT OF RECORDS. ImmunoGen shall, and shall procure that any
         sub-contractors or agents shall, keep at its usual place of business
         complete and proper records and books of account showing the quantity
         and COGS of Licensed Compound and/or Product Manufactured and supplied
         whether inside or outside the Territory. Upon [*] prior written notice
         from BB, ImmunoGen shall permit an independent certified public
         accounting firm of nationally recognized standing selected by BB and
         approved by ImmunoGen (such approval not to be unreasonably withheld or
         delayed) at BB's expense, to examine pertinent books and records of
         ImmunoGen, its sub-contractors or agents at their respective usual
         places of business as may be reasonably necessary to verify the
         accuracy of the price payable for clinical supplies or COGS in respect
         of commercial supplies hereunder and ImmunoGen shall procure the
         cooperation of its sub-contractors or agents in this regard. The
         provisions of Clauses 12.1, 12.2 and 12.3 shall equally apply with
         regard to the rights and obligations of the parties in connection with
         audits under this Clause.

29.2     AUDIT OF FACILITIES. BB, or at BB's choice a third party auditor agreed
         with ImmunoGen, shall be entitled to audit ImmunoGen's, or any
         authorized sub-contractors', premises and processes for compliance with
         the principles of GMP, and to audit the validation of facilities,
         equipment, and production and tests to ensure their suitability for the
         particular stage of development. ImmunoGen shall procure that BB has
         equivalent rights to audit any additional approved sub-contractor(s)
         that may be appointed by ImmunoGen. At BB's option, a written
         questionnaire may be substituted for an audit. Audits may be made
         before commencement of the operations specified in this Agreement,
         during their execution and/or upon completion of such activities. Such
         audit or audits shall be during normal plant or laboratory working
         hours, and shall be on a date or dates agreed between the parties.
         Under normal circumstances BB shall not audit ImmunoGen more than once
         a year, however additional audits may be requested:

         29.2.1   in the event of major process or facility change, to verify
                  corrective actions for significant deficiencies identified in
                  prior audits; or

         29.2.2   in the event of process failures.

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         In all cases BB shall provide a copy of any audit report to ImmunoGen.
         ImmunoGen shall provide a response to the report within [*] of receipt
         thereof, including a description of agreed corrective actions plus a
         timetable for their implementation.

                      PART F - COMMERCIALIZATION PROVISIONS

30       BB'S DILIGENCE OBLIGATIONS

30.1     BB warrants and undertakes that it shall, at BB's expense, use
         reasonable commercial efforts to diligently develop Licensed Compound
         and obtain Regulatory Approval for the Product in the Territory and to
         promote, market and sell Product in the Territory. The parties
         acknowledge and agree that, subject to the terms of this Agreement, all
         business decisions including, without limitation, decisions relating to
         BB's research, development, registration, manufacture, sale,
         commercialization, design, price, distribution, marketing and promotion
         of Licensed Compound and/or Product covered under this Agreement and
         relating to the Territory, shall be within the sole discretion of BB.
         ImmunoGen acknowledges that nothing in this Agreement shall be
         construed as imposing on BB the duty to market and/or sell and exploit
         Product for which royalties are payable hereunder to the exclusion of,
         or in preference to, any other product of comparable potential, or in
         any way other than in accordance with its normal commercial practices.
         The parties further acknowledge that the use of Independent Third Party
         contractors in connection with obtaining Regulatory Approvals and/or
         marketing of Product may facilitate the performance of such activities,
         and that BB shall have the right, in its sole discretion, to utilize
         Independent Third Party contractors for such purposes and to grant
         appropriate sublicenses to such Independent Third Party subcontractors,
         provided that BB first complies with the sublicensing obligations set
         forth in Clause 3.

30.2     If, in ImmunoGen's reasonable opinion, BB fails to comply with any of
         its diligence obligations under Clause 30.1, then, without limiting any
         other remedies that may be available to ImmunoGen, ImmunoGen shall have
         the right to give BB written notice thereof stating in reasonable
         detail the particular failure(s). BB shall have a period of [*] from
         the receipt of such notice to correct the failure or, in the event that
         the failure cannot be reasonably cured within a [*] period, then BB
         shall initiate actions reasonably expected to cure the failure within
         [*] days of receiving notice and shall thereafter diligently pursue
         such actions to cure the failure, even if requiring longer than the
         time period specified in Clause 46. In the event of a dispute as to
         whether or not BB has failed to exercise due diligence under Clause
         30.1, or whether BB is diligently pursuing actions reasonably expected
         to cure such failure under this Clause, such dispute shall be resolved
         in accordance with Clause 51.

30.3     In addition to the provisions of Clause 65, the diligence obligations
         of BB with respect to Product under this Agreement are expressly
         conditioned upon the continuing absence of any adverse condition or
         event which warrants a delay in Product commercialization including,
         but not limited to, an adverse condition or event relating to Product
         safety or efficacy or unfavorable labeling, pricing or pricing
         reimbursement approvals, or lack of Regulatory Approval, and the
         obligation of BB to develop or market Product shall be delayed or
         suspended so long as in BB's reasonable opinion any such condition or
         event exists. In the event that the parties disagree as to whether any
         such condition or event exists or is continuing, the matter shall be
         referred to the Collaboration Committee. If the Collaboration Committee
         is unable to resolve the matter within

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         [*] of the submission of the matter to it for resolution, the matter
         shall be resolved in accordance with Clauses 51 and 52.

31       OBTAINING REGULATORY APPROVAL IN THE TERRITORY

31.1     BB shall be responsible, at its cost, for obtaining Regulatory Approval
         for Product in the Field in the Territory where there is adequate
         clinical data to support Regulatory Approval from each competent health
         authority in the Territory.

31.2     Each Regulatory Approval and each pricing and reimbursement approval in
         the Territory shall be placed in BB's name or the name of BB's
         Affiliate(s) unless applicable law requires, or ImmunoGen and BB
         otherwise agree, that an approval be solely or jointly in the name of
         ImmunoGen or a designated ImmunoGen Affiliate. ImmunoGen agrees that
         notwithstanding such Regulatory Approval or pricing and reimbursement
         approval in its name, BB remains solely responsible for and retains the
         exclusive rights to import, export, keep (whether for disposal or
         otherwise), market, distribute, use, promote, offer for sale and sell
         Licensed Compound and/or Product as granted to BB under the terms of
         this Agreement.

31.3     BB agrees that, prior to the commencement of any appropriate pivotal
         Agreed Clinical Study(ies) (as set out in the Development Plan), it
         shall consult with the EMEA and/or the FDA to determine whether such
         regulatory body is of the opinion that should such study(ies) meet its
         primary protocol objective, when considered in conjunction with the
         clinical data existing at that time, such combined data would be
         reasonably sufficient to gain Regulatory Approval.

31.4     ImmunoGen shall provide BB, at BB's expense, with such assistance in
         obtaining Regulatory Approval for Product as may be reasonably
         requested by BB, including all reasonable assistance in relation to the
         preparation and submission of Regulatory Approval dossiers to the
         regulatory authorities and the response to questions posed by such
         authorities, in respect of any Pre-Clinical Studies conducted by
         ImmunoGen, CMC Development, Manufacture or such other data that has
         been generated by ImmunoGen. ImmunoGen shall provide all such
         assistance to enable BB to respond to the Regulatory Authorities in
         accordance with any timetable that BB may set for filing such a
         response.

31.5     Each party shall keep the other party fully informed of all its
         interactions with Regulatory Authorities including the provision of all
         material submissions, minutes of any meetings and all relevant
         correspondence from the Regulatory Authorities.

31.6     BB shall take all commercially reasonable steps to maintain, at its
         cost, and to comply with all necessary regulatory requirements relating
         to the maintenance of, any Regulatory Approval granted for Product in
         the Field in the Territory.

32       REGULATORY APPROVALS OUTSIDE THE TERRITORY

32.1     It is acknowledged that with regard to obtaining Regulatory Approvals
         in countries outside the Territory, the general expectation is that
         such applications will be based upon either upon an application for
         Regulatory Approval or a Regulatory Approval filed with either the FDA
         or the EMEA ("Primary Dossiers"). Schedule 6 sets out a summary of the
         parties' current understanding with regard to ImmunoGen's ability to
         rely on Primary Dossiers outside the Territory.

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32.2     In recognition of the fact that ImmunoGen will be relying on data,
         Technical Information and/or Primary Dossiers generated by BB, BB shall
         provide ImmunoGen, at ImmunoGen's expense, with such assistance in
         obtaining Regulatory Approval for Product outside the Territory as may
         be reasonably requested by ImmunoGen, including all reasonable
         assistance in relation to the preparation and submission of Regulatory
         Approval dossiers to the Regulatory Authorities and the response to
         questions posed by such authorities, in respect of any Agreed Clinical
         Studies, Technical Information, Primary Dossiers or other data that has
         been generated by BB. BB shall provide all such assistance to enable
         ImmunoGen to respond to the Regulatory Authorities in accordance with
         any timetable that ImmunoGen may set for filing such a response.

32.3     It is further agreed that to the extent that any Regulatory Authority
         outside the Territory requires ImmunoGen to conduct local Clinical
         Studies (with the exception of the USA, where the conduct of Agreed
         Clinical Studies shall be the responsibility, and at the cost, of BB)
         or Commercialization Studies to support any application for Regulatory
         Approval, such studies shall be the responsibility of ImmunoGen at its
         cost. It is specifically acknowledged that in the event that ImmunoGen
         applies for Regulatory Approval in China, due to the recognized
         physiological differences that exist in the Chinese population, local
         Clinical Studies and/or Commercialization Studies are likely to be
         required.

32.4     In recognition of the fact that payment of the fee by ImmunoGen
         pursuant to Clause 10.2 is dependant on the grant of a BLA in the USA,
         ImmunoGen agrees that, following the generation of sufficient data from
         the Agreed Clinical Studies, it will use reasonable commercial efforts
         to apply for and obtain Regulatory Approval for Product in the USA

32.5     To the extent reasonably required, BB will provide ImmunoGen, at
         ImmunoGen's cost, with advice and assistance in the design of any local
         Clinical Studies or Commercialization Studies that ImmunoGen is
         required to conduct outside the Territory.

