<PAGE>

                  COLLABORATIVE RESEARCH AND LICENSE AGREEMENT

                                 by and between

                                 IMMUNOGEN, INC.

                                       and

                           RAVEN BIOTECHNOLOGIES, INC.

                                 March 28, 2001

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.

<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<CAPTION>

                                                                            PAGE

<S>                                                                           <C>
ARTICLE 1       DEFINITIONS....................................................1

      1.1    "Affiliate".......................................................1

      1.2    "Agreement".......................................................2

      1.3    "Antigen".........................................................2

      1.4    "Candidate Designation Period"....................................2

      1.5    "Candidate Mab"...................................................2

      1.6    "Candidate Target"................................................2

      1.7    "Combination Product".............................................2

      1.8    "Commercial Field"................................................2

      1.9    "Commercially Reasonable Efforts".................................2

      1.10   "Confidential Information"........................................2

      1.11   "Control", "Controls" and "Controlled"............................3

      1.12   [     *             ].............................................3

      1.14   "Derivative"......................................................3

      1.15   "Effective Date"..................................................4

      1.16   "Europe"..........................................................4

      1.17   "FDA".............................................................4

      1.18   "First Commercial Sale"...........................................4

      1.19   "[                                 ]".............................4

      1.20   "ImmunoGen".......................................................4

      1.21   "ImmunoGen Background Technology".................................4

      1.22   "ImmunoGen Mab"...................................................4

      1.24   "ImmunoGen Program Technology"....................................4

      1.25   "ImmunoGen Technology"............................................4

      1.26   "IND".............................................................4

      1.27   "Initial Research Term"...........................................5

      1.28   "Joint Know-How"..................................................5

      1.29   "Joint Patents"...................................................5

      1.30   "Know-How"........................................................5

      1.31   "Licensed Know-How"...............................................5

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       1.
<PAGE>

      1.32   "Licensed Mab"....................................................5

      1.33   "Licensed Patents"................................................5

      1.34   "Licensed Product"................................................5

      1.35   "Licensed Target".................................................5

      1.36   "Mab".............................................................5

      1.37   "Mab Criteria"....................................................5

      1.38   "Major Indication"................................................5

      1.39   "Major Market"....................................................5

      1.40   "Marketing Approval"..............................................6

      1.41   "Molecular Target"................................................6

      1.42   "Naked Mab Licensed Product"......................................6

      1.43   "[     *             ]"...........................................6

      1.44   "Non-Program Molecule"............................................6

      1.45   "Party" and "Parties".............................................6

      1.46   "Patents".........................................................6

      1.47   "Phase II Clinical Trials"........................................7

      1.48   "Phase III Clinical Trials".......................................7

      1.49   "Program Mab".....................................................7

      1.50   "Program Target"..................................................7

      1.51   "Raven"...........................................................7

      1.52   "Raven Technology"................................................7

      1.53   "Regulatory Authority"............................................7

      1.54   "Research"........................................................7

      1.55   "Research Plan"...................................................7

      1.56   "Research Term"...................................................7

      1.57   [     *             ].............................................7

      1.58   [     *             ].............................................7

      1.59   [     *             ].............................................8

      1.60   [     *             ].............................................8

      1.61   [     *             ].............................................8

      1.62   "RMC".............................................................8

      1.63   "Royalty Term"....................................................8

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       2.
<PAGE>

      1.64   "Selected Mab"....................................................8

      1.65   "Selected Target".................................................8

      1.66   "Selection Period"................................................8

      1.67   "Subsequent Research Period"......................................8

      1.68   "Term"............................................................8

      1.69   "Territory".......................................................8

      1.70   "Third Party".....................................................8

      1.71   "Valid Claim".....................................................8

ARTICLE 2       RESEARCH COLLABORATION MANAGEMENT..............................9

      2.1    Formation.........................................................9

      2.2    Functions and Powers of the RMC...................................9

      2.3    RMC Governance....................................................9

      2.4    RMC Access.......................................................10

ARTICLE 3       CONDUCT OF RESEARCH...........................................11

      3.1    General..........................................................11

      3.2    Screening and Evaluation of Mabs.................................11

      3.3    Candidate Mabs...................................................11

      3.4    Selected Mabs....................................................12

      3.5    ImmunoGen Mabs...................................................12

      3.6    Combination Products.............................................12

      3.7    Subsequent Research Period.......................................12

      3.8    Reporting........................................................13

      3.9    Other Research Programs..........................................13

ARTICLE 4       OWNERSHIP OF INTELLECTUAL PROPERTY AND LICENSE GRANTS.........13

      4.1    Ownership of Intellectual Property...............................13

      4.2    Screening Licenses...............................................13

      4.3    Development and Commercialization License........................14

      4.4    Sublicensing.....................................................14

      4.5    Right of Negotiation for Non-Program Molecules...................14

      4.6    [     *             ]............................................15

      4.7    Third Party Licenses.............................................15

ARTICLE 5       FINANCIAL TERMS...............................................16

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       3.
<PAGE>

      5.1    Technology Access Fees...........................................16

      5.2    Research Funding.................................................16

      5.3    Raven Research Performance Milestones............................16

      5.4    Milestone Payments...............................................16

      5.5    Royalty on Licensed Products.....................................17

      5.6    Royalty [   *    ] in Case of [    *    ] Third Parties..........18

      5.7    Payment of Royalties.............................................18

      5.8    Currency Conversion..............................................18

      5.9    Records and Audit................................................18

      5.10   Late Payments....................................................19

      5.11   Sublicensee Record Keeping Obligations...........................19

      5.12   Payment Currency and Method......................................19

ARTICLE 6       Development and Commercialization.............................19

      6.1    Development......................................................19

      6.2    Commercialization................................................19

      6.3    Subcontracting...................................................20

      6.4    General Development and Commercialization Diligence..............20

      6.5    Communications with Raven........................................21

      6.6    Compliance with Laws.............................................22

ARTICLE 7       PATENT PROSECUTION AND EXPENSES...............................22

      7.1    Prosecution of Patents...........................................22

      7.2    Infringement of Licensed Patents by Third Parties................23

      7.3    Infringement of Third Party's Rights.............................24

ARTICLE 8       REPRESENTATIONS AND WARRANTIES................................25

      8.1    Mutual Representations and Warranties............................25

      8.2    No Other Representations.........................................25

ARTICLE 9       INDEMNIFICATION...............................................25

      9.1    Indemnification by Raven.........................................26

      9.2    Indemnification by ImmunoGen.....................................26

      9.3    Mechanics........................................................26

      9.4    Insurance Coverage...............................................26

      9.5    Limitation of Liability..........................................27

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       4.
<PAGE>

ARTICLE 10      CONFIDENTIALITY...............................................27

      10.1   Confidential Information.........................................27

      10.2   Authorized Disclosure............................................27

      10.3   Publication......................................................28

      10.4   Terms of Agreement...............................................28

      10.5   Return of Confidential Information...............................28

      10.6   Publicity........................................................28

      10.7   Disclosure of Terms of Agreement.................................28

      10.8   Use of Names, Logos or Symbols...................................29

ARTICLE 11      TERM AND TERMINATION..........................................29

      11.1   Term.............................................................29

      11.2   Termination for Breach...........................................29

      11.3   Licenses upon Expiration.........................................29

      11.4   Product Rights upon Termination..................................29

      11.5   Accrued Rights and Obligations; Survival.........................30

ARTICLE 12      MISCELLANEOUS.................................................31

      12.1   Dispute Resolution...............................................31

      12.2   Governing Law....................................................31

      12.3   No Agency........................................................31

      12.4   Assignment.......................................................31

      12.5   Amendment........................................................32

      12.6   Covenant of Further Assurances...................................32

      12.7   Notices..........................................................32

      12.8   Rights in Bankruptcy.............................................33

      12.9   Force Majeure....................................................33

      12.10  Counterparts.....................................................33

      12.11  Captions.........................................................34

      12.12  Severability.....................................................34

      12.13  Cumulative Rights................................................34

      12.14  Waiver...........................................................34

      12.15  Entire Agreement.................................................34
</TABLE>


Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       5.
<PAGE>

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       6.
<PAGE>

      THIS COLLABORATIVE RESEARCH AND LICENSE AGREEMENT (the "Agreement") is
entered into and made effective as of March 28, 2001 (the "Effective Date") by
and between IMMUNOGEN, INC., a Massachusetts corporation having an address of
128 Sidney Street, Cambridge, MA 02139 ("ImmunoGen") and RAVEN BIOTECHNOLOGIES,
INC., a Delaware corporation having an address at 305 Old County Road, San
Carlos, CA 94070 ("Raven"). ImmunoGen and Raven are referred to herein each
individually as a "Party" and collectively as the "Parties."

                                    RECITALS

      WHEREAS, Raven has developed and has expertise in applying proprietary
immunization and other methods for generating Mabs against cell surface
proteins, and possesses certain proprietary cell lines or will acquire cell
lines which have cell surface proteins with relevance to ovarian cancer;

      WHEREAS, ImmunoGen is in the business of developing anti-cancer products
based upon monoclonal antibodies that bind selectively to tumor cells, many of
which products couple one of ImmunoGen's proprietary small molecule cytotoxic
compounds to such antibodies, and wishes to develop monoclonal antibody-based
products for the treatment of ovarian cancer and possibly other cancers;

      WHEREAS, the Parties wish to embark upon a collaborative research program
wherein Raven would apply its proprietary technologies to identify proteins
expressed on the surface of ovarian cancer cells, identify monoclonal antibodies
targeted against those proteins and perform certain screening and evaluation of
such antibodies, and ImmunoGen would perform various in vitro and in vivo
evaluations of selected antibodies as further specified herein; and

      WHEREAS, Raven wishes to grant ImmunoGen, and ImmunoGen wishes to accept,
a license to develop and commercialize certain of the antibodies produced and
evaluated by the Parties pursuant to such collaborative relationship and/or
other antibodies generated against the target proteins identified within the
collaboration and selected by ImmunoGen for development, and the Parties desire
for Raven to have all rights to those targets and antibodies coming out of the
collaborative research that ImmunoGen does not elect to further develop, all on
the terms set forth and as more specifically described herein.