33       MARKETING AND SALE OF PRODUCT

33.1     AGREEMENT OF MARKETING PLAN. Commencing in the year in which either BB
         files an application for Regulatory Approval in any of the Major EU
         Markets or ImmunoGen files an application for Regulatory Approval in
         the USA, but in any event no earlier than [*] after such filing, the
         parties shall, via the Collaboration Committee, meet to agree a
         marketing plan to coordinate the worldwide commercialization (to the
         extent legally permissible in accordance with applicable anti-trust
         legislation in and outside the Territory) for Product which shall
         include the following information:

         33.1.1   a sales target in respect of the sale of Product;

         33.1.2   anticipated or actual pricing of Product, as applicable, on a
                  country-by-country basis in each geographic market;

         33.1.3   the branding and labeling of Product, which it is intended
                  shall be consistent across all countries in which Product is
                  marketed by the parties to the extent reasonably practicable
                  in all the circumstances;

         33.1.4   the proposed detailing to be assigned to the sale of Product;
                  and

         33.1.5   the major marketing programs and marketing assumptions;

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         (referred to as the "Marketing Plan").

33.2     ANNUAL UPDATES. The Marketing Plan shall be updated annually thereafter
         in the fourth calendar quarter of each year and in this respect the
         parties shall meet, via the Collaboration Committee, to agree any
         revisions thereto.

33.3     OTHER AMENDMENTS. It is agreed that:

         33.3.1   the Marketing Plan may be subject to amendments, adjustments
                  and/or revisions by the parties to take into account current
                  market conditions and/or any events which may occur during the
                  period to which the Marketing Plan relates provided always
                  that amending party shall notify the Collaboration Committee
                  as soon as reasonably practicable in the event that such party
                  proposes to make any such amendment, adjustment or revision;
                  and

         33.3.2   to the extent that any changes are made to the Marketing Plan
                  which have not been notified to the Collaboration Committee
                  pursuant to Clause 33.3.1, they shall be discussed and
                  explained at the annual meetings to take place between the
                  parties pursuant to Clause 33.2.

33.4     INTENTION TO OUT-LICENSE. In the event that either party elects to
         license an Independent Third Party to conduct the marketing and sale of
         Product in the whole of its territorial market it shall notify the
         other party. If both parties agree that the Product should be
         out-licensed to an Independent Third Party for the relevant indication
         on a worldwide basis the parties agree to cooperate, through the
         Collaboration Committee, in the identification of a suitable
         pharmaceutical entity to conduct such worldwide commercialization on
         their behalf. If after [*] (or such longer period as may be agreed by
         the Collaboration Committee at the time), the parties have been unable
         to interest a suitable pharmaceutical entity, and encourage such entity
         to submit preliminary business terms, or if both parties decide that
         any terms so submitted are unacceptable to them, each of BB and
         ImmunoGen shall be entitled, in their sole discretion, to appoint a
         licensee or licensees in respect of their respective territories or
         part thereof. If both parties decide that the terms are acceptable, it
         is agreed that [*]

33.5     BB's FIRST RIGHT TO NEGOTIATE. In the event that ImmunoGen elects to
         appoint a licensee in any country(ies) outside the Territory and BB
         intends to commercialize Product in the Territory, ImmunoGen agrees to
         grant BB first right to enter discussions with it with regard to BB
         being licensed to commercialize Product in such country(ies) outside
         the Territory. If after [*] good faith negotiations, ImmunoGen
         reasonably determines that BB would not be a suitable licensee, it
         shall be entitled to initiate discussions with Independent Third
         Parties.

33.6     ImmunoGen's FIRST RIGHT TO NEGOTIATE. In the event that BB elects to
         appoint a licensee in any country(ies) of the Territory and ImmunoGen
         intends to commercialize Product outside the Territory, BB agrees to
         grant ImmunoGen first right to enter discussions with it with regard to
         ImmunoGen being licensed to commercialize Product in such country(ies)
         in the Territory. If after [*] good faith negotiations, BB reasonably
         determines that ImmunoGen would not be a suitable licensee, it shall be
         entitled to initiate discussions with Independent Third Parties.

34       TRADE MARKS

34.1     To the extent reasonably practicable and allowable, the parties shall
         endeavor to ensure that that marketing and selling of Product shall
         occur under the same trade marks and logos both in and outside the
         Territory it being agreed that BB shall be the owner of any such trade
         marks or logos

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         in the Territory and ImmunoGen shall be the owner of any such trade
         marks or logos outside the Territory. To the extent that it is not
         possible for Product to be marketed and sold under a global trade mark
         or logo, BB shall be entitled to select its own trade marks and logos
         in the Territory and ImmunoGen shall be entitled to select its own
         trade marks outside the Territory.

                           PART G - PATENT PROVISIONS

35       PATENT PROSECUTION AND MAINTENANCE

35.1     ImmunoGen, [*], shall file, prosecute and do all such acts and things
         as may be reasonably necessary to obtain the grant of the Patents for
         their full term. In addition, ImmunoGen, [*] shall file and prosecute
         applications for patents covering any ImmunoGen Improvement that it
         adjudges in its reasonable opinion has potential commercial application
         and that constitutes a ImmunoGen Improvement or a Joint Improvement in
         accordance with the following principles:

         35.1.1   applications shall be filed expeditiously at the appropriate
                  time in all countries in which ImmunoGen in its absolute
                  discretion considers that patent protection for a ImmunoGen
                  Improvement or a Joint Improvement is necessary or desirable;

         35.1.2   ImmunoGen shall notify BB of its proposed filing list at least
                  [*] prior to the deadline for filing; and

         35.1.3   in the event that BB wishes a patent application to be filed
                  in respect of the said ImmunoGen Improvement or Joint
                  Improvement in a particular country that does not appear upon
                  ImmunoGen's filing list it shall request the addition of such
                  country to ImmunoGen's filing list. If ImmunoGen refuses to
                  include such country in the said list [*] . ImmunoGen shall
                  allow BB to apply for patent protection in its own name in
                  respect of such country and shall allow BB to take the benefit
                  of ImmunoGen's priority date and, [*] give all reasonable
                  assistance required by BB.

         For purposes of clarity the parties acknowledge and agree that patent
         applications claiming ImmunoGen Improvements or Joint Improvements
         filed pursuant to this Clause, and any patents issuing therefrom, shall
         be included in the definition of Patents.

35.2     At BB's request, ImmunoGen shall,[*] provide BB with copies of:

         35.2.1   granted Patents;

         35.2.2   pending applications for Patents and amendments thereto; and

         35.2.3   such material correspondence to and from patent offices within
                  the Territory concerning pending applications for Patents to
                  allow for review by, and consultation with, BB reasonably in
                  advance of any submission to a patent office which could
                  materially affect the scope or extent or validity of any
                  Patent that may result.

35.3     Save as provided in Clause 35.4, ImmunoGen shall during the term of
         this Agreement [*] and do all such acts and things as may be reasonably
         necessary to maintain any granted Patents for their full term in any
         part of the Territory.

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35.4     In the event that ImmunoGen intends to abandon any of the Patents or
         allow any of the Patents to lapse in any part of the Territory, it
         shall notify BB in writing of such intention at least [*] prior to such
         proposed abandonment or lapse and offer BB an opportunity to acquire
         such Patents to the extent they apply to such part of the Territory for
         nominal consideration. BB shall have a period of[*] days to respond to
         such written offer by ImmunoGen. In the event that no response is
         received from BB within such thirty [*] period, ImmunoGen shall be
         entitled to allow such Patents to become abandoned or lapse without
         further notice to BB. In the event that BB does elect to acquire the
         Patent(s), ImmunoGen shall, [*] give all reasonable assistance required
         by BB to assign all rights relating to any such Patent(s) which shall
         thereafter not be considered Patents under this Agreement and ImmunoGen
         shall have no further rights or obligations with respect thereto,
         provided that in the event that this Agreement is terminated, except
         for any termination by BB under Clause 46 for material breach by
         ImmunoGen, BB will grant ImmunoGen a royalty-free non-exclusive license
         thereunder.

35.5     Unless the term of any Patent has already been extended by ImmunoGen at
         the time of the granting of Regulatory Approval in a particular part of
         the Territory, ImmunoGen shall be entitled to apply in such part of the
         Territory for an extension of the term of any such Patent(s) capable of
         extension in such part of the Territory. In this event, BB shall, and
         shall use reasonable endeavors to procure that any Sub-licensees shall,
         make available to ImmunoGen [*] all relevant documentation and other
         materials which may be in the possession or under the control of BB or
         Sub-licensees which in ImmunoGen's reasonable opinion are necessary to
         procure such extension. If in order to obtain such extension it is in
         ImmunoGen's reasonable opinion necessary for BB to join with ImmunoGen
         in making the application for extension, BB shall at [*] promptly do so
         and shall fully cooperate with ImmunoGen in connection with any such
         application.

35.6     ImmunoGen shall provide BB or BB's nominated agent with an annual
         status report of patent applications filed by ImmunoGen under this
         Clause 35. In addition, ImmunoGen shall promptly notify BB in writing
         in the event that ImmunoGen becomes aware of any facts, information or
         circumstances that may cause any of the Patents to be deemed invalid or
         unenforceable, in whole or in part, and shall thereafter undertake
         diligent efforts to defend the validity and enforceability of such
         Patents.

36       INVENTIONS OF BB

36.1     BB, [*], shall file and prosecute applications for patents covering any
         BB Improvement that it adjudges in its reasonable opinion has potential
         commercial application and that constitutes a BB Improvement in
         accordance with the following principles:

         36.1.1   applications shall be filed expeditiously at the appropriate
                  time in all countries in which BB in its absolute discretion
                  considers that patent protection for a BB Improvement is
                  necessary or desirable;

         36.1.2   BB shall notify ImmunoGen of its proposed filing list at least
                  [*] prior to the deadline for filing;

         36.1.3   In the event that ImmunoGen wishes a patent application to be
                  filed in respect of the said BB Improvement in a particular
                  country that does not appear upon BB's filing list it shall
                  request the addition of such country to BB's filing list. If
                  BB refuses to include such country in the said list then [*]
                  BB shall allow ImmunoGen to apply for patent protection

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                  in its own name in respect of such country and shall allow
                  ImmunoGen to take the benefit of BB's priority date and, [*],
                  give all reasonable assistance required by ImmunoGen; and

         36.1.4   BB shall not abandon any such patent application or permit any
                  patent issuing therefrom to lapse without first having given
                  to ImmunoGen at least [*] prior notice in writing of its
                  intention to permit such application to be abandoned or lapse
                  and permitting ImmunoGen to continue the prosecution or
                  maintenance of such application or patent (as appropriate).
                  ImmunoGen shall have[*] to respond to such written notice from
                  BB. In the event that no response is received from ImmunoGen
                  within such [*] period, BB shall be entitled to allow such
                  patent or patent application to become abandoned or lapse. In
                  the event that ImmunoGen does elect to acquire the patent or
                  patent application, BB shall [*] give all reasonable
                  assistance required by ImmunoGen to assign all rights relating
                  to any such patent application or patent to ImmunoGen to
                  enable it to prosecute or maintain the said patent application
                  or patent (as appropriate) which shall thereafter be
                  considered Patents under this Agreement and except as
                  expressly granted under this Agreement BB shall have no
                  further rights or obligations with respect thereto, provided
                  that in the event that this Agreement is terminated by BB
                  under Clause 46 for material breach by ImmunoGen, ImmunoGen
                  will grant BB a royalty-free non-exclusive license thereunder.