       NOW, THEREFORE, in consideration of the foregoing premises and the
covenants and obligations set forth in this Agreement, the Parties hereby agree
as follows:

                                    AGREEMENT

                                    ARTICLE 1

                                   DEFINITIONS

      As used herein, the following terms shall have the following meanings:

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.

<PAGE>

      1.1 "Affiliate" shall mean an entity that, directly or indirectly, through
one or more intermediaries, controls, is controlled by or is under common
control with Raven or ImmunoGen. A legal entity will be regarded as in "control"
of another legal entity if it directly or indirectly owns at least fifty percent
(50%) (or such lesser percentage which is the maximum allowed to be owned by a
foreign entity in a particular jurisdiction) of the voting stock or other
ownership interest of the owned entity, or if it directly or indirectly
possesses the ability to direct or cause the direction of the management and
policies of the owned entity by lawful means.

      1.2 "Agreement" shall have the meaning as defined in the first paragraph
above.

      1.3 "Antigen" shall mean any protein, peptide or carbohydrate or other
composition, and/or any fragment, peptide and/or epitope thereof which causes an
immune response that produces antibodies.

      1.4 "Candidate Designation Period" shall have the meaning as defined in
Section 3.3.

      1.5 "Candidate Mab" shall mean a Program Mab designated by ImmunoGen
pursuant to Section 3.3, and any Derivative thereof.

      1.6 "Candidate Target" shall mean any Molecular Target to which a
Candidate Mab binds.

      1.7 "Combination Product" shall mean any product that contains both a
Licensed Product and one or more compounds having independent therapeutic
activity in the Commercial Field.

      1.8 "Commercial Field" shall mean therapeutic products employing Mabs,
alone or in conjunction with other ingredients, to treat cancer in humans.

      1.9 "Commercially Reasonable Efforts" shall mean those efforts that are
required to perform an obligation to develop or commercialize a Licensed Product
in a sustained manner consistent with the those efforts that [ * ] as ImmunoGen,
but in any event [ * ] which are [ * ] in the market for [ * ] would typically
devote to products of similar nature, market potential, profit potential or
strategic value, in view of conditions prevailing at the time, and evaluated
taking into account all relevant factors, including without limitation the
competitiveness of the marketplace, the proprietary position of the product, the
relative potential safety and efficacy of the product, the regulatory
requirements involved in its development and commercialization, and the seeking
of Marketing Approval therefor, and other technical, legal scientific or medical
factors. With respect to a particular Licensed Product, such efforts include,
without limitation, the assignment of appropriate personnel to develop and
commercialize such product, the establishment of specific and meaningful goals
regarding the development and commercialization of such product and the
allocation of sufficient resources to achieve such goals.

      1.10 "Confidential Information" shall mean, subject to the limitations set
forth below, all Know-How and other confidential or proprietary information
received by a Party (a

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       2
<PAGE>

"Receiving Party") from the other Party (a "Disclosing Party") pursuant to this
Agreement, including, without limitation, any patent application or drawing, any
trade secret, information, invention, idea, samples, assay components, process,
formula, or test data relating to any research project, work in progress or
development, engineering, manufacturing, regulatory, marketing, financing or
personnel information relating to the Disclosing Party, its present or future
products, business plans, sales, suppliers, clients, customers, employees,
investors or business, whether in oral, written, graphic or electronic form.

      Confidential Information shall not include any information which, as shown
by competent proof:

            (a) is now, or hereafter becomes, through no act or failure to act
on the part of the Receiving Party in breach hereof, generally known or
available;

            (b) is known by the Receiving Party at the time of receiving such
information, as shown by contemporaneous written records, other than by virtue
of a prior confidential disclosure by the Disclosing Party;

            (c) is hereafter furnished to the Receiving Party by a Third Party,
as a matter of right and without restriction on disclosure by the Disclosing
Party;

            (d) is independently developed by the Receiving Party without use of
or reference to Confidential Information of the Disclosing Party, as shown by
independent, contemporaneous written records; or

            (e) is the subject of a prior, express, written permission to
disclose provided by the Disclosing Party.

      1.11 "Control", "Controls" and "Controlled" shall mean, with respect to a
particular item of information or intellectual property right, that the
applicable Party owns or has a license to such item or right and has the ability
to grant to the other Party access to and a license or sublicense (as
applicable) under such item or rights as provided for herein without violating
the terms of any agreement or other arrangement with any Third Party.

      1.12 [ * ] shall mean any [ * ], and all [ * ] thereof, whether produced
by a [ * ] or [ * ], which is, in any case, [ * ].

      1.13 [ * ] shall mean any [ * ] the [ * ] within which is [ * ] with a [ *
] or with any other molecule or compound that increases the [ * ] of the [ * ]
compared to a product the [ * ] within which is solely the [ * ].

      1.14 "Derivative" shall mean, with respect to a Mab, (i) any molecule or
compound that is derived from, or that is designed based upon, the amino acid
sequence of or nucleotide sequence coding for such Mab, in each case including
without limitation via the humanization or

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       3
<PAGE>

deimmunization of such Mab; and (ii) any molecule or compound that is or
contains any fragment or portion of such Mab or any molecule or compound
described in (i) above.

      1.15 "Effective Date" shall have the meaning as defined in the first
paragraph above.

      1.16 "Europe" shall mean all countries on the European continent, and
includes for purposes of this definition the entire territory of the Russian
Federation and Turkey.

      1.17 "FDA" shall mean the United States Food and Drug Administration, or
any successor thereto.

      1.18 "First Commercial Sale" shall mean, with respect to any particular
country and any particular Licensed Product, the first sale of such Licensed
Product in such country by ImmunoGen, or any of its Affiliates or sublicensees,
after Marketing Approvals have been granted in such country for such Licensed
Product.

      1.19 "[ * ]" shall have the meaning as defined in [ * ].

      1.20 "ImmunoGen" shall have the meaning as defined in the first paragraph
above.

      1.21 "ImmunoGen Background Technology" shall mean any Know-How that is,
and all Patents claiming inventions that are (a) useful or necessary to generate
and to screen Mabs binding to a Molecular Target Controlled by ImmunoGen, (b)
Controlled by ImmunoGen as of the Effective Date, and (c) disclosed by ImmunoGen
to Raven during the Term.

      1.22 "ImmunoGen Mab" shall mean any Mab, other than a Selected Mab,
generated by or for ImmunoGen against a Selected Target or which binds to a
Selected Target, and any Derivative thereof.

      1.23 "ImmunoGen Non-Program Technology" shall mean any invention, Know-How
or Patent which is developed or conceived solely by employees of or consultants
to ImmunoGen in the conduct of the Research which is not ImmunoGen Program
Technology.

      1.24 "ImmunoGen Program Technology" shall mean any invention, Know-How or
Patent developed or conceived solely by employees of or consultants to ImmunoGen
in the conduct of the Research and/or the exercise by ImmunoGen of any of the
licenses granted to ImmunoGen under Sections 4.2 or 4.3(a)(i) which is necessary
to practice the Raven Technology.

      1.25 "ImmunoGen Technology" shall mean, collectively, ImmunoGen Background
Technology, ImmunoGen Non-Program Technology and ImmunoGen Program Technology.

      1.26 "IND" shall mean an investigational new drug application filed with
the FDA (pursuant to 21 CFR ss.312.3) prior to beginning clinical trials in
humans or any comparable application filed with a Regulatory Authority of a
country other than the United States prior to beginning clinical trials in
humans in that country. If no application is required to be filed prior

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       4
<PAGE>

to commencing human clinical trial in a particular regulatory jurisdiction, an
IND is deemed to have been filed upon the administration of the first dose of a
Licensed Product to the first human.

      1.27 "Initial Research Term" shall mean the time period beginning on the
Effective Date and ending on the [ * ] anniversary of the Effective Date.

      1.28 "Joint Know-How" shall mean any Know-How (a) jointly invented by both
ImmunoGen and Raven in the conduct of the Research, (b) invented by Raven in the
conduct of the Research as a direct result of its use of ImmunoGen Technology or
(c) invented by ImmunoGen in the conduct of the Research as a direct result of
its use of Raven Technology.

      1.29 "Joint Patents" shall mean Patents claiming inventions that are Joint
Know-How.

      1.30 "Know-How" shall mean all proprietary inventions; discoveries;
technology; trade secrets; data; methods; techniques; clinical and preclinical
results; physical, chemical or biological material; cell lines; know-how or
other information.

      1.31 "Licensed Know-How" shall mean all Know-How Controlled by Raven
during the Term and disclosed to ImmunoGen, including, without limitation,
Raven's interest in any Joint Know-How, that is, in each case, useful or
necessary to develop and commercialize Licensed Products.

      1.32 "Licensed Mab" shall mean a Selected Mab or an ImmunoGen Mab and all
Derivatives of such Selected Mab or ImmunoGen Mab.

      1.33 "Licensed Patents" shall mean (a) all Patents Controlled by Raven
during the Term and (b) Raven's interest in any Joint Patents, to the extent
that they contain a Valid Claim which would, but for the license granted
hereunder, be infringed in any country in the Territory by the manufacture, or
by the use or sale of a Licensed Product or a Licensed Target.

      1.34 "Licensed Product" shall mean any pharmaceutical product containing a
Licensed Mab.

      1.35 "Licensed Target" shall mean a Molecular Target to which a Licensed
Mab binds.

      1.36 "Mab" shall mean a monoclonal antibody with binding affinity for an
Antigen.

      1.37 "Mab Criteria" shall mean those criteria defined in Appendix 1 of the
Research Plan.

      1.38 "Major Indication" shall mean any one of [ * ].

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       5
<PAGE>

      1.39 "Major Market" shall mean the United States, Germany, the United
Kingdom, France, Italy or Spain or, if an application for Regulatory Approval is
filed through the centralized procedure, the European Union.

      1.40 "Marketing Approval" shall mean the act of a Regulatory Authority
necessary for the marketing and sale of Licensed Products in a regulatory
jurisdiction in the Territory in compliance with applicable laws.

      1.41 "Molecular Target" shall mean any Antigen to which a Mab binds.

      1.42 "Naked Mab Licensed Product" shall mean any Licensed Product that is
not a Cytotoxin-Coupled Licensed Product.

            1.43 "[ * ]" shall mean the [ * ] for the [ * ] of [ * ] in [ * ]
form made by [ * ], as applicable, [ * ] the following [ * ]:

            (a) [ * ] or [ * ] by reason of [ * ] or [ * ];

            (b) [ * ] upon or measured by the [ * ];

            (c) [ * ] of [ * ] for [ * ], and [ * ] and [ * ], including [ * ];
and

[ * ] granted to [ * ], including [ * ] to [ * ] or [ * ], if applicable;
provided, however that in the case of [ * ], similar programs or [ * ] on [ * ]
of [ * ], all [ * ] and the like shall be [ * ] to the [* ] of such [ * ] when [
* ].

[ * ] received by [ * ] for the [ * ] among [ * ] for [ * ] shall not be [ * ]
hereunder.

For purposes of [ * ] in accordance with the foregoing in this [ * ], use of [ *
] or other [ * ], or [ * ] of [ * ] as part of a [ * ] at [ * ] in the industry
shall [ * ].

      1.44 "Non-Program Molecule" shall have the meaning as defined in Section
4.5.

      1.45 "Party" and "Parties" shall have the meaning as defined in the first
paragraph above.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       6
<PAGE>

      1.46 "Patents" shall mean (i) United States issued patents,
re-examinations, reissues, renewals, extensions, patent term restorations, (ii)
any foreign counterparts of each of the foregoing; and (iii) pending
applications for any of the foregoing.

      1.47 "Phase II Clinical Trials" shall mean, as to a particular Licensed
Product, those trials on sufficient numbers of patients that are designed to
establish the preliminary safety, dosage and biological activity of such
Licensed Product for its intended use, and to define warnings, precautions and
adverse reactions that are associated with such Licensed Product in the dosage
range to be prescribed, in any case to generate sufficient data to commence a
Phase III Clinical Trial of such Licensed Product.

      1.48 "Phase III Clinical Trials" shall mean, as to a particular Licensed
Product, a clinical trial (or set of clinical trials) usually initiated after
completion of a Phase II or IIa Clinical Trial and after such date as the FDA
(or equivalent Regulatory Agency) has indicated that the Party conducting the
trial may reasonably continue such trials on sufficient numbers of patients to
establish definitively that such Licensed Product is safe and efficacious for
its intended use, and to define warnings, precautions and adverse reactions that
are associated with such Licensed Product in the dosage range to be prescribed,
and to support Marketing Approval of such Licensed Product or label expansion of
such Licensed Product.

      1.49 "Program Mab" shall mean a Mab generated by Raven during the Research
Term that meets the Mab Criteria and any Derivative thereof.

      1.50 "Program Target" shall mean a Molecular Target to which a Program Mab
binds.

      1.51 "Raven" shall have the meaning as defined in the first paragraph
above.

      1.52 "Raven Technology" shall mean any Know-How that is and all Patents
claiming inventions that are (a) useful or necessary to generate and to screen
Mabs binding to a Molecular Target, including without limitation methods of
immunization, methods for maintaining and handling cell lines expressing Mabs,
production and freezing media, and antibody screening facilitation tools
including without limitation CellArray(TM) (b) Controlled by Raven as of the
Effective Date and during the Term, and (c) disclosed by Raven to ImmunoGen
during the Term.

      1.53 "Regulatory Authority" shall mean a supranational, regional, federal,
state, provincial or other local regulatory agency, department, bureau or other
governmental authority with jurisdiction over Marketing Approvals.

      1.54 "Research" shall have the meaning as defined in Section 3.1.

      1.55 "Research Plan" shall mean the research plan attached hereto as
Exhibit A, as it may be updated from time to time by the Parties by mutual
written agreement.

      1.56 "Research Term" shall mean the Initial Research Term together with
any Subsequent Research Period to which the Parties may agree pursuant to the
mechanism of Section 3.7.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       7
<PAGE>

      1.57 [ * ] shall have the meaning as defined in Section 6.4(a)(ii).

      1.58 [ * ] shall have the meaning as defined in Section 11.4.

      1.59 [ * ] shall have the meaning as defined in Section 11.4(d).

      1.60 [ * ] shall have the meaning as defined in Section 11.4(b).

      1.61 [ * ] shall have the meaning as defined in Section 6.4(a)(ii)(A).

      1.62 "RMC" shall have the meaning as defined in Section 2.1.

      1.63 "Royalty Term" shall mean, with respect to a particular Licensed
Product and country, the [ * ] of (i) the period of time commencing on the First
Commercial Sale of a Licensed Product or Reverted Product, as the case may be,
in a country and ending upon the date of expiration of the last to expire of the
Licensed Patents containing a Valid Claim which would, but for the license
granted hereunder, be infringed by the manufacture, or by the use or sale in
such country, of such Licensed Product or Reverted Product, as the case may be,
and (ii) [ * ] from First Commercial Sale in such country of such Licensed
Product or Reverted Product, as the case may be.

      1.64 "Selected Mab" shall mean a Candidate Mab designated by ImmunoGen
pursuant to Section 3.4, and any Derivative thereof.

      1.65 "Selected Target" shall mean a Molecular Target to which a Selected
Mab binds.

      1.66 "Selection Period" shall have the meaning as defined in Section 3.4.

      1.67 "Subsequent Research Period" shall mean a [ * ] period not part of
the Initial Research Term into which the Parties may agree to extend their
collaborative research pursuant to Section 3.7.

      1.68 "Term" shall mean the term of this Agreement as further described in
Section 11.1.

      1.69 "Territory" shall mean Canada, Mexico, the United States and all of
Europe.

      1.70 "Third Party" shall mean any entity or person other than Raven,
ImmunoGen, or an Affiliate of either of them.

      1.71 "Valid Claim" shall mean a claim (a) that has been allowed or is
contained in an issued patent, which claim has not lapsed, been canceled, or
become abandoned and which claim has not been declared invalid or unenforceable
by an unappealable court of competent jurisdiction, or (b) of a patent
application that has been pending for less than [ * ] from the date it was
filed.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       8
<PAGE>

                                   ARTICLE 2

                        RESEARCH COLLABORATION MANAGEMENT

      2.1 Formation. Within [ * ] after the Effective Date, the Parties will
establish a research management committee to oversee and coordinate the Parties'
research hereunder during the Research Term, composed of an equal number of
representatives of each Party and having the powers enumerated below in this
Article 2 (the "RMC").

      2.2 Functions and Powers of the RMC. During the Research Term, the RMC
will determine the overall strategy and direction of the Research, coordinate
and oversee the activities of the Parties in the performance of the Research.
The RMC will have no power to amend, modify or waive compliance with this
Agreement and will have only such powers as are specifically delegated to it
hereunder.

      2.3 RMC Governance.

            (a) Membership. The RMC shall consist of three (3) representatives
of each Party. ImmunoGen shall select one (1) person appointed by it to the RMC
to serve as chairperson to the RMC. Said representatives have initially been
designated by the Parties as follows:

                        For ImmunoGen: [ * ]
                                       [ * ]
                                       [ * ]

                        For Raven:     [ * ]
                                       [ * ]
                                       [ * ]

Raven shall select one (1) person appointed by it to the RMC to serve as
assistant chairperson to the RMC. Either Party may designate substitutes for its
RMC representatives to participate if one or more of such Party's designated
representatives is unable to be present at a meeting. A Party may replace its
representatives serving on the RMC from time to time by written notice to the
other Party specifying the prior representative(s) to be replaced and the
replacement(s) therefor. The RMC will have the power to form subcommittees or
working groups with appropriate representation from ImmunoGen and Raven, their
Affiliates, and Third Parties acceptable to both Parties. The RMC chairperson
shall be responsible for preparing and issuing minutes of each meeting within
thirty (30) days thereafter, which minutes shall not be effective unless and
until ratified by the RMC's assistant chairperson to the RMC.

            (b) Meetings. During the Research Term, the RMC shall hold meetings
at such times as it elects to do so, but in no event shall such meetings be held
less frequently than once every three (3) months during the Research Term. The
RMC shall meet alternately at ImmunoGen's facilities in Cambridge, Massachusetts
and Raven's facilities in San Carlos, California, or at such locations as the
Parties may otherwise agree. With the consent of the representatives of each
Party serving on the RMC, other representatives of each Party may attend

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       9
<PAGE>

meetings of the RMC as nonvoting participants. Meetings of the RMC may be held
by audio or video teleconference with the consent of each Party, provided that
at least two (2) meetings per year shall be held in person. Each Party shall be
responsible for all of its own expenses of participating in the RMC.

            (c) Decision-Making Process; Limited Authority.

                  (i) In performing its functions and exercising its powers
under this Agreement, all decisions by the RMC will be made, in person or by
proxy, with unanimous consent of all members of the RMC. In the absence of
consensus of RMC members with respect to any matter before the RMC, such matter
shall be deemed not to have been approved or resolved by the RMC.