36.2     At ImmunoGen's request, BB shall, [*] furnish ImmunoGen with copies of:

         36.2.1   granted patents obtained by BB pursuant to this Clause 36;

         36.2.2   pending applications for such patents and amendments thereto;
                  and

         36.2.3   such material correspondence to and from patent offices to
                  allow for review by, and consultation with, ImmunoGen
                  reasonably in advance of any submission to a patent office
                  which could materially affect the scope or extent or validity
                  of the patent that may result.

36.3     BB shall provide ImmunoGen or ImmunoGen's nominated agent with an
         annual status report of patent applications filed by BB under this
         Clause 36.

37       INFRINGEMENT AND INVALIDITY PROCEEDINGS

37.1     In the event that either BB or ImmunoGen becomes aware of any third
         party infringement within the Territory of any Patents, it will notify
         the other party in writing to that effect. Any such notice shall
         include any evidence in the notifying party's possession to support an
         allegation of infringement by such third party. ImmunoGen shall have a
         period of [*] from the date of said notice to obtain a discontinuance
         of such infringement and failing the obtaining of such discontinuance,
         ImmunoGen shall have the right but not the obligation to bring legal
         proceedings against the third party infringer. ImmunoGen shall bear all
         the expenses of any suit brought by it (except to the extent otherwise
         provided in this Clause 37).

37.2     In the event that ImmunoGen elects to bring suit against the third
         party infringer, BB shall have the right, [*], to be represented in an
         such action by ImmunoGen by counsel of BB's own choice; provided, that
         under no circumstances shall the foregoing affect the right of
         ImmunoGen to control the suit as described in Section 37.1.

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37.3     BB will reasonably cooperate with ImmunoGen in any such suit or action
         and shall have the right to consult with ImmunoGen and be independently
         advised by independent counsel, provided that ImmunoGen shall [*] with
         ImmunoGen. ImmunoGen shall indemnify and hold BB harmless for any
         monetary judgment or award against or penalty levied upon ImmunoGen or
         BB arising out of ImmunoGen's acts in the enforcement of such Patents
         except to the extent that any such monetary judgment award or penalty
         is caused by the negligent acts or omissions of BB.

37.4     If ImmunoGen does not take any action within [*] after written notice
         from BB of such infringement, then BB shall have the right, but not the
         obligation, to bring suit against such infringer under the Patents and
         to the extent required by law, join ImmunoGen as a party plaintiff,
         [*](except to the extent otherwise provided in this Clause). ImmunoGen
         will reasonably cooperate with BB in any such suit for infringement of
         a Patent brought by BB against a third party, and shall have the right
         to consult with BB and be independently advised by independent counsel
         in such litigation [*]. [*] in cooperating with BB. BB shall incur no
         liability to ImmunoGen as a consequence of such litigation or any
         unfavorable decision resulting therefrom, including any decision
         holding any of the Patents invalid or unenforceable, except that BB
         shall indemnify and hold ImmunoGen harmless for any monetary judgement
         or award against or penalty levied upon either ImmunoGen or BB arising
         out of BB's acts in the enforcement of such Patents except to the
         extent that any such monetary judgment award or penalty is caused by
         the negligent acts or omissions of ImmunoGen.

38       THIRD PARTY INFRINGEMENT PROCEEDINGS

38.1     In the event that a third party sues BB alleging that BB's or its
         Sub-licensees' importing, exporting, keeping (whether for sale or
         otherwise), using, distributing, marketing, promoting, offering for
         sale or selling Licensed Compound or Product in one or more countries
         in the Territory infringes or will infringe said third party's patent,
         the parties shall meet as soon as reasonably practicable to discuss the
         future conduct thereof. If following such discussions:

         38.1.1   ImmunoGen determines that it wishes to participate in the
                  conduct of such action, it is hereby agreed that [*].

         38.1.2   ImmunoGen determines that it does not wish to participate in
                  the conduct of such action then [*].

         38.2     Upon the institution of an action referred to in Clause 38.1,
                  [*].

         38.3     [*].

         38.4     [*]

39       MISCELLANEOUS PROVISIONS

39.1     The parties shall keep one another informed of the status of their
         respective activities regarding any litigation or settlement thereof
         concerning Licensed Compound or Product or any actual or threatened
         infringement of the Patents.

39.2     Any damages obtained as a result of the proceedings contemplated by
         Clause 37 [*]:

         39.2.1   [*];

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         39.2.2   [*]:

                  39.2.2.1  [*]

                  39.2.2.2  [*].

39.3     ImmunoGen shall promptly notify BB in writing in the event that
         ImmunoGen becomes aware of:

         39.3.1   any actual or potential claim by a third party alleging an
                  ownership interest in the Licensed IP or any actual or
                  potential liens, charges or encumbrances with respect to the
                  Licensed IP;

         39.3.2   any information suggesting that the Licensed IP and the
                  development, manufacture, use, distribution, marketing,
                  promotion and sale of Licensed Compound and/or Product may
                  interfere or infringe on any intellectual property rights
                  owned or possessed by any Independent Third Party;

         39.3.3   any pending or threatened claims or litigation against
                  ImmunoGen relating to Licensed Compound, Product or the
                  Licensed IP;

         39.3.4   any circumstances that would render BB liable to an
                  Independent Third Party for patent infringement as a
                  consequence of BB's sale of the Licensed Compound or Product
                  or use of the Licensed IP; or

         39.3.5   any Patents which have not been properly registered, or for
                  which applications for registration have not been properly
                  made, on behalf of and in the name of ImmunoGen as sole
                  proprietor, or of any grounds for refusing any existing
                  application for registration of any of the Patents.

         Following such notice to BB, ImmunoGen shall provide BB with reasonable
         assistance to resolve such situation in a manner reasonably acceptable
         to BB and in accordance with the terms of this Agreement.

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40       EFFECT OF INTERIM OR FINAL RESTRAINING ORDER OR INJUNCTION ON AGREEMENT

40.1     If at any stage an interim or final restraining order or interlocutory
         injunction or similar order of the Court is granted whereby BB or
         ImmunoGen is restrained from manufacturing, supplying, importing,
         exporting, keeping (whether for disposal or otherwise), selling or
         otherwise dealing with or in Licensed Compound and/or Product in a
         particular part of the Territory then BB's obligations to
         commercialize, use, sell, distribute and/or market, and ImmunoGen's
         obligations to supply, Licensed Compound or Product in the Field in
         such part of the Territory to which the said order or injunction
         applies pursuant to this Agreement and to pay any future royalty or
         other sums in connection therewith shall be suspended in respect of
         such part of the Territory only.

40.2     The provisions of Clause 40.1 shall in no way affect BB's obligations
         to pay ImmunoGen any accrued payments which relate to its activities
         prior to any such order or injunction being granted or payments in
         respect of any part of the Territory unaffected by the order or
         injunction. In the event that any temporary restraining order or
         injunction is removed, BB's obligations under this Agreement in respect
         of such part of the Territory to which the order or injunction related
         shall automatically recommence from the date that such removal is
         effective.

41       FURTHER THIRD PARTY LICENSES

41.1     In the event that further patent licenses from third parties
         (including, without limitation, those set out in Schedule 10) are
         reasonably required by either (a) ImmunoGen in order for it to carry
         out Pre-Clinical Studies, to fulfil its CMC Development obligations or
         to Manufacture Licensed Compound or Product; or (b) BB or its
         Sub-licensees in order to develop, import, export, keep (whether for
         disposal or otherwise) use, distribute, promote, market, offer for sale
         or sell Licensed Compound and/or Product (hereinafter "Further Third
         Party Licenses"), [*]. Upon the conclusion of a Further Third Party
         License, [*].

42       COMPULSORY LICENSES

42.1     If a compulsory license (as such term is construed pursuant to section
         48 of the UK Patents Act 1977 as amended by the Patents and Trademarks
         (World Trade Organization) Regulations 1999, or such equivalent
         statutory provision in any other country of the Territory) is granted
         under the Patents to an Independent Third Party with respect to
         Licensed Compound and/or Product in any country in the Territory [*].

43       DIRECT AFFILIATE LICENSES

43.1     Whenever BB shall reasonably demonstrate to ImmunoGen that, in order to
         facilitate direct royalty payments by an Affiliate, it is desirable
         that a separate license agreement be entered into between ImmunoGen and
         such Affiliate, ImmunoGen will grant such licenses directly to such
         Affiliate by means of an agreement which shall be consistent with all
         of the provisions hereof, provided that BB guarantees the Affiliate's
         obligations thereunder.

                         PART H - TERMINATION PROVISIONS

44       BB'S RIGHT TO TERMINATE

44.1     BB shall have the right, but not the obligation, to terminate this
         Agreement [*]

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         44.1.1   [*];

         44.1.2   [*]

                  44.1.2.1  [*]

                  44.1.2.2  [*]

                  44.1.2.3  [*].

         44.1.3   [*]:

                  44.1.3.1  [*]

                  44.1.3.2  [*]

         In the event that ImmunoGen disagrees that the opinion formed by BB in
         Section 44.1.2 above is reasonable, the matter shall be referred to the
         Collaboration Committee. If the Collaboration Committee is unable to
         resolve the matter within [*] of the submission of the matter to it for
         resolution, BB shall be entitled to terminate the Agreement and all
         licensed rights shall return to ImmunoGen pursuant to the provisions of
         Clause 47.2.