                  (ii) The RMC members will attempt to resolve any disagreement
among them within the RMC in accordance with the goal of conducting the Research
as efficiently and effectively as possible. If the RMC cannot reach agreement as
to any matter that it is empowered to address, either Party may refer the
dispute to Raven's CEO and ImmunoGen's CEO for resolution in accordance with
Section 12.1.

            (d) Specific Responsibilities. In addition to its overall
responsibility for overseeing the Parties' activities under this Agreement,
during the Research Term, the RMC shall in particular:

                  (i) once notified by either Party pursuant to Section 3.7,
that such Party wishes to extend the Parties' collaborative research activities
into a Subsequent Research Period, discuss and agree upon a budget and research
plan for such potential Subsequent Research Period to present to the Parties for
their consideration;

                  (ii) discuss any Third Party intellectual property issues that
may arise relating to the Research and/or any Licensed Product;

                  (iii) review and monitor the progress of the Research;

                  (iv) discuss and approve any updates and amendments to the
Research Plan proposed by either of the Parties; and

                  (v) coordinate and monitor publication of research results
obtained from and the exchange of information and materials that relate to the
Research Program during the Research Term and for [ * ] thereafter.

            (e) Meeting Agendas. Each Party will disclose to the other Party its
final agenda items along with appropriate related information at least five (5)
business days in advance of each meeting of the RMC.

      2.4 RMC Access. During the Research Term, Raven and ImmunoGen will provide
the RMC and its authorized representatives with reasonable access during regular
business hours to all records and documents of the respective Parties specific
to the Research that the RMC may

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       10
<PAGE>

reasonably require in order to perform its obligations hereunder, subject to any
bona fide obligations of confidentiality to a Third Party.

                                   ARTICLE 3

                               CONDUCT OF RESEARCH

      3.1 General. During the Initial Research Term, the Parties will conduct a
program of research in accordance with the Research Plan (such research, the
"Research"). As more specifically described below in this Article 3, each Party
will carry out its responsibilities as outlined in the Research Plan and as
directed by the RMC diligently and in good scientific manner in accordance with
accepted laboratory practices and in compliance with any and all laws and
regulations applicable to the jurisdiction in which those activities take place.

      3.2 Screening and Evaluation of Mabs.

            (a) Raven shall generate and evaluate Mabs and Molecular Targets
according to the Research Plan with the goal of identifying Program Mabs and
Program Targets. Raven shall report promptly the results of the evaluations
pursuant to this Section 3.2(a) to the RMC on an on-going basis, which results
shall include the data and information demonstrating that a Program Mab meets
the Mab Criteria. In addition, at the request of the RMC, Raven will provide the
RMC with such additional data as the RMC shall reasonably request. Raven may
delay reporting data and information in accordance with the foregoing in this
Section 3.2(a) for a reasonable period of time to enable it to file patent
applications claiming such Molecular Targets and Mabs prior to such reporting;
provided that it hereby agrees to file such patents as promptly as is reasonably
practicable.

            (b) All data and results generated by Raven pursuant to this Section
3.2 shall be deemed to be Know-How of Raven.

      3.3 Candidate Mabs.

            (a) Within [ * ] of the RMC receiving the results reported by Raven
pursuant to Section 3.2 with respect to a particular Program Mab or (as
applicable) its receipt of the requested additional data ("Candidate Designation
Period"), ImmunoGen may designate, by written notice to Raven, such Program Mab
to be a Candidate Mab for further evaluation by ImmunoGen. ImmunoGen may choose
up to [ * ] such [ * ] and corresponding [ * ] for further evaluation, except
that ImmunoGen may not choose as [ * ] or [ * ] or [ * ] as part of an [ * ]
which was [ * ] prior to the [ * ].

            (b) ImmunoGen shall perform evaluations of each Candidate Mab at
least to the extent provided in the Research Plan and promptly report the
results thereof to the RMC. In connection therewith, for each Candidate Mab,
Raven shall provide ImmunoGen with (i) quantities of such Candidate Mab
sufficient for ImmunoGen to carry out its evaluation of such Candidate Mab as
called for in the Research Plan, or, alternatively, (ii) the means for

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       11
<PAGE>

ImmunoGen to generate sufficient quantities of such Candidate Mab, for example,
a cell line expressing such Candidate Mab.

            (c) If ImmunoGen reasonably determines, based upon the data
resulting from screening and evaluation of the Program Mabs conducted by
ImmunoGen, not to designate as Candidate Mabs any of the Program Mabs resulting
from the [ * ] immunizations and fusions performed by Raven in the course of the
Research Program, then it may request in writing that Raven perform, and Raven
shall perform, up to [ * ] additional immunizations and fusions to produce
potential Program Mabs. Raven shall report the results of such additional work
as provided in Section 3.2. Thereafter, [ * ] under the [ * ] even if [ * ] do
not [ * ] or if [ * ] none of the [ * ] .

      3.4 Selected Mabs. At any time on or before the [ * ] of (i) [ * ] after
the expiration of the Research Term and (ii) [ * ] after reporting to the RMC
the results of secondary screening of the Mabs produced by the last immunization
and fusion in accordance with the Research Plan (the "Selection Period"),
ImmunoGen may designate, by written notice to Raven, up to [ * ] Candidate Mabs
to be Selected Mabs which ImmunoGen has the right to develop and commercialize
as provided in this Agreement. Notwithstanding the foregoing, in the event that
(i) Raven has not performed its supply obligations under Section 3.3(b)(i)
hereof with respect to any Candidate Mab, then the Selection Period applicable
to such Candidate Mab shall be extended for an additional period of time that
Raven or ImmunoGen requires to generate such quantities of such Candidate Mab
and (ii) if neither Raven nor ImmunoGen are able to generate the quantities of
Candidate Mab contemplated in Section 3.3(b), or neither Raven nor ImmunoGen
have identified the Antigens targeted by the Candidate Mab, then the RMC will
discuss the impact of the same on the Selection Period applicable to such
Candidate Mabs.

      3.5 ImmunoGen Mabs. ImmunoGen shall have the right to use the Selected
Targets to identify ImmunoGen Mabs. ImmunoGen shall notify Raven promptly upon
deciding to proceed with preclinical development of any ImmunoGen Mab.
Commencing on the date that ImmunoGen notifies Raven of the discovery of an
ImmunoGen Mab pursuant to this Section 3.5, ImmunoGen shall include descriptions
of its research and development activities with respect to such ImmunoGen Mab in
its regular development reports pursuant to Section 6.5.

      3.6 Combination Products. If ImmunoGen desires to develop a Combination
Product, it shall inform Raven thereof in writing. Upon receipt of such notice,
the Parties shall meet to discuss and agree on any adjustments to the
calculation of Net Sales or to the royalty rates as may be appropriate under the
circumstances.

      3.7 Subsequent Research Period. Either Party may propose to extend the
Research Term into a Subsequent Research Period by submitting to the RMC a
written proposal for additional collaborative research to be conducted by the
Parties during such Subsequent Research Period. To the extent desired by the
Parties, the Parties may discuss the proposal via their participation in the
RMC, and the RMC may prepare for the Parties' consideration a written

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       12
<PAGE>

proposal incorporating the Parties' comments in the RMC's discussions.
Notwithstanding the foregoing, neither Party shall be required to agree to
extend the Parties' collaborative research hereunder to cover additional subject
matter beyond that which is described in the Research Plan as attached hereto as
of the Effective Date or to extend the Research Term into a Subsequent Research
Period.

      3.8 Reporting. Subject to Section 3.2(a), during the Research Term
ImmunoGen and Raven will prepare and provide to each other quarterly reports
summarizing the status of all Mabs and Molecular Targets that are subject to the
Research.

      3.9 Other Research Programs. Beginning upon [ * ] and continuing through [
* ], Raven shall not [ * ] any [ * ] with [ * ] directed towards [ * ].

                                   ARTICLE 4

              OWNERSHIP OF INTELLECTUAL PROPERTY AND LICENSE GRANTS

      4.1 Ownership of Intellectual Property.

            (a) Subject to the rights and licenses granted to ImmunoGen the
other terms in this Agreement, Raven shall own all Raven Technology and all
Know-How and Patents generated or conceived solely by employees of or
consultants to Raven in the performance of this Agreement, except to the extent
that such Know-How and Patents fall within the definition of Joint Know-How and
Joint Patents.

            (b) Subject to the rights and licenses granted to Raven and the
other terms in this Agreement, ImmunoGen shall own all ImmunoGen Technology and
all Know-How and Patents generated or conceived solely by employees of or
consultants to ImmunoGen in the performance of this Agreement, except to the
extent that such Know-How and Patents fall within the definition of Joint
Know-How and Joint Patents.

            (c) Subject to the rights and licenses granted to Raven and
ImmunoGen and the other terms in this Agreement, any Joint Know-How and Joint
Patents shall be owned jointly, with each Party having an undivided 50%
ownership interest in such Joint Know-How and Joint Patents, with the right to
practice and to grant licenses under such Joint Know-How and Joint Patents
without obligation to account to the other Party.

      4.2 Screening Licenses.

            (a) Raven hereby grants to ImmunoGen a non-exclusive, royalty-free,
fully paid-up license under the Raven Technology and Raven's interest in any
Joint Know-How in the United States solely as necessary to carry out the
Research hereunder during the Research Term. Such license shall not be
sublicensable.

            (b) ImmunoGen hereby grants to Raven a non-exclusive, royalty-free,
fully paid-up license under the ImmunoGen Technology and ImmunoGen's interest in
any Joint

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       13
<PAGE>

Know-How in the United States solely as necessary to carry out the Research
hereunder during the Research Term. Such license shall not be sublicensable.