44.2     If in the reasonable opinion of BB, [*]

45       IMMUNOGEN'S RIGHTS TO TERMINATE

45.1     Save in the circumstances expressly referred to herein, in the event
         that [*].

45.2     ImmunoGen shall have the right, but not the obligation, to terminate
         this Agreement [*]:

         45.2.1   [*]

         45.2.2   [*]

46       GENERAL RIGHTS OF TERMINATION BY EITHER PARTY

46.1     INSOLVENCY. If at any time either party shall become insolvent or shall
         cease to carry on its business or shall go into liquidation, whether
         compulsory or voluntary (other than a voluntary liquidation for the
         purpose of reconstruction or amalgamation), or shall have a receiver
         appointed over the whole or any part of its assets or shall enter into
         any arrangement or composition with its creditors or become bankrupt or
         enter into a corporate rehabilitation or corporate reorganization then,
         and in any of the foregoing events, the other party shall be entitled
         to terminate this Agreement forthwith by notice in writing.

46.2     MATERIAL BREACH. This Agreement may be terminated by either party if
         the other party is in breach of its material obligations hereunder and
         has not cured such breach within [*] after written notice requesting
         cure of the breach with reasonable detail of the particulars of the
         alleged breach, or within [*] of receiving notice initiated actions
         reasonably expected to cure the cited failure and thereafter diligently
         pursued such actions to cure the failure (even if requiring longer than
         the ninety [*] set forth in this subsection). To the extent that the
         party receiving the notice

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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

         disputes the existence of a material breach, the provisions of Clause
         51 shall apply and the notice shall be suspended until the outcome
         thereof.

47       EFFECT OF TERMINATION

47.1     GENERAL PROVISIONS. The termination of this Agreement either in whole
         or in part shall:

         47.1.1   be without prejudice to the accrued obligation of either party
                  to pay to the other party all sums due and payable either in
                  whole or in part, which sums shall be paid within forty five
                  (45) days of the date of termination;

         47.1.2   be without prejudice to any right of, or remedy available to,
                  either party against the other in respect of anything done or
                  omitted hereunder prior to such termination; and

         47.1.3   not release either party from the inability to supply,
                  confidentiality, non-solicitation, standstill or
                  indemnification obligations set forth in Clauses 27, 53, 55,
                  56 and 62 hereof.

         47.1.4   not relieve BB or ImmunoGen from complying with the applicable
                  terms and conditions of this Agreement.

47.2     EFFECT OF TERMINATION[*] If this Agreement is terminated [*]:

         47.2.1   [*];

         47.2.2   [*];

         47.2.3   [*];

         47.2.4   [*];

         47.2.5   [*]

         47.2.6   [*].

47.3     EFFECT OF TERMINATION BY [*]. If this Agreement is terminated by[*]:

         47.3.1   [*];

         47.3.2   [*];

         47.3.3   [*]

         47.3.4   [*]

                  PART I - PUBLICITY AND PUBLICATION PROVISIONS

48       PUBLICITY

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                       CONFIDENTIAL TREATMENT REQUESTED.
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

48.1     Except as provided in Clause 53, a party may not use the name of the
         other party in any publicity, advertising or in any other public way
         and, may not issue press releases or otherwise publicize or disclose
         any information related to the existence of this Agreement, the terms
         or conditions of this Agreement, or any information relating to the
         subject matter hereof, without the prior written consent of the other
         party. The parties may agree upon an initial press release to announce
         the execution of this Agreement, together with a corresponding Q&A
         outline for use in responding to inquiries about the Agreement.
         Following such initial press release, either party may use the specific
         information contained therein, or in any subsequent public
         announcements or publications made by the other party or by mutual
         agreement of the parties, in its investor relations and public
         relations activities. Neither party shall make public announcements,
         either written, oral or in any medium relating to the safety of
         Licensed Compound and/or Product, except for statements in official
         correspondence with government patent authorities in support of Patents
         as provided for in this Agreement. Nothing in the foregoing, however,
         shall prohibit a party from making disclosures to the extent required
         under applicable federal or state securities laws or any rule or
         regulation of any nationally recognized securities exchange, provided
         same is accurate and complete. In such event, however, the disclosing
         party shall use good faith efforts to consult with the other party
         prior to such disclosure and, where applicable, shall request
         confidential treatment to the extent available.

49       PUBLICATIONS

49.1     BB and ImmunoGen each acknowledge the potential benefit in publishing
         results of certain studies to obtain recognition within the scientific
         community and to advance the state of scientific knowledge. Each party
         also recognizes the mutual interest in obtaining valid patent
         protection and in protecting business interests and trade secret
         information. No publication of Manufacturing Information, Patents or
         Technical Information or other Confidential Information (collectively,
         "Protected Information") may be made without mutual written consent.
         The parties agree that BB, its employees or consultants shall be free
         to make any publication which does not disclose any of the Protected
         Information. In the event that any proposed publication (as defined
         below) discloses Protected Information, the following procedure shall
         apply: Either party, its employees or consultants wishing to make a
         publication shall deliver to the other party a copy of the proposed
         written publication or an outline of an oral disclosure at least [*]
         prior to submission for publication or presentation. For purposes of
         this Agreement, the term "publication" shall include, without
         limitation, abstracts and manuscripts for publication, slides and texts
         of oral or other public presentations, and texts of any transmission
         through any electronic media, e.g. any computer access system such as
         the Internet, including the World Wide Web. The reviewing party shall
         have the right (i) to propose modifications to the publication for
         patent reasons, trade secret reasons or business reasons or (ii) to
         request delay of the publication or presentation in order to protect
         patentable information. If the reviewing party requests a delay, the
         publishing party shall delay submission or presentation for a period
         not less than [*] from the filing date of the first patent application
         in the Territory covering the information contained in the proposed
         publication or presentation. If the reviewing party requests
         modifications to the publication, the publishing party may edit such
         publication to prevent disclosure of trade secret or proprietary
         business information prior to submission of the publication or
         presentation. Following Regulatory Approval, BB shall have the right to
         use any clinical data arising out of this Agreement, to the extent that
         it relates to the Product(s) in respect of which Regulatory Approval
         has been obtained.

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                       CONFIDENTIAL TREATMENT REQUESTED.
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                           PART J - DISPUTE PROVISIONS

50       GOVERNING LAW AND JURISDICTION

50.1     This Agreement shall be governed by and construed and interpreted in
         accordance with the laws of the State of New York and the United States
         without regard to that body of law known as conflicts of law; provided
         that issues relating to the validity and enforceability of patents
         shall be governed by the laws of the jurisdiction by which such patent
         was granted. Any proceeding between the parties shall be conducted in
         the English language.

51       DISPUTE RESOLUTION

51.1     In the event that a dispute arises between the parties as to the
         interpretation or performance of any of the provisions of this
         Agreement or as to matters related to but not covered by this
         Agreement, the parties shall consult initially to try and resolve the
         matter amicably. If they shall not be capable of resolving the matter
         within [*]of the dispute arising, or such other period as may be
         provided by this Agreement, it shall be referred to the respective
         Chief Executive Officers or Chief Operating Officers of the parties.

51.2     If the Chief Executive Officers or Chief Operating Officers cannot
         resolve the dispute within [*] days of it being referred to them, to
         the extent that the parties mutually agree, the matter shall be
         referred to binding or non-binding arbitration or mediation. Such
         arbitration or mediation proceedings shall be held in Boston,
         Massachusetts in the event that the dispute originated from BB and in
         London, England in the event that the dispute originated from
         ImmunoGen. To the extent that the parties do not mutually agree upon
         the matter being referred to arbitration or mediation or to the extent
         that the arbitration or mediation proceedings are non-binding and one
         of the parties disputes the outcome thereof, the matter shall be dealt
         with in accordance with the provisions of Clause 52.

52       LEGAL PROCEEDINGS

52.1     In the event that disputes between the parties have not been settled in
         accordance with the provisions of Clause 51, the parties shall be
         entitled to bring court proceedings in the Courts of New York, which
         courts shall have the exclusive jurisdiction in respect thereof.

      PART K - CONFIDENTIALITY, NON-SOLICITATION AND STANDSTILL PROVISIONS

53       CONFIDENTIALITY

53.1     During the term of this Agreement and, subject to the provisions of
         Clause 4, for [*] thereafter, each party shall hold in confidence any
         confidential information (including but not limited to Technical
         Information, Manufacturing Information or BB Technical Information)
         generated by the other party or received from the other party pursuant
         to this Agreement ("Confidential Information") and shall not without
         the prior written consent of the other party disclose any part of the
         same to any Independent Third Party except such of its employees and
         consultants and those of its Sub-licensees and contractors who have a
         need to know in order to effect the work of the

                                       45
<PAGE>   46
         Development Plan or the obtaining of Regulatory Approval or marketing
         of Product and who are bound by equivalent obligations of
         confidentiality as those contained in this Clause.

53.2     BB and its Sub-licensees shall only use the Confidential Information
         obtained from ImmunoGen in pursuit of its rights and obligations under
         this Agreement including for the purposes of carrying out the
         Development Plan or Marketing Plan in respect of Product.

53.3     ImmunoGen, its licensees and its contractors shall only use the
         Confidential Information obtained from BB for the purpose of assisting
         it in registering, marketing and selling Product outside the Territory.

53.4     The provisions of this Clause shall not apply to information received
         by either ImmunoGen or BB or Sub-licensees hereunder which:

         53.4.1   as of the date of receipt is in the public domain or comes
                  into the public domain through no breach of this Agreement by
                  the receiving party of any confidentiality obligation; or

         53.4.2   is received at any time in good faith from a third party
                  lawfully in possession of the same and having no restriction
                  disclosing the same; or

         53.4.3   was known to the receiving party prior to its receipt from the
                  disclosing party as can be demonstrated by the receiving party
                  through written evidence; or

         53.4.4   is developed by the receiving party its Affiliate or
                  Sub-licensees without aid, application or use of any of the
                  Confidential Information to which the obligation of
                  confidentiality applies by employee(s) of the receiving party
                  who were not given access to the Confidential Information
                  received hereunder.

53.5     Nothing contained herein shall prevent:

         53.5.1   BB from disclosing Confidential Information generated by or on
                  behalf of ImmunoGen for the purpose of procuring the requisite
                  Regulatory Approval from the appropriate authority and other
                  governmental agencies in the Territory for BB or Sub-licensees
                  to undertake clinical trials upon Product (including any
                  institutional review board of any Entity conducting clinical
                  studies) or to market Product in the Territory or any part
                  thereof; provided however that all reasonable steps are taken
                  to require the appropriate authority and said other
                  governmental agencies in the Territory or any part thereof to
                  treat the Confidential Information as confidential and
                  proprietary information of ImmunoGen; or

         53.5.2   ImmunoGen from disclosing Confidential Information generated
                  by BB for the purpose of procuring the requisite Regulatory
                  Approval from the appropriate authority and other governmental
                  agencies outside the Territory for ImmunoGen to undertake
                  clinical trials upon Product (including any institutional
                  review board of any Entity conducting clinical studies) or to
                  market Product outside the Territory or any part thereof;
                  provided however that all reasonable steps are taken to
                  require the appropriate governmental agencies to treat the
                  Confidential Information as confidential and proprietary
                  information; or

         53.5.3   Confidential Information that is required by law, regulation,
                  rule, act or order of any governmental authority or agency to
                  be disclosed by a party, provided that notice is promptly
                  delivered to the other party in order to provide an
                  opportunity to seek a

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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                  protective order or other similar order with respect to such
                  Confidential Information and thereafter the disclosing party
                  discloses to the requesting entity only the minimum
                  Confidential Information required to be disclosed in order to
                  comply with the request, whether or not a protective order or
                  other similar order is obtained by the other party.

53.6     The parties agree that the commercial and other terms of this Agreement
         [*] shall, unless otherwise agreed, be treated as Confidential
         Information.

54       ADDITIONAL PROVISION OF INFORMATION BY IMMUNOGEN

54.1     ImmunoGen agrees to provide BB with [*]

55       NON-SOLICITATION OF STAFF

55.1     For a period of [*] from the Effective Date, neither of the parties
         shall do or permit any of the following without the prior written
         consent of the other party:

         55.1.1   solicit or entice away, or endeavor to solicit or entice away,
                  any key person employed by the other party in a managerial,
                  supervisory, technical, scientific or sales capacity.

         55.1.2   cause or permit any Affiliate, and in the case of BB, any
                  Sub-licensee, to do any of the acts or things specified in
                  Clause 55.1.1.

55.2     Whilst the undertaking in Clause 55.1 is considered by the parties to
         be reasonable in all the circumstances, if one or more is held invalid
         as an unreasonable restraint of trade or for any other reason but would
         have been held valid if part of the wording had been deleted, the
         period reduced or the range of activities or area dealt with reduced in
         scope, the undertakings shall apply with such modifications as may be
         necessary to make them valid.

56       STANDSTILL PROVISIONS

56.1     For the duration of this Agreement and for a period of [*] from the
         date on which this Agreement terminates for whatever reason neither
         ImmunoGen nor BB will:

         56.1.1   [*]

         56.1.2   [*]

                   PART L - WARRANTY AND INDEMNITY PROVISIONS

57       REPRESENTATIONS AND WARRANTIES OF EACH PARTY

57.1     Each of ImmunoGen and BB hereby represents, warrants and covenants to
         the other party that as at the Effective Date:

         57.1.1   it is a corporation or entity duly organized and validly
                  existing under the laws of the state or other jurisdiction of
                  its incorporation or formation;

                                       47
<PAGE>   48
         57.1.2   the execution, delivery and performance of this Agreement by
                  such party has been duly authorized by all requisite corporate
                  action;

         57.1.3   it has the power and authority to execute and deliver this
                  Agreement and to perform their obligations hereunder;

         57.1.4   the execution, delivery and performance by such party of this
                  Agreement and its compliance with the terms and provisions
                  hereof does not and will not conflict with or result in a
                  breach of any of the terms and provisions of or constitute a
                  default under (i) a loan agreement, guaranty, financing
                  agreement, agreement affecting a product or other agreement or
                  instrument binding or affecting it, its Affiliates, or their
                  property; (ii) the provisions of its or its Affiliates'
                  charter or operative documents or bylaws; or (iii) any order,
                  writ, claim form, injunction or decree of any court or
                  governmental authority entered against it or its Affiliates or
                  by which any of its or their property is bound;

         57.1.5   except for the governmental and Regulatory Approvals required
                  to market Product in the Territory, the execution, delivery
                  and performance of this Agreement by such party does not
                  require the consent, approval or authorization of, or notice,
                  declaration, filing or registration with, any governmental or
                  regulatory authority and the execution, delivery or
                  performance of this Agreement will not violate any law, rule
                  or regulation applicable to such party;

         57.1.6   this Agreement has been duly authorized, executed and
                  delivered and constitutes such party's legal, valid and
                  binding obligation enforceable against it in accordance with
                  its terms subject, as to enforcement, to bankruptcy,
                  insolvency, reorganization and other laws of general
                  applicability relating to or affecting creditors' rights and
                  to the availability of particular remedies under general
                  equity principles;

         57.1.7   there is no action, suit, notice of violation, proceeding or
                  investigation pending or, to the best of its knowledge,
                  threatened against it or any of its Affiliates or any of their
                  respective properties before or by any court, governmental or
                  administrative agency or regulatory authority which:

                  57.1.7.1  relates to or challenges the legality, validity or
                            enforceability of this Agreement; or

                  57.1.7.2  could, individually or in aggregate, materially
                            impair its ability to perform fully on a timely
                            basis its obligations under this Agreement.

                  Furthermore, the board of directors of each party does not
                  have knowledge of any fact or circumstance which is likely to
                  lead to any such action, suit, notice of violation, proceeding
                  or investigation; and

         57.1.8   the information set out in the Schedules is true and accurate
                  in all material respects insofar as it is aware there is no
                  matter which renders any such information untrue, inaccurate,
                  incomplete or misleading.

57.2     Each of ImmunoGen and BB represents, warrants and covenants to the
         other party that during the term of this Agreement it shall comply with
         all applicable material laws and regulations relating to its activities
         under this Agreement.

                                       48
<PAGE>   49
58       REPRESENTATIONS AND WARRANTIES OF IMMUNOGEN

58.1     ImmunoGen represents, warrants and covenants to BB, except as set forth
         in Schedule 10, in respect of the Territory and, where appropriate,
         only to the extent that Licensed IP relates to the rights specifically
         granted to BB in respect of the Licensed Compound or Product, that:

         58.1.1   to the best of its knowledge, to the extent that they have
                  been granted, the Patents are valid, subsisting and
                  enforceable, in whole or in part, and there are no facts which
                  would as a matter of law preclude the issuance of Patents for
                  which patent applications are pending;

         58.1.2   it has the full right, power and authority to grant all of the
                  right, title and interest in the licenses granted to BB under
                  Clause 2;

         58.1.3   it and its Affiliates have not previously assigned,
                  transferred, conveyed or otherwise encumbered its right, title
                  and interest in the Licensed Compound, or the Licensed IP;

         58.1.4   to the best of its knowledge, the Patents and the development,
                  manufacture, importation, exportation, keeping (whether for
                  disposal or otherwise) marketing, distribution, use,
                  promotion, offer for sale and sale of Licensed Compound or the
                  Product do not interfere or infringe on any intellectual
                  property rights owned or possessed by any Independent Third
                  Party;

         58.1.5   to the best of ImmunoGen's knowledge, there are no claims,
                  judgements or settlements against or amounts with respect
                  thereto owed by ImmunoGen or pending or threatened claims or
                  litigation against ImmunoGen relating to Licensed Compound or
                  Licensed IP and no grounds exist which may support any such
                  claims;

         58.1.6   it is in compliance in all material respects with any Third
                  Party Licenses relating to Licensed IP;

         58.1.7   to the best of its knowledge, there are no circumstances that
                  would render BB liable to an Independent Third Party for
                  patent infringement as a consequence of BB's sale of Product
                  or use of the Licensed IP and there has been no claim
                  concerning such infringement made or considered by ImmunoGen;

         58.1.8   to the best of its knowledge, all data summaries provided in
                  writing to BB by ImmunoGen relating to pre-clinical studies of
                  the Licensed Compound accurately represent the raw data
                  underlying such summaries;

         58.1.9   to the best of ImmunoGen's knowledge, there is no fact
                  undisclosed by ImmunoGen which materially adversely affects or
                  would materially adversely affect the rights granted to BB
                  under this Agreement;

         58.1.10  during the term of this Agreement, it and its Affiliates will
                  use reasonable commercial efforts not to diminish the rights
                  under the Licensed IP granted to BB hereunder, including
                  without limitation, by committing or permitting any actions or
                  omissions which would cause the breach of any agreements
                  between itself and Independent Third Parties which provide for
                  intellectual property rights applicable to the development,
                  manufacture,

                                       49
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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                  use or sale of Licensed Compound and/or Product(s), and that
                  it will provide BB promptly with notice of any such breach.

58.2     Nothing in this Agreement or any license granted hereunder is to be
         construed as a representation or warranty that Licensed Compound shall
         be or is capable of being successfully developed or granted Regulatory
         Approval in the Territory or any part thereof for the treatment of any
         disease within the Field.

58.3     BB acknowledges that ImmunoGen has not given and BB is not relying on
         any warranty, covenant or representation of any kind (other than
         ImmunoGen's warranties, covenants and representations under Clauses 57
         and 58.1) express or implied, in relation to Licensed Compound or its
         use including but not limited to implied warranties for merchantability
         or fitness for a particular purpose. BB further acknowledges that it
         has made its own due and careful inquiries before entering this
         Agreement.

59       [*]

59.1     [*]

60       NO INCONSISTENT AGREEMENTS

60.1     Neither party has in effect and after the Effective Date neither party
         shall enter into any oral or written agreement or arrangement that
         would be inconsistent with its obligations under this Agreement.

61       REPRESENTATION BY LEGAL COUNSEL

61.1     Each party hereto represents that it has been represented by legal
         counsel in connection with this Agreement and acknowledges that it has
         participated in the drafting hereof. In interpreting and applying the
         terms and provisions of this Agreement, the parties agree that no
         presumption shall exist or be implied against the party which drafted
         such terms and provisions.

62       INDEMNIFICATION

62.1     BB shall indemnify and hold harmless ImmunoGen, its Affiliates and
         their employees, officers, directors and agents (each, a "ImmunoGen
         Indemnified Party") from and against any and all claims, demands,
         lawsuits, proceedings, settlement amounts, liability, loss, damage,
         cost and expense (including reasonable attorneys' fees), but subject to
         the limitations in Clause 62.5 (collectively, a "Liability") which may
         be asserted against the ImmunoGen Indemnified Party or which the
         ImmunoGen Indemnified Party may incur, suffer or be required to pay
         resulting from or arising out of (i) the research, discovery,
         development, manufacture, importing, exporting, keeping (whether for
         disposal or otherwise), promotion, distribution, use, testing,
         marketing, sale of Licensed Compound and/or Product(s) by BB or
         Sub-licensees (including without limitation any personal injury, death,
         or other injuries suffered by users of Product), or (ii) the breach by
         BB of any covenant, representation or warranty contained in this
         Agreement; or (iii) the successful enforcement by a ImmunoGen
         Indemnified Party of its rights under this Clause. Notwithstanding the
         foregoing, BB shall have no obligation under this Agreement to
         indemnify or hold harmless any ImmunoGen Indemnified Party with respect
         to any Liability which results from the willful misconduct or negligent
         acts or omissions of ImmunoGen, its Affiliates or any of their
         employees, officers, directors or agents.