      4.3 Development and Commercialization License.

            (a) Subject to the terms of this Agreement, Raven hereby grants
ImmunoGen an exclusive, royalty-bearing license, with the right to sublicense
solely as provided in Section 4.4, under the Licensed Know-How and the Licensed
Patents to (i) generate and screen ImmunoGen Mabs; and (ii) create Derivatives
of Selected Mabs and ImmunoGen Mabs, and (iii) develop, make, have made, use,
sell, offer for sale, have sold, import and have imported Licensed Products for
use in the Commercial Field in the Territory during the Term.

            (b) Subject to the terms of this Agreement, Raven hereby grants
ImmunoGen an exclusive, royalty-bearing license, with the right to sublicense
solely as provided in Section 4.4, under the Licensed Know-How and the Licensed
Patents to use the Selected Targets to (i) generate and screen ImmunoGen Mabs,
and (ii) develop Licensed Mabs in the Commercial Field in the Territory during
the Term.

            (c) ImmunoGen shall not use for any purpose or perform any
activities with the Licensed Mabs or Selected Targets other than as expressly
licensed pursuant to this Section 4.3 above.

      4.4 Sublicensing. The licenses granted to ImmunoGen by Raven pursuant to
Section 4.3 shall be sublicensable by ImmunoGen; provided that any sublicense
shall conform to the terms of this Agreement; and provided further that the
granting of any such sublicense shall not relieve ImmunoGen of any of its
responsibilities hereunder.

      4.5 Right of Negotiation for Non-Program Molecules.

            (a) If Raven, during the Research Term, decides that [ * ] the [ * ]
that (i) satisfies [ * ] as its [ * ] and (ii) is discovered [ * ] but not in
the course of [ * ], it shall promptly notify ImmunoGen in writing.

            (b) If, within [ * ] days after its receipt of such notice,
ImmunoGen provides written notice (the "Negotiation Notice") to Raven of its
desire to commence negotiations with Raven regarding the terms upon which Raven
would grant ImmunoGen a license to develop and commercialize a particular
Non-Program Molecule, then the Parties shall negotiate such terms in good faith
for a period of [ * ] days following the date of the Negotiation Notice, as such
period may be extended by mutual written agreement of the Parties. If the
Parties are unable to agree to such terms within such [ * ] day period, as such
period may be extended by mutual written agreement of the Parties, then Raven
shall thereafter be free to itself commercialize, or to contract with a Third
Party to commercialize, such Non-Program Molecule without further obligation
relating thereto to ImmunoGen.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       14
<PAGE>

            (c) This Section 4.5 shall not apply to any Non-Program Molecules
discovered in the course of research undertaken by Raven with or for a Third
Party and with respect to which Raven has an obligation to such Third Party
which would conflict with negotiating or granting a license relating to such
Non-Program Molecule to ImmunoGen. Except as provided in Section 3.9, nothing in
this Agreement shall be construed to limit Raven's right to enter into
agreements with Third Parties for the discovery, development and
commercialization of Mabs meeting the Mab Criteria or Molecular Targets.

      4.6 [ * ]. In recognition of the fact that the rights that are granted to
ImmunoGen pursuant to the licenses of Section 4.2(a) and 4.3 are rights to use
technology that is central to Raven's intellectual property portfolio and
proprietary position, [ * ] hereby [ * ] a [ * ] under all [ * ] to [ * ] shall
have [ * ] and [ * ] of [ * ] under the [ * ] it pursuant to [ * ].

      4.7 Third Party Licenses.

            (a) ImmunoGen shall be responsible for obtaining a license under any
intellectual property it deems necessary or desirable for the humanization or
other alteration of any Licensed Mab, and shall solely bear the costs of any
such license.

            (b) Either Party may propose to the RMC that ImmunoGen seek a
license under Third Party intellectual property not described by Section 4.7(a),
and [ * ] will discuss whether [ * ] is [ * ] to avoid successful Third Party
claims of patent infringement or trade secret misappropriation in relation to
the manufacture, use, sale, offer for sale or importation of any Licensed
Product (a "Necessary" license). If [ * ] that a particular license is
Necessary, then ImmunoGen shall seek such license and be [ * ] thereunder [ * ]
Raven hereunder to the extent permitted pursuant to Section 5.6. If [ * ]
reaches [ * ] that [ * ] is not [ * ] shall remain [ * ] but [ * ] to [ * ] due
[ * ] against [ * ] hereunder. In all other cases, either Party may, upon notice
to the other Party, submit the matter of whether a license is Necessary to an
independent patent counsel reasonably acceptable to both Parties to render a
decision as to whether such license is Necessary. If such patent counsel
determines that the license is Necessary, ImmunoGen shall be [ * ] thereunder [
* ] Raven to the extent provided pursuant to Section 5.6; in all other
circumstances such payments shall [ * ] Raven hereunder.

            (c) ImmunoGen shall be free at any time to seek licenses under Third
Party intellectual property to cover its or its Affiliate's or sublicensee's
development, manufacture, use, sale, offer for sale or importation of Licensed
Products hereunder (whether or not described elsewhere in this Section 4.7), and
shall solely bear the costs of all such licenses except as provided in Section
4.7(b).

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       15
<PAGE>

                                   ARTICLE 5

                                 FINANCIAL TERMS

      5.1 Technology Access Fees.

            (a) ImmunoGen shall pay Raven [ * ] in cash within [ * ] after [ *
].

            (b) ImmunoGen shall pay Raven [ * ] in cash within [ * ] after the
beginning of any [ * ].

      5.2 Research Funding. In support of the Research to be conducted during
the Initial Research Term, ImmunoGen shall pay Raven a total of [ * ], the first
such payment being due within [ * ] after the Effective Date, and each other
such payment being due on or before [ * ] anniversary of [ * ] during the [ * ].

      5.3 Raven Research Performance Milestones. ImmunoGen shall pay Raven [ * ]
within [ * ] of [ * ] in accordance with [ * ]. For each [ * ] that ImmunoGen
designates pursuant to [ * ], ImmunoGen shall pay Raven [ * ] within [ * ] of
such designation.

      5.4 Milestone Payments.

            (a) [ * ]. With respect to [ * ] and [ * ] is developed to [ * ],
ImmunoGen shall pay Raven the milestone payments set forth below:

   Milestone Event     Milestone Payment Amount
   -----------------   ------------------------
   1. [ * ]                     [ * ]

   2  [ * ]                     [ * ]

   3. [ * ]                     [ * ]

   4. [ * ]                     [ * ]

   5. [ * ]                     [ * ]

            (b) [ * ]. With respect to each [ * ] and [ * ], ImmunoGen shall pay
Raven the milestone payments set forth below:

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       16
<PAGE>

   Milestone Event     Milestone Payment Amount
   ---------------     ------------------------
   1. [ * ]                     [ * ]

   2  [ * ]                     [ * ]

   3. [ * ]                     [ * ]

   4. [ * ]                     [ * ]

   5. [ * ]                     [ * ]

            (c) Notification of Milestones. ImmunoGen shall promptly notify
Raven in writing of its achievement of each event referenced under the heading
"Milestone Event" set forth in Section 5.4(a) and (b).

            (d) Payment of Milestones. ImmunoGen shall pay Raven the milestone
payments as set forth in this [ * ] within [ * ] after the achievement of the
corresponding milestone. Such milestone payments shall be nonrefundable and
noncreditable against royalties or other payments hereunder.

            (e) Major Indications. For the avoidance of doubt, it is hereby
acknowledged and agreed that a separate milestone payment will be made for each
of the foregoing milestone events ImmunoGen or its Affiliates or sublicensees
hereunder achieves with a given Licensed Product, the first time each such event
is achieved with respect to each Major Indication such Licensed Product is
developed to treat.

      5.5 Royalty on Licensed Products. In consideration of the grant of the
license by Raven hereunder, and subject to the other terms of this Agreement
(including the remainder of this Section 5), commencing on [ * ] of [ * ] during
the Royalty Term applicable thereto, ImmunoGen shall pay Raven the following
royalties based on [ * ] ImmunoGen, its Affiliates or sublicensees on an
incremental basis, according to the following rates:

              For [ * ]                              Royalty Rate
 in any Calendar Year During the Term                   [ * ]
 ------------------------------------                ------------
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]

          For [ * ] of [ * ]                         Royalty Rate
 in any Calendar Year During the Term                   [ * ]
 ------------------------------------                ------------
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]
                [ * ]                                   [ * ]

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       17
<PAGE>

      As an example, for the avoidance of doubt, in the event that ImmunoGen has
[ * ] equal [ * ] the [ * ] would be [ * ] calculated as follows: [ * ] , [ * ]
and [ * ] [ * ] .

      Upon expiration of the Royalty Term in a country with respect to a
Licensed Product, ImmunoGen shall thereafter have a paid-up, royalty-free,
non-exclusive license under any and all Know-How and Patents of Raven covering
such Licensed Patent for purposes of the manufacture, use, sale or import
thereof.

      5.6 Royalty [ * ] in Case of [ * ] Third Parties. ImmunoGen [ * ] owed to
Raven for sales of a particular Licensed Product, [ * ], on a country-by-country
basis, [ * ] by ImmunoGen [ * ] in respect of the Net Sales of such Licensed
Product under licenses entered into in accordance with [ * ]; provided however,
that in no event shall [ * ] paid to Raven [ * ] pursuant to this [ * ] by an
amount that is [ * ] of [ * ] of such Licensed Product if such [ * ] is a [ * ]
of [ * ] of such [ * ] if such [ * ] is a [ * ].

      5.7 Payment of Royalties. ImmunoGen shall pay [ * ] owed pursuant to [ * ]
within [ * ] after the end of each calendar quarter in which [ * ] are
generated. Such payments shall be accompanied by a statement showing the [ * ]
of each Licensed Product by ImmunoGen, its Affiliates or any of their
sublicensees in each country, the applicable royalty rate for such Licensed
Product, and a calculation of the amount of royalty due, including any offsets
and deductions.