                                       50
<PAGE>   51
62.2     ImmunoGen shall indemnify and hold harmless BB, its Affiliates and
         their employees, officers, directors and agents (each, a "BB
         Indemnified Party") from and against any Liability which the BB
         Indemnified Party may incur, suffer or be required to pay resulting
         from or arising out of (i) the development, manufacture, importing,
         exporting, keeping (whether for disposal or otherwise), use or testing
         of Licensed Compound and/or Product(s) by ImmunoGen (including without
         limitation) any personal injury, death or other injuries suffered by
         users of Product; (ii) the breach by ImmunoGen of any covenant,
         representation or warranty contained in this Agreement; or (iii) the
         successful enforcement by a BB Indemnified Party of its rights under
         this Clause. Notwithstanding the foregoing, ImmunoGen shall have no
         obligation under this Agreement to indemnify or hold harmless any BB
         Indemnified Party with respect to any Liability which results from
         willful misconduct or negligent acts or omissions of BB, its Affiliates
         or their employees, officers, directors or agents.

62.3     Each party agrees to promptly give the other party notice of any claim
         for which indemnification may be sought. Failure of an indemnified
         party to provide notice of a claim to the indemnifying party shall
         affect the indemnified party's right to indemnification only to the
         extent that such failure has a material adverse effect on the
         indemnifying party's ability to defend or the nature or the amount of
         the Liability. Subject to the provisions of Section G (Patent
         Provisions) of this Agreement, the indemnifying party shall have the
         right to assume the defense of any suit or claim related to the
         Liability if it has assumed responsibility for the suit or claim in
         writing; provided, however, that if in the reasonable judgement of the
         indemnified party, such suit or claim involves an issue or matter which
         could have a materially adverse effect on the business operations or
         assets of the indemnified party, the indemnified party may waive its
         rights to indemnity under this Agreement and control the defense or
         settlement thereof, but in no event shall any such waiver be construed
         as a waiver of any indemnification rights such party may have at law or
         in equity. If the indemnifying party defends the suit or claim, the
         indemnified party may participate in (but not control) the defense
         thereof at its sole cost and expense.

62.4     Subject to the provisions of Part G (Patent Provisions) of this
         Agreement, neither party may settle a claim or action related to a
         Liability without the consent of the other party if such settlement
         would impose any monetary obligation on the other party or require the
         other party to submit to an injunction or otherwise limit the other
         party's rights under this Agreement, provided that such consent shall
         not be unreasonably withheld or delayed. Any payment made by a party to
         settle any such claim or action shall be at its own cost and expense.

62.5     With respect to any claim by one party against the other arising out of
         the performance or failure of performance of the other party under this
         Agreement, the parties expressly agree that the liability of such party
         to the other party for such breach shall be limited under this
         Agreement or otherwise at law or equity to direct damages only and in
         no event shall a party be liable for, punitive, exemplary or
         consequential damages suffered or incurred by the other party.

62.6     Each party acknowledges and agrees that during the term of this
         Agreement it shall maintain adequate insurance for contractual
         liability insurance to cover such party's obligations under this
         Agreement (including product liability). The Collaboration Committee
         shall review the level of each party's insurance coverage annually to
         ensure that it is, in their reasonable opinion, adequate, taking into
         account all relevant circumstances. Each party shall provide the other
         party with evidence of such insurance upon request.

                                       51
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                           PART M - GENERAL PROVISIONS

63       BANKRUPTCY PROTECTION

63.1     RETENTION OF LICENSE RIGHTS. All licenses and rights granted under or
         pursuant to this Agreement by ImmunoGen to BB are, and shall otherwise
         be deemed to be, for purposes of Section 365(n) of the U.S. Bankruptcy
         Code, licenses of rights to "intellectual property" as defined under
         Section 101(52) of the U.S. Bankruptcy Code. The parties agree that BB,
         as a licensee of such rights under this Agreement, shall retain and may
         fully exercise all of its rights and elections under the U.S.
         Bankruptcy Code, subject to performance by BB of its pre-existing
         obligations under this Agreement. The parties further agree that, in
         the event of the commencement of a bankruptcy proceeding by or against
         ImmunoGen under the U.S. Bankruptcy Code, BB shall be entitled to a
         complete duplicate of (or complete access to, as appropriate) any such
         intellectual property and all embodiments of such intellectual
         property, and same, if not already in its possession, shall be promptly
         delivered to BB (a) upon any such commencement of a bankruptcy
         proceeding upon written request therefor by BB, unless ImmunoGen elects
         to continue to perform all of its obligations under this Agreement, or
         (b) if not delivered under (a) above, upon the rejection of this
         Agreement by or on behalf of ImmunoGen upon written request therefor by
         BB, provided, however, that upon ImmunoGen's (or its successor's)
         written notification to BB that it is again willing and able to perform
         all of its obligations under this Agreement, BB shall promptly return
         all such tangible materials to ImmunoGen, but only to the extent that
         BB does not require continued access to such materials to enable BB to
         perform its obligations under this Agreement.

64       NOTICES

64.1     ADDRESS DETAILS. Any notice required or permitted under this Agreement
         shall be delivered by hand or sent by courier, by first class postage
         prepaid and receipt requested or by facsimile transmission to the
         following addresses of the parties:

         BB's Address:        British Biotech Pharmaceuticals Limited
                              Watlington Road
                              Oxford OX4 6LY
                              England
                              Fax no: +44-1865-781128
                              For the attention of the Legal Department

         ImmunoGen's Address: ImmunoGen Inc.
                              333 Providence Highway
                              Norwood, MA 02062
                              USA
                              Fax no: (781) 255-9679
                              For the attention of  Chief Executive Officer

         With copies to:      Mintz. Levin, Cohn, Ferris, Glovsky
                                and Popeo, P.C.
                              One Financial Center
                              Boston, MA 02111
                              Attn: Jeffrey M. Wiesen, Esq.
                              Fax no.: (617) 542-2241

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<PAGE>   53
64.2     TIME OF RECEIPT. Any notice required or permitted to be given
         concerning this Agreement shall be deemed to have been received by the
         party to whom it is addressed:

         64.2.1   if delivered by hand, upon receipt at the premises referred to
                  above; or

         64.2.2   if sent by post or by courier, on the date it was received as
                  recorded on the return receipt; or

         64.2.3   if sent by facsimile (confirmed by letter sent by first class
                  post), at the time of receipt shown on the transmission
                  confirmation report.

64.3     LANGUAGE OF NOTICE. Any notice or other document served by one party on
         the other in accordance with the terms of this Agreement shall be in
         the English language and shall not be validly served unless this
         condition is complied with.

65       FORCE MAJEURE

65.1     Failure of any party to perform its obligations under this Agreement
         (except the obligation to make payments when properly due) shall not
         subject such party to any liability or place them in breach of any term
         or condition of this Agreement to the other party if such failure is
         due to any cause beyond the reasonable control of such non-performing
         party ("force majeure"), unless conclusive evidence to the contrary is
         provided. Causes of non-performance constituting force majeure shall
         include, without limitation, acts of God, fire, explosion, flood,
         drought, war, riot, sabotage, embargo, strikes or other labor trouble,
         failure in whole or in part of suppliers to deliver on schedule
         materials, equipment or machinery, interruption of or delay in
         transportation, a national health emergency or compliance with any
         order or regulation of any government entity acting with color of
         right. The party affected shall promptly notify the other party of the
         condition constituting force majeure as defined herein and shall exert
         reasonable efforts to eliminate, cure and overcome any such causes and
         to resume performance of its obligations with all possible speed;
         provided, however, that nothing contained herein shall require any
         party to settle on terms unsatisfactory to such party any strike,
         lock-out or other labor difficulty, any investigation or proceeding by
         any public authority, or any litigation by any third party. If a
         condition constituting force majeure as defined herein exists for more
         than ninety (90) consecutive days, the parties shall meet to negotiate
         a mutually satisfactory resolution to the problem, if practicable.

66       ASSIGNMENTS

66.1     Neither this Agreement nor any or all of the rights and obligations of
         a party hereunder shall be assigned, delegated, sold, transferred,
         sublicensed (except as expressly permitted hereunder) or otherwise
         disposed of, by operation of law or otherwise, to any Independent Third
         Party (other than an Affiliate of an assigning party under the
         condition that the assignor remain responsible to the other party under
         this Agreement), without the prior written consent of the other party.
         Any attempted assignment, delegation, sale, transfer, sublicense or
         other disposition, by operation of law or otherwise, of this Agreement
         or of any rights or obligations hereunder contrary to this Clause 66.1
         shall be a material breach of this Agreement by the attempting party,
         and shall be void and without force or effect; provided, however,
         either party may, without such consent, assign the Agreement and its
         rights and obligations hereunder to an Affiliate or in connection with
         the transfer or sale of all or substantially all of its assets related
         to the division or the subject business, or in the event of its merger
         or consolidation or change in control or similar transaction.

                                       53
<PAGE>   54
66.2     This Agreement shall be binding upon, and inure to the benefit of, each
         party and its permitted successors and assigns. Each party shall be
         responsible for the compliance by its Affiliates with the terms and
         conditions of this Agreement.

67       SEVERABILITY

67.1     In the event that any Clause or any part of any Clause contained in
         this Agreement is declared invalid or unenforceable by the judgment or
         decree by consent or otherwise of a Court of competent jurisdiction not
         subject to appeal, all other Clauses or parts of Clauses contained in
         this Agreement shall remain in full force and effect and shall not be
         affected thereby for the term of this Agreement.

68       WAIVER

68.1     No relaxation, forbearance, delay or indulgence by either party in
         enforcing any of the terms and conditions of this Agreement or the
         granting of time by either party to the other shall prejudice, affect
         or restrict the rights and powers of that said party hereunder nor
         shall any waiver by either party of any breach hereof operate as a
         waiver of or in relation to any subsequent or any continuing breach
         hereof.