      5.8 Currency Conversion. The [ * ] used for computing the royalties
payable to Raven by ImmunoGen shall be computed in U.S. dollars. For purposes of
determining the amount of royalties due, the amount of [ * ] in any foreign
currency shall be computed by converting such amount into U.S. dollars at the
prevailing commercial rate of exchange for purchasing dollars with such foreign
currency as published in the Wall Street Journal for the close of the last
business day of the calendar quarter for which the relevant royalty payment is
to be made by ImmunoGen.

      5.9 Records and Audit. ImmunoGen shall keep complete and accurate records
pertaining to the sale or other disposition of the Licensed Product and of the
royalty payments and other amounts payable under this Agreement [ * ] [ * ].
Raven shall have the right to [ * ], [ * ] to [ * ] to confirm [ * ] for the
preceding year. Such [ * ] may be exercised no more often than once a year,
within [ * ] after the calendar quarter to which [ * ], upon reasonable notice
to ImmunoGen and during normal business hours. Raven will [ * ] [ * ] unless [ *
] an [ * ] of [ * ] from the amount of [ * ]. In such case, ImmunoGen shall [ *
] of such [ * ]. The terms of this Section 5.10 shall survive any termination or
expiration or termination of this Agreement for a period of [ * ].

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       18
<PAGE>

      5.10 Late Payments. Any amounts not paid by ImmunoGen when due under this
Agreement shall be subject to interest from and including the date payment is
due through and including the date upon which Raven has collected immediately
available funds in an account designated by Raven. Such interest shall (a) with
respect to royalty payments, be at a rate equal to [ * ], and (b) with respect
to all other payments due hereunder, be at an annual rate equal to the sum of [
* ] plus the prime rate of interest quoted in the Money Rates section of The
Wall Street Journal (online edition found at http://www.interactive.wsj.com),
or, if lower, the highest rate permitted under applicable law.

      5.11 Sublicensee Record Keeping Obligations. Any sublicenses granting the
right to sell Licensed Product shall include obligations for the sublicensee to
account for and report its Net Sales of Licensed Products, and provide that
Raven shall have [ * ] corresponding to this Article 5 for any payment report
received by ImmunoGen from any ImmunoGen sublicensee on the same basis as if the
information contained in such report were Net Sales of Licensed Products by
ImmunoGen, and ImmunoGen shall [ * ] as if the [ * ] of the [ * ] were [ * ].

      5.12 Payment Currency and Method. All payments due to Raven under this
Agreement shall be made in U.S. dollars by wire transfer in immediately
available funds to a U.S. account designated by Raven, or by other mutually
agreed upon means.

                                   ARTICLE 6

                        DEVELOPMENT AND COMMERCIALIZATION

      6.1 Development. Beginning upon the designation pursuant to [ * ] of each
Candidate Mab and upon the initiation of work aimed at discovery of each
ImmunoGen Mab, ImmunoGen shall be responsible, at its sole expense, for all
further research and development of such Candidate Mabs or ImmunoGen Mabs and
Licensed Products based upon such Candidate Mabs or ImmunoGen Mabs, including,
without limitation, the conduct of any pre-clinical and clinical development of
such Licensed Products and making all regulatory filings necessary to obtain
Marketing Approvals of such Licensed Products in the Territory. ImmunoGen may
carry out such development in its sole discretion except as provided in [ * ]
and [ * ].

      6.2 Commercialization. ImmunoGen shall be responsible, at its sole
expense, for all commercialization (including marketing, promotion, sales and
distribution activities) of Licensed Products in the Territory beginning on the
Effective Date and may carry out such efforts in its sole discretion, except as
provided in [ * ] and [ * ].

      6.3 Subcontracting. Raven hereby acknowledges and agrees that ImmunoGen
shall have the right to subcontract with any Third Party to perform ImmunoGen's
obligations pursuant to Sections 6.1 and 6.2 hereof on behalf of ImmunoGen;
provided, however, that entering into any such subcontract, or the failure by
ImmunoGen's subcontractor to perform under such a subcontract, shall not relieve
ImmunoGen of its obligations hereunder.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       19
<PAGE>

      6.4 General Development and Commercialization Diligence.

            (a) Diligence in Selection of Mabs and Commencement of Development.

                  (i) ImmunoGen shall designate Program Mabs as Candidate Mabs
within the time period specified in [ * ], and designate Candidate Mabs as
Selected Mabs within the time period specified in [ * ]. Further, within [ * ]
after designating a Selected Mab, ImmunoGen shall (A) begin research and
development activities designed to humanize at least one Selected Mab for the
Selected Target associated with such Selected Mab or otherwise enable the filing
of an IND for a Licensed Product containing such a Selected Mab or (B) perform a
first immunization in a system designed to generate fully human ImmunoGen Mabs
that bind to the Selected Target for such Selected Mab.

                  (ii)

                        (A) Raven shall have the right, by written notice to
ImmunoGen, to [ * ] any [ * ] or [ * ] as a [ * ], and its [ * ] as a [ * ], if
ImmunoGen does not comply with the provisions of [ * ] with respect to such [ *
] or [ * ].

                        (B) ImmunoGen hereby grants Raven, under all Know-How
and Patents Controlled by ImmunoGen as of the date of such designation that (i)
were employed or developed by ImmunoGen in connection with the research,
development or commercialization of such [ * ] and such [ * ] and (ii) would,
but for the [ * ] in [ * ], be infringed or misappropriated by the research,
development, making, having made, use, sale, offer for sale and importation by
Raven (or its sublicensees) of such [ * ], such [ * ] or products directed
against such [ * ] in the Commercial Field, an [ * ] to [ * ] such [ * ] such [
* ] and products directed against such [ * ] in the Commercial Field. Further,
ImmunoGen shall [ * ] relating to such [ * ] and [ * ] which is reasonably
necessary to [ * ]. Any [ * ] shall thereafter [ * ] and shall be thereafter
deemed [ * ].

                        (C) Raven shall [ * ] on [ * ] of any product covered by
Patents or Know-How, or approved pursuant to a regulatory filing which [ * ].
Raven's obligation to [ * ] shall expire at such time as the [ * ] in the
development of such Patents, Know-How, data and results have been [ * ]. The [ *
] referenced in this Section 6.4(a)(ii)(C) shall include (a) the [ * ] and [ * ]
, [ * ]; (b) any [ * ] under the terms of [ * ] to the extent covering such [ *
]; and (c) the [ * ] of [ * ] and [ * ] [ * ]. The provisions of [ * ], and [ *
] through [ * ] shall apply [ * ] to Raven's [ * ] [ * ] under this [ * ].

            (b) Diligence in Development and Commercialization.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       20
<PAGE>

                  (i) ImmunoGen shall devote Commercially Reasonable Efforts (1)
to performing research and pre-clinical development of at least [ * ] Licensed
Mab for each Licensed Target, and clinical development of at least [ * ]
Licensed Product for each Licensed Target, (2) to obtaining Marketing Approvals
for [ * ] Licensed Products in [ * ], (3) to achieving a First Commercial Sale
on a timely basis thereafter in those of the countries of the Territory in which
Marketing Approval is obtained, and (4) to market, promote and sell each
Licensed Product during the Term.

                  (ii) Raven shall have the right, by written notice to
ImmunoGen, to terminate this Agreement with respect to any given Licensed Mab,
Licensed Product or Licensed Target in any or all countries of the Territory if
ImmunoGen does not comply with the provisions of subsection (i) above, subject
to ImmunoGen's ability to cure as provided in subsection (iii) below.

                  (iii) The notice of termination of subsection (ii) shall be
effective [ * ] after receipt thereof by ImmunoGen, unless (1) ImmunoGen has
cured its failure to comply with the diligence obligations and has provided
evidence of such cure which is acceptable to Raven within such [ * ] period, or
(2) if such failure is not curable within such [ * ] , ImmunoGen has provided a
written plan acceptable to Raven for curing such failure within a reasonable
period, and thereafter cures such failure in accordance with such plan.

            (c) Diligence of ImmunoGen Affiliates and Sublicensees The efforts
of ImmunoGen Affiliates and sublicensees shall be treated as efforts of
ImmunoGen for purposes of this Section 6.4.

      6.5 Communications with Raven.

            (a) General Reports and Meeting. Beginning with the [ * ]
anniversary of the expiration of the Research Term, on or before each [ * ]
anniversary of such date, ImmunoGen will submit to Raven written reports
summarizing the status and progress of the preclinical and clinical development,
marketing and commercialization efforts for Licensed Product in detail at a
minimum sufficient to allow Raven to monitor ImmunoGen's compliance with [ * ]
and the progress towards achieving milestone events. Such reports shall be
deemed to be ImmunoGen's Confidential Information. Upon request of Raven, the
Parties shall meet to discuss the reports provided by ImmunoGen under this
subsection (a) and ImmunoGen shall reasonably consider any concerns raised or
suggestions made by Raven during such discussions.

            (b) Regulatory Filings and Matters. As between Raven and ImmunoGen,
ImmunoGen shall own all applications for Marketing Approval of Licensed Products
and be responsible for all communications with all Regulatory Agencies in
connection with those filings that may be necessary to obtain Marketing
Approvals of Licensed Products within the Territory, subject to Section 11.4.
ImmunoGen will keep Raven informed of the status of such filings in each
country. ImmunoGen will promptly advise Raven each time that it obtains
Marketing Approval of a Licensed Product anywhere in the Territory. As between
Raven and ImmunoGen,

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       21
<PAGE>

ImmunoGen shall be responsible for the reporting of adverse events related to
the use of Licensed Products marketed by ImmunoGen, its Affiliates or
sublicensees in the Territory.

      6.6 Compliance with Laws. Each Party shall carry out its activities
pursuant to this Agreement in compliance with all applicable supranational,
national, state, provincial and other local laws, rules, regulations and
guidelines.

                                   ARTICLE 7

                         PATENT PROSECUTION AND EXPENSES

      7.1 Prosecution of Patents.

            (a) By Raven. Subject to Section 7.1(b), Raven shall, at its sole
expense, file, prosecute and maintain the Licensed Patents using commercially
reasonable efforts, except as otherwise provided in this Section 7.1. Raven
shall keep ImmunoGen reasonably informed of the status of each Licensed Patent
the claims of which are directed primarily to any Licensed Mab, Licensed Target
or the use of either of the foregoing in the Commercial Field in the Territory
(each, a "Product-Specific Licensed Patent"). Actions to keep ImmunoGen so
informed shall include without limitation notifying ImmunoGen reasonably in
advance of any action or filing with respect to, providing ImmunoGen copies of
all correspondence with governmental patent authorities regarding and providing
ImmunoGen copies of proposed filings with respect to each Product-Specific
Licensed Patent. Raven shall give reasonable consideration to any suggestions or
recommendations of ImmunoGen regarding the preparation, filing, prosecution and
maintenance of the Product-Specific Licensed Patents.

            (b) Raven Election not to Prosecute. In the event that Raven elects
not to file, prosecute or maintain any Product-Specific Licensed Patent or not
to file any patent term extensions thereto which may be appropriate to obtain an
extended period of market exclusivity for a Licensed Product, Raven shall give
ImmunoGen notice of such election promptly but in any event at least [ * ] days
before any filing or payment of fees is required for such prosecution,
maintenance or filing. If after receiving such notice, ImmunoGen gives notice to
Raven that ImmunoGen wishes to take responsibility for such any Product-Specific
Licensed Patents, Raven shall (i) promptly provide ImmunoGen with all pertinent
files, correspondence, records, information and other documents relating thereto
in its Control, and (ii) take all other actions reasonably necessary to transfer
to ImmunoGen the authority to prosecute, maintain and file for patent term
extension for such any Product-Specific Licensed Patents. In the event that
ImmunoGen wishes to have filed a patent application in respect of a
Product-Specific Licensed Patent in a particular country, it shall notify Raven.
If Raven elects not to file in such country within [ * ] of ImmunoGen's notice,
then the responsibility for, and the costs of, filing and prosecuting that
application in that country shall be borne entirely by ImmunoGen, who shall be
entitled to use for that purpose patent attorneys or agents of its choice.
ImmunoGen's filing, prosecution and maintenance of any Product-Specific Licensed
Patent pursuant to this Section 7.1(b) shall be at ImmunoGen's sole expense. If
ImmunoGen elects to discontinue the filing, prosecution or maintenance of any
Product-Specific Licensed Patent or not to file any patent term extensions
thereto which may be appropriate to obtain an extended period of market

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       22
<PAGE>

exclusivity for a Licensed Product, Raven shall have the right to assume control
of such activities and this subsection (e) shall apply mutatis mutandis to such
right of Raven.

            (c) Licensed Patent Status. Once prosecution of any Product-Specific
Licensed Patent has been transferred to ImmunoGen pursuant to Section 7.1(b),
such Patent shall continue to be deemed to be a Licensed Patent for purposes of
this Agreement.

      7.2 Infringement of Licensed Patents by Third Parties.

            (a) Notification. Each Party shall promptly notify the other Party
in writing of any alleged, threatened or actual infringement in the Commercial
Field of the Licensed Patents of which it becomes aware and provide any
information available to that Party relating to such infringement.

            (b) Enforcement of Licensed Patents.

                  (i) If any Licensed Patent is infringed by a Third Party,
Raven shall have the first right, but not the obligation, to initiate, prosecute
and control any action with respect to such infringement, by counsel of its own
choice, to secure the cessation of the infringement or to bring suit against the
infringer. ImmunoGen shall have the right, but not the obligation (subject to
Section 7.2(c)), to participate in any such action with respect to the Licensed
Patent and to be represented by counsel of its own choice at its own expense if
ImmunoGen provides Raven with written notice that ImmunoGen will join such suit
within [ * ] after Raven first brings suit and joins the suit reasonably
promptly thereafter, or to itself bring suit if Raven fails to bring suit within
[ * ] after either Party's notice pursuant to Section 7.2(a).

                  (ii) Notwithstanding the foregoing, if the infringement
primarily relates to a product which would compete directly with a Licensed
Product, ImmunoGen shall have the first right, but not the obligation, to
initiate, prosecute and control any action with respect to such infringement, by
counsel of its own choice, to secure the cessation of the infringement or to
bring suit against the infringer, and Raven shall have the right, but not the
obligation (subject to Section 7.2(c), to participate in any such action with
respect to the Licensed Patent and to be represented by counsel of its own
choice and at its own expense if Raven provides ImmunoGen with written notice
that Raven will join such suit within [ * ] days after ImmunoGen first brings
suit and joins the suit reasonably promptly thereafter, or to itself bring suit
if ImmunoGen fails to bring suit within [ * ] days after either Party's notice
pursuant to Section 7.2(a).

            (c) Cooperation. If a Party brings any such action or proceeding as
permitted under Section 7.2(b), the other Party agrees to be joined as a party
plaintiff and to give the first Party reasonable assistance and authority to
control, file and prosecute the suit as necessary.

            (d) Recoveries. In the event that either Party or the Parties shall
undertake an enforcement action pursuant to Section 7.2(b), any award or
compensation (including the fair market value of non-monetary compensation) paid
by Third Parties as a result of such an infringement action (whether by way of
settlement or otherwise) shall be applied as follows: (i)

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       23
<PAGE>

first, to reimbursement of each Party for their respective costs and expenses
(including reasonable attorneys' fees and costs) incurred by each in connection
with such action, on a pro rata basis, and (ii) second, [ * ] as [ * ] for [ * ]
associated with [ * ] and to [ * ] as [ * ] for [ * ] to the [ * ] the [ * ] is
attributable [ * ] associated with [ * ]; [ * ]; (iii) third, any amounts
remaining shall be allocated as follows: (a) if ImmunoGen is the Party
prosecuting such action, [ * ], (b) if Raven is the Party prosecuting such
action, [ * ] and (c) if both Parties are prosecuting such action, [ * ].

      7.3 Infringement of Third Party's Rights.

            (a) If the practice of the Licensed Patents through the manufacture,
use or sale of Licensed Products by ImmunoGen, its Affiliates or sublicensees
results in a claim of patent infringement against ImmunoGen, its Affiliates or
sublicensees, the Party first having notice of that claim shall promptly notify
the other Party in writing. The notice shall set forth the facts of the claim in
reasonable detail.

            (b) If a Third Party asserts that a Patent or other right owned by
or licensed to it is infringed by the practice of the Licensed Patents through
the manufacture, use or sale of Licensed Products by ImmunoGen, ImmunoGen's
Affiliates or sublicensees, ImmunoGen shall have the exclusive right to resolve
the problem raised by the asserted infringement. ImmunoGen shall have the sole
right, but not the obligation, to defend any such claim. Raven shall have the
right, but not the obligation, to participate in any such suit at its sole
option and at its own expense, and if it elects to so participate, it shall
reasonably cooperate with ImmunoGen in conducting the defense of the claim.
Additionally, Raven shall reasonably cooperate with ImmunoGen in any suit being
prosecuted by ImmunoGen hereunder and in which Raven is not otherwise
participating pursuant to the foregoing sentence at ImmunoGen's expense and to
the extent reasonably requested by ImmunoGen. If, as a consequence of such an
action, suit or proceeding by a Third Party, either Party is prohibited or is
only allowed in a restricted manner or subject to some conditions, financial or
other, to discover, develop, manufacture, use, sell, offer for sale and/or
import a Licensed Product, the Parties shall examine and discuss in good faith
(i) the consequences of such prohibition or restriction or other conditions on
activities governed by this Agreement and (ii) possible modifications this
Agreement.

                                   ARTICLE 8

                         REPRESENTATIONS AND WARRANTIES

      8.1 Mutual Representations and Warranties. Each Party hereby represents
and warrants to the other Party as follows:

            (a) Corporate Existence and Power. It is a corporation duly
organized, validly existing and in good standing under the laws of the state in
which it is incorporated, and has full corporate power and authority and the
legal right to own and operate its property and

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       24
<PAGE>

assets and to carry on its business as it is now being conducted and as
contemplated in this Agreement, including, without limitation, the right to
grant the licenses granted hereunder.

            (b) Authority and Binding Agreement. As of the Effective Date, (a)
it has the corporate power and authority and the legal right to enter into this
Agreement and perform its obligations hereunder; (b) it has taken all necessary
corporate action on its part required to authorize the execution and delivery of
the Agreement and the performance of its obligations hereunder; and (c) the
Agreement has been duly executed and delivered on behalf of such Party, and
constitutes a legal, valid and binding obligation of such Party and is
enforceable against it in accordance with its terms.

            (c) Title. As of the Effective Date, it has sufficient legal and/or
beneficial title under its intellectual property rights necessary to perform
activities contemplated under this Agreement and to grant the licenses contained
in this Agreement.

            (d) No Conflict. It has not entered, and will not enter, into any
agreement with any Third Party which is in conflict with the rights granted to
the other Party under this Agreement, and has not taken and will not take any
action that would in any way prevent it from granting the rights granted to the
other Party under this Agreement, or that would otherwise materially conflict
with or adversely affect the rights granted to the other Party under this
Agreement.