68.2     A waiver by one party of a breach by the other of any term of this
         Agreement shall not prevent the subsequent enforcement of that term and
         shall not be deemed a waiver of any subsequent breach.

69       VAT

69.1     Any amount payable under this Agreement shall be deemed to be exclusive
         of Value Added Tax.

70       COSTS OF PREPARATION

70.1     The parties hereto shall pay their own respective legal costs incurred
         in the preparation of this Agreement.

71       GOVERNMENT CONSENT

71.1     Insofar as this Agreement requires the consent of any official body of
         the Government of either the UK or the USA, each party shall use its
         reasonable endeavors to obtain the approval of such body in such
         country and notify the other thereof promptly.

72       INDEPENDENT DISCOVERIES BY BB

72.1     Subject to the provisions of this Agreement, ImmunoGen acknowledges
         that BB has ongoing research programs which may now or in the future
         independently discover, develop and/or acquire technologies and/or
         products relating to treatment and prevention of any disease, disorder
         or condition in humans or animals. ImmunoGen agrees that such
         technologies and products, to the extent discovered without use of
         Patents or Technical Information, will not be deemed to be BB Technical
         Information or Improvements and will fall outside the scope of this
         Agreement.

                                       54
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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

73       INDEPENDENT RELATIONSHIP

73.1     Nothing in this Agreement shall be deemed to create an employment,
         agency, joint venture or partnership relationship between the parties
         hereto or any of their respective agents or employees, or any other
         legal arrangement that would impose liability upon one party for the
         act or failure to act of the other party. Neither party shall have any
         power to enter into any contracts or commitments or to incur any
         liabilities in the name of, or on behalf of, the other party, or to
         bind the other party in any respect whatsoever. Further, nothing in
         this Agreement shall entitle either party to make any representation or
         give a warranty on behalf of the other party.

74       COUNTERPARTS

74.1     This Agreement shall become binding when any one or more counterparts
         hereof, individually or taken together, shall bear the signatures of
         each of the parties hereto. This Agreement may be executed in any
         number of counterparts, each of which shall be an original as against
         either party whose signature appears thereon, but all of which taken
         together shall constitute but one and the same instrument.

75       RECORDING

75.1     Each party shall have the right, at any time, to record register, or
         otherwise notify this Agreement in appropriate governmental or
         regulatory offices anywhere in the world, and each party shall provide
         reasonable assistance to the other in effecting such recording,
         registering or notifying. Notwithstanding the foregoing, prior to
         recording, registering, or otherwise notifying this Agreement, the
         party desiring to so record, register, or notify shall provide a copy
         of all materials to be filed for review, comment and approval by the
         other party, such approval not unreasonably to be withheld or delayed.

76       FURTHER ACTIONS

76.1     Each party agrees to execute, acknowledge and deliver such further
         instruments, and to do all other acts, as may be necessary or
         appropriate in order to carry out the purposes and intent of this
         Agreement including, without limitation, any filings with any antitrust
         agency which may be required.

77       ENTIRE AGREEMENT

77.1     This Agreement (including its Exhibits and Schedules) [*] set forth the
         entire agreement and understanding of the parties relating to the
         subject matter hereof, and merge all prior discussions between them and
         all prior memoranda of intent or understanding. Neither party shall be
         bound by any definition, condition or representation other than as
         expressly stated in this Agreement or as subsequently agreed by the
         parties in writing, signed by a duly authorized officer of each party.

                                       55
<PAGE>   56

                                   SCHEDULE 1

                     PART 1 - CHEMICAL STRUCTURE OF huN901

AMINO ACID SEQUENCES OF CDR-GRAFTED V(L) AND V(H) OF huN901. Numbering follows
the Kabat system. The constant regions are human (kappa) for the light chain and
human (gamma)-1 for the heavy chain.

V(L)

----.---10----.---20--- -.--ABCDEF-30---- .---40----.----50----.- --60----.
DVVMTQSPLSLPVTLGQPASISC RSSQIIIHSDGNTY-LE WFQQRPGQSPRRLIY KVSNRFS GVPDRFSGS
                        -----------------                 -------
                              CDR L1                       CDR L2

---70----.---80----.--- 90----.-- 100----.--
GSGTDFTLKISRVEAEDVGVYYC FQGSHVPHT FGQGTKVEIK
                        ---------
                         CDR L3

V(H)

----.---10----.---20----.---30 ----. ---40----.----50--A--.--- 60--- -- .
QVQLVESGGGVVQPGRSLRLSCAASGFTFS SFGMH WVRQAPGKGLEWVA YISSGSFTIY YADSVKG
                               -----                ----------
                               CDR H1                 CDR H2

---70----.---80--ABC--. -- --90---- . -- -100A-- --. -- -110--
RFTISRDNSKNTLYLQMNSLRAEDTAVYYCAR MRKGYAMDY WGQGTLVTVS
                                 ---------
                                   CDR H3

 CDR = Complementarity-Determining Regions
V(L) = variable region, light chain
V(H) = variable region, heavy chain

                                       56
<PAGE>   57
                                   SCHEDULE 1

          PART 2 - CHEMICAL STRUCTURE OF LICENSED COMPOUND, huN901-DM1

                [2 GRAPHICS OMITTED EACH DEPICTING THE MOLECULAR
                STRUCTURE OF THE LICENSED COMPOUND, huN9901-DM1]

                                       57
<PAGE>   58
                                   SCHEDULE 2

                                     PATENTS

MAYTANSINOID CONJUGATES

<TABLE>
<CAPTION>
-----------------------------------------------------------------------------------------------------------------------------------
Attorney        Country          Appl. No.        Filing Date   Priority     Patent No.      Issue Date   Exp. Date    Next Annuity
Reference No.                                                   Date                                                       Date
-----------------------------------------------------------------------------------------------------------------------------------
<S>             <C>              <C>              <C>           <C>          <C>             <C>         <C>           <C>
A-5567          U.S.             07/426,247       10/25/89                   Abandoned

A-5567-1        U.S.
                Rule 62          07/911,380       07/13/92      10/25/89     5,208,020       05/04/93     05/04/10    11/04/00
                Continuation

F89903          Europe*          0 90 311 590.5   10/23/90      10/25/89     0 425 235 B1    09/25/96     10/23/10    10/23/00
                *National
                Patents in AT,
                BE, CH, DE,
                DK, ES, FR,
                GB, IT, LI,
                LU, NL, SE
                DE number: G
                690 28678.3-08

F89902          Canada           2,026,147-1      09/25/90      10/25/89     Pending                                  9/25/00

F89904          Japan            2-290,625        10/25/90      10/25/89     Pending
-----------------------------------------------------------------------------------------------------------------------------------
</TABLE>

N901 ANTIBODY

<TABLE>
<CAPTION>
-----------------------------------------------------------------------------------------------------------------------------------
Attorney          Country        Appl. No.        Filing Date    Priority     Patent No.      Issue Date   Exp. Date    Next Annuity
Reference No.                                                    Date                                                        Date
-----------------------------------------------------------------------------------------------------------------------------------
<S>               <C>            <C>              <C>           <C>          <C>              <C>          <C>          <C>
DFCI #72
00530/028         U.S.           06/603,181       04/23/84                    4,772,552       09/20/88     09/20/05     03/17/00

00530/028EP1      Europe*        0 85 302 806.6   04/22/85       04/23/84     160486          01/02/92     04/22/05     04/22/00
                  *National
                  Patents in AT,
                  BE, CH, DE,
                  FR, GB, IT,
                  LI, LU, NL,
                  SE

00530/028CA1      Canada         479685           04/22/85       04/23/84     1271715         07/17/90     04/22/05     07/17/00

00530/028JP1      Japan          87354/85         04/23/85       04/23/84     1875957         10/07/94     04/23/05     12/21/00

00530/028JP2      Japan
                  (Divisional)   323131/92        04/23/85       04/23/84     1999925         12/08/95     04/23/05     04/19/00
-----------------------------------------------------------------------------------------------------------------------------------
</TABLE>

N901 SYNERGY

<TABLE>
<CAPTION>
-----------------------------------------------------------------------------------------------------------------------------------
Attorney          Country        Appl. No.        Filing Date    Priority     Patent No.      Issue Date   Exp. Date    Next Annuity
Reference No.                                                    Date                                                        Date
-----------------------------------------------------------------------------------------------------------------------------------
<S>               <C>            <C>              <C>           <C>          <C>              <C>          <C>          <C>
104322.198        U.S.
                  (Provisional)  60/157,051       10/01/99                   Pending
-----------------------------------------------------------------------------------------------------------------------------------
</TABLE>

                                       58
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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                                   SCHEDULE 3

                                DEVELOPMENT PLAN

                 REGULATORY AND CLINICAL STRATEGY FOR huN901-DM1

[*]:

                                       59
<PAGE>   60
                                   SCHEDULE 4

                 PRINCIPAL TERMS OF COMMERCIAL SUPPLY AGREEMENT

1        PARTIES

1.1      The Agreement shall be between British Biotech Pharmaceuticals Limited
         and ImmunoGen Inc.

2        MANUFACTURE AND SUPPLY

2.1      British Biotech will appoint ImmunoGen to manufacture and supply, and
         ImmunoGen will manufacture and supply to British Biotech, Product for
         commercial use in the Territory.

3        WARRANTIES AND COVENANTS

3.1      ImmunoGen will warrant, amongst other things, that:

         -        all Product manufactured will comply with the manufacturer's
                  licenses, the Regulatory Approvals, the Specifications and all
                  applicable laws and regulations in force from time to time in
                  the relevant part of the Territory in respect of which Product
                  will be sold in;

         -        it will promptly disclose to British Biotech any comments by
                  Regulatory Authorities concerning its manufacturer's licenses
                  and that it will not seek to vary its manufacturer's licenses
                  without the consent of British Biotech (not to be unreasonably
                  withheld);

         -        all Product supplied will have a minimum shelf life, to be
                  agreed by the parties;

         -        all Product will be batch marked in accordance with agreed
                  marking procedures;

         -        all raw materials and Product will be stored in accordance
                  with the terms of the manufacturer's licenses and the
                  Regulatory Approvals pending delivery to British Biotech.

3.2      ImmunoGen will covenant, amongst other things, that:

         -        it will have and will maintain all necessary manufacturer's
                  licences in accordance with all applicable laws and
                  regulations to manufacture and supply Product for use in the
                  Territory;

         -        it will conduct the manufacture in accordance with the
                  Technical Agreement to be agreed by the parties;

         -        any raw materials employed by ImmunoGen in the Manufacture
                  will comply with the Specifications;

         -        it will allow, during normal business hours and upon
                  reasonable notice, authorised representatives of British
                  Biotech and representatives of any Government or regulatory
                  bodies to inspect the premises where the manufacture of the
                  Product is carried out or the Product or raw materials are
                  stored and to inspect the process of manufacture;

                                       60
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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

         -        it will upon written request supply BB with reasonable
                  quantities of samples of the Product manufactured by it;

         -        it shall retain a quantity of samples of each production batch
                  of the Products equal to twice the amount reasonably required
                  to conduct relevant analysis;

         -        it shall retain all manufacturing, analytical and distribution
                  records and shall retain such samples of the Products as are
                  required by, and in the manner and for the duration specified
                  by, all applicable laws and regulations, including GMP. Such
                  records will be made available to BB upon reasonable notice.
                  Records and samples shall not be destroyed without notice to
                  BB;

         -        it will have in place and will continue to have in place, and
                  will procure that any approved sub-contractors have in place,
                  appropriate health and safety procedures in compliance with
                  all applicable laws in the Territory.

3.3      BB will covenant that it will have and will maintain throughout the
         term of this Agreement appropriate Regulatory Approvals for the Product
         in the Territory or part(s) thereof for which the Product is to be
         marketed distributed sold or used.

4        TECHNICAL AGREEMENT

4.1      The respective responsibilities of ImmunoGen and BB relating to the
         manufacture of Product shall be as specified in the Technical Agreement
         to be agreed by the parties which agreement shall include, amongst
         other things, terms relating to:

         -        the way in which each batch of Product is to be manufactured
                  and checked for compliance with and adherence to the
                  appropriate Specifications and GMP

         -        the responsibility for purchasing materials

         -        testing and releasing materials

         -        undertaking production and quality control including
                  in-process controls as well as sampling and analysis

5        DURATION

5.1      The Agreement shall continue until the expiry or termination of the
         Collaboration, Development and License Agreement subject to the
         specific exceptions contained therein. Further, at least [*] prior to
         the expiry of the Collaboration, Development and License Agreement, the
         parties shall meet to discuss terms for the continued supply of Product
         to British Biotech.

                                       61
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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

6        FORECASTS

6.1      The parties shall agree upon appropriate provisions regarding the
         submission by British Biotech of non-binding and binding estimates for
         Product. Procedures relating to the provision by ImmunoGen of the
         confirmation of orders shall be agreed, which confirmations shall
         specify, amongst other things, quantity to be supplied and delivery
         times.

6.2      The parties shall agree upon a suitable amount of Product in respect of
         which British Biotech shall be entitled to order on a binding basis in
         addition to amount estimated (for example, up to fifty per cent. (50%)
         more than the quantity specified in the non-binding forecast for such
         month).

6.3      In addition to the above, if so requested, ImmunoGen will use
         reasonable efforts but with no obligation in respect of the quantity
         thereof, to supply to British Biotech additional Product in excess of
         that ordered or contained in the relevant forecast having due regard
         for ImmunoGen production capacity and other manufacturing commitments.

7        SUPPLY

7.1      Product shall be supplied by ImmunoGen in final form, fully packaged
         and ready for sale and shall be delivered to British Biotech or its
         designee [*]. In addition, ImmunoGen shall supply British Biotech with
         those documents specified in the Technical Agreement with each batch of
         the Product.

7.2      [*] Notwithstanding [*], title to the Product shall be and remain with
         ImmunoGen unless and until BB has paid in full for such Product.

7.3      Procedures relating to inspection and/or testing of Product by British
         Biotech following delivery shall be agreed, which procedures shall also
         regulate the rejection and/or acceptance of Product by British Biotech
         together with a dispute resolution procedure in the event of
         disagreement. Product inspection and testing shall be to ensure
         compliance with the Specifications.

7.4      If ImmunoGen accepts that the Product does not conform to the
         Specifications due to the negligence or default of ImmunoGen then
         ImmunoGen shall manufacture and deliver to BB (at no additional cost to
         British Biotech) a sufficient quantity of the Product to replace the
         defective batch or batches. If BB accepts that the relevant batches of
         Product were Manufactured in accordance with the Specifications or that
         any defect did not arise due to ImmunoGen negligence or default
         ImmunoGen shall have no liability or obligation to BB in respect of
         such Batches.

7.5      To the extent that [*]

                                       62
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                       CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS INDICATED BY "*", HAVE BEEN OMITTED AND FILED SEPARATELY
                  WITH THE SECURITIES AND EXCHANGE COMMISSION

8        INTELLECTUAL PROPERTY

8.1      Save as specifically provided, neither party shall acquire any rights
         in respect of any of the other party's intellectual property in
         relation to the Products or the manufacture thereof or of the goodwill
         associated therewith.

9        PRICES

9.1      The Agreement shall state a price to be charged for the Product, which
         price shall be effective for a stated period (for example twelve (12)
         months) subject only to variations due to direct increases or savings
         (for example raw material costs, economies of scale etc.) during such
         period.

9.2      A mechanism for the review of price of Product shall be agreed which
         shall regulate annual price increases/decreases following the initial
         period.

         In addition, ImmunoGen may review the price to take account of any
         increased cost of production and raw material costs provided that such
         review [*]

9.3      Invoices will be submitted to BB when the Product has been delivered.
         BB will pay such within [*] after the date of receipt of the invoice
         unless there is any dispute relating to the conformity of the Product
         with the Specifications, in which case payment will be made within [*]
         of receipt by British Biotech of conforming Product or agreement that
         the original Product was conforming.

9.4      If, as a result of any inspection of the manufacturing premises used by
         ImmunoGen, a requirement is imposed by a competent authority which
         leads to or requires a change in the cost of production or to the
         Specification of the Product then, upon written notification by
         ImmunoGen to BB, ImmunoGen and BB will meet to discuss any increase to
         the price of the Product, the date upon which such price increase will
         take effect and any other consequences arising from such changes.
         Failure to reach agreement within an agreed time, shall mean the matter
         being subject to the agreed dispute resolution provisions.

9.5      ImmunoGen shall agree that [*]

10       INDEMNITY

10.1     Each party (the "indemnifying party") shall indemnify other party (the
         "indemnified party") against legal liability to third parties in
         respect of all claims, actions, judgements, damages, lawsuits, costs or
         expenses or professional fees incurred or arising out of (a) any breach
         of contract by or any act or omission of the indemnifying party, its
         employees, sub-contractors or agents; (b) any claim for death or
         personal injury incurred by the indemnified party in relation to or
         arising out of any breach of contract by the indemnifying party or any
         negligent act or omission of the indemnifying party, its employees,
         sub-contractors or agents.

10.2     A notification procedure shall be agreed to regulate how an indemnified
         party may claim the benefit of any indemnity, to include notification
         of any claims, and agreement not to compromise the conduct of any such
         claims or take any material steps in relation to such claims without
         the prior consent of the indemnifying party and co-operation with the
         indemnifying party in the handling of any such claims.

11       CONTRACT MANUFACTURE AND ALTERNATE SUPPLY

                                       63
<PAGE>   64
11.1     ImmunoGen shall be entitled to appoint sub-contractors to perform its
         obligations under the Supply provided that it complies with the
         provisions of Clause 25 of the License Agreement and provided that
         British Biotech is entitled to audit such proposed sub-contractor prior
         to appointment and, to the extent that any such proposed third party
         fails such audit, to require all corrective steps to be taken by such
         third party, to British Biotech's reasonable satisfaction, prior to
         appointment.

11.2     To the extent that sub-contractors are appointed by ImmunoGen, it shall
         ensure that an appropriate technical agreement is put in place

11.3     ImmunoGen shall use commercially reasonable efforts to cause at least
         two sources of supply to become and remain pre-qualified during the
         continuance of the Agreement.

12       INABILITY TO SUPPLY

12.1     The Agreement shall incorporate provisions equivalent to those set out
         in Clause 27 of the License Agreement, enabling British Biotech to
         assume responsibility for manufacture of Product in the event that
         ImmunoGen is unable for whatever reason to deliver British Biotech's
         Product requirements. In such event, ImmunoGen shall co-operate with
         British Biotech, in terms of transfer of information and provision of
         assistance, to enable British Biotech to assume manufacturing
         responsibility.

13       STOCK REPORTING

13.1     The parties shall agree upon the timetable for ImmunoGen to report to
         British Biotech on stock held and work-in-progress, the form of which
         report shall be agreed upon by the parties.

                                       64
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                       CONFIDENTIAL TREATMENT REQUESTED.
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                                   SCHEDULE 5

                          LIST OF THIRD PARTY LICENSES

[*]

                                       65
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                                 SCHEDULE 6

                     REGISTRATION IN THE REST OF THE WORLD,
                       BASED ON AN EU/USA DEVELOPMENT PLAN

                                       [*]

                                       66
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                       CONFIDENTIAL TREATMENT REQUESTED.
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                                   SCHEDULE 7

                    CMC DEVELOPMENT ACTIVITIES FOR huN901-DM1

1. MANUFACTURING PROCESS

Develop the manufacturing process to:
[*]

2. PRODUCT DEVELOPMENT

[*]

                                       67
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

3. ANALYTICAL METHODS

[*]

4. STABILITY STUDIES

[*]

                                       68
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                       CONFIDENTIAL TREATMENT REQUESTED.
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

5. COMPATIBILITY TESTING

[*]

6. CLINICAL TRIAL SUPPLIES (CTS)

[*]

7. DOCUMENTATION

[*]

                                       69
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                                   SCHEDULE 8

                                COGS CALCULATION

[*]

                                       70
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                  WITH THE SECURITIES AND EXCHANGE COMMISSION

                                   SCHEDULE 9

                    SCHEDULE OF COSTS FOR BIOANALYTICAL WORK
                 TO BE CONDUCTED BY IMMUNOGEN UNDER CLAUSE 14.2

[*]

                                       71
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                                   SCHEDULE 10

                          FURTHER THIRD PARTY LICENSES

                                       72
<PAGE>   73
IN WITNESS whereof each of the parties intend this Agreement to be a deed and
have caused it to be executed as such by executed by their duly authorized
representatives the day and year first before written.

------------------------------------------
Signed and delivered as a deed on behalf of
BRITISH BIOTECH PHARMACEUTICALS LIMITED
by:

Name:

Title:

------------------------------------------
Signed and delivered as a deed on behalf of
IMMUNOGEN, INC
by:

Name:

Title:

                                       73

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