      8.2 No Other Representations. THE EXPRESS REPRESENTATIONS AND WARRANTIES
STATED IN THIS ARTICLE 8 ARE IN LIEU OF ALL OTHER REPRESENTATIONS AND
WARRANTIES, EXPRESS, IMPLIED, OR STATUTORY, INCLUDING WITHOUT LIMITATION,
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE,
NON-INFRINGEMENT OR NON-MISAPPROPRIATION OF THIRD PARTY INTELLECTUAL PROPERTY
RIGHTS, TITLE, CUSTOM OR TRADE.

                                   ARTICLE 9

                                 INDEMNIFICATION

      9.1 Indemnification by Raven. Raven will indemnify, hold harmless and
defend (collectively, "Indemnify") ImmunoGen, its Affiliates and their
respective employees and agents (each an "ImmunoGen Indemnitee") against any and
all losses, damages, liabilities, judgments, fines, amounts paid in settlement,
expenses and costs of defense (including without limitation reasonable
attorneys' fees and witness fees) ("Losses") resulting from any claim, action or
proceeding brought or initiated by a Third Party ("Third Party Claim") against
them to the extent that such Third Party Claim arises out of (i) the breach or
alleged breach of any representation or warranty by Raven in Article 8, or (ii)
the gross negligence or willful misconduct of Raven, its Affiliates, or their
respective employees or agents; provided that such indemnity shall not apply to
the extent ImmunoGen has an indemnification obligation pursuant to Section 9.2,
and provided further, that such indemnity shall not apply to the extent arising
from the gross negligence or willful misconduct of any ImmunoGen Indemnitee.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       25
<PAGE>

      9.2 Indemnification by ImmunoGen. ImmunoGen will Indemnify Raven, its
Affiliates, and their respective employees and agents (each, a "Raven
Indemnitee") against any and all Losses resulting from any Third Party Claim
against them to the extent that such Third Party Claim arises out of (i) the
development, manufacture, use, sale or other commercialization of Licensed
Products by ImmunoGen, its Affiliates, sublicensees or distributors; (ii) the
breach or alleged breach of any representation or warranty by ImmunoGen in
Article 8; or (iii) the gross negligence or willful misconduct of ImmunoGen, its
Affiliates, or their respective employees or agents; provided that such
indemnity shall not apply to the extent Raven has an indemnification obligation
pursuant to Section 9.1 for such Loss; and provided, further that such indemnity
shall not apply to the extent arising from the gross negligence or willful
misconduct of any Raven Indemnitee.

      9.3 Mechanics. A Party entitled to be Indemnified pursuant to this Article
9 (either for itself or its ImmunoGen Indemnitee, as the case may be) (the
"Indemnified Party") shall give prompt notice of the Third Party Claim to the
other Party (the "Indemnifying Party") and subject to Section 7.3, the
Indemnifying Party shall defend against such Third Party Claim, with the
reasonable cooperation of the Party; provided that the Indemnified Party will
not settle any such Third Party Claim for anything other than money damages
without the prior written consent of the Indemnified Party, which consent shall
not be unreasonably withheld, conditioned or delayed. The Indemnified Party
shall have the right to be present in person or through counsel at substantive
legal proceedings relating to the Third Party Claim giving rise to the
Indemnified Party's right to indemnification hereunder. In the event that the
Parties cannot agree as to the application of Sections 9.1 and 9.2 to any Loss
or Third Party Claim, the Parties may conduct separate defenses of such Third
Party Claim. In such case, each Party further reserves the right to claim
indemnity from the other in accordance with Sections 9.1 and 9.2 upon resolution
of such underlying Third Party Claim.

      9.4 Insurance Coverage. ImmunoGen represents and warrants that it
maintains and shall continue to maintain adequate comprehensive general
liability insurance which covers ImmunoGen's activities and obligations
hereunder (including product liability) in accordance with reasonable
pharmaceutical industry standards. ImmunoGen will maintain such insurance
program, or other program with comparable coverage, beyond the expiration or
termination of this Agreement during the period in which any Licensed Product is
being commercially distributed or sold, and for a commercially reasonable period
thereafter.

      9.5 Limitation of Liability. In no event shall either Party or its
respective Affiliates and permitted sublicensees be liable for special,
exemplary, consequential or punitive damages, whether in contract, warranty,
tort, strict liability or otherwise, except to the extent such Party may be
required to Indemnify the other Party under this Article 9 from such damages
claimed by Third Parties.

                                   ARTICLE 10

                                 CONFIDENTIALITY

      10.1 Confidential Information.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       26
<PAGE>

            (a) Each Party will maintain all Confidential Information of the
other Party received by it under this Agreement in trust and confidence and will
not disclose any such Confidential Information to any Third Party. Each Party
may use the Confidential Information of the other Party only to the extent
required to exercise the rights expressly granted and perform the obligations
imposed by this Agreement. Neither Party will use any Confidential Information
of the other Party for any purpose or in any manner that would constitute a
violation of any laws or regulations, including without limitation the export
control laws of the United States. Neither Party will reproduce any Confidential
Information of the other Party in any form except as required to accomplish the
intent of this Agreement.

            (b) Neither Party will disclose Confidential Information of the
other Party to any employee, agent, consultant, Affiliate, or sublicensee who
does not have a reasonable need for such information for purposes of performance
under this Agreement or who is not subject to binding obligations of
confidentiality and limited use at least as restrictive as those of this Article
10. Each Party will use at least the same standard of care as it uses to protect
its own Confidential Information of a similar nature to prevent unauthorized
disclosures or uses of Confidential Information of the requesting Party, but no
less than reasonable care. Each Party will promptly notify the other Party upon
discovery of any unauthorized use or disclosure of the Confidential Information
of the other Party.

      10.2 Authorized Disclosure. Notwithstanding any other provision of this
Agreement, each Party may disclose Confidential Information of the other Party:

            (a) to the extent and to the persons and entities required by an
applicable governmental law, rule, regulation or order; provided, however, that
the responding Party shall first have given prompt notice to the other Party
hereto to enable it to seek any available exemptions from or limitations on such
disclosure requirement and shall reasonably cooperate in such efforts by the
other Party, except as provided in Section 10.4;

            (b) to the extent and to the persons and entities required by rules
of the National Association of Securities Dealers; or

            (c) as necessary to file or prosecute patent applications, prosecute
or defend litigation or otherwise establish rights or enforce obligations under
this Agreement, but only to the extent that any such disclosure is necessary.

      10.3 Publication. Notwithstanding any matter set forth with particularity
in this Agreement to the contrary, results obtained in the course of the
Research Program may be submitted for publication following scientific review by
the RMC and subsequent approval by the management of both Parties, which
approval shall not be unreasonably withheld. After receipt of the proposed
publication by both Parties' management, written approval or disapproval shall
be provided within [ * ] for a manuscript, within [ * ] for an abstract for
presentation at, or inclusion in the proceedings of a scientific meeting, and
within [ * ] for a transcript of an oral presentation to be given at a
scientific meeting. Upon request of either Party, the other Party shall delete
from any proposed publication any Confidential

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       27
<PAGE>

Information of the other Party, and shall delay the publication by up to [ * ]
to enable the filing of patent applications on subject matter to be published in
the proposed publication.

      10.4 Terms of Agreement. The Parties agree that the material financial
terms of this Agreement will be deemed to be Confidential Information of both
Parties. Notwithstanding the foregoing, either Party may disclose such terms to
bona fide potential corporate partners, potential investors or merger or
acquisition partners, and to financial underwriters and legal and financial
advisors, provided that all such disclosures shall be made only to such parties
under an obligation of confidentiality.

      10.5 Return of Confidential Information. In the event that the Agreement
is terminated for breach according to the provisions of Section 11.2, both
Parties will use diligent efforts (including without limitation a diligent
search of files and computer storage devices) to return all Confidential
Information received by it from the other Party except to the extent such
Confidential Information is necessary to exercise any license or other right
surviving termination of this Agreement. Additionally, each Party will be
allowed to keep one archival copy of any Confidential Information of the other
Party's Confidential Information for record keeping purposes only.

      10.6 Publicity. The Parties shall mutually agree on a press release
announcing the execution of this Agreement. Prior to any public disclosure
regarding this Agreement and/or the terms hereof, including but not limited to
press releases, the releasing Party shall provide a copy of the proposed release
to the other Party for comment prior to release. Once any written statement is
approved for disclosure by both Parties, either Party may make subsequent public
disclosures of the contents of such statement without further approval of the
other Party.

      10.7 Disclosure of Terms of Agreement. Either Party may file this
Agreement as may be required pursuant to applicable securities laws or
regulations of the Securities and Exchange Commission ("SEC"), provided that the
Party filing this Agreement shall first confer with the non-filing Party
regarding for which of the provisions of this Agreement the filing Party will
seek confidential treatment from the SEC and shall reasonably consider the
non-filing Party's comments relating thereto.

      10.8 Use of Names, Logos or Symbols. Neither Party shall use the name,
trademarks, logos, physical likeness, employee names or owner symbol of the
other Party for any purpose without the prior written consent of the affected
Party. Such consent shall not be unreasonably withheld, conditioned or delayed
so long as such use of name is limited to objective statements of fact regarding
this Agreement, rather than for endorsement purposes; provided, however, that
nothing contained herein shall be construed to prevent either Party from using
the name of the other Party for purposes of preparing necessary filings for the
SEC or complying with SEC regulations, including preparing proxy statements or
prospectuses. Nothing contained herein shall be construed as granting either
Party any rights or license to use any of the other Party's trademarks or trade
names without separate, express written permission of the owner of such
trademark or trade name.

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       28
<PAGE>

                                   ARTICLE 11

                              TERM AND TERMINATION

      11.1 Term. The term of this Agreement shall commence upon the Effective
Date and, unless sooner terminated as provided in this Article 11, expire on the
expiration of the last-to-expire Royalty Term for any Licensed Product.

      11.2 Termination for Breach. Each Party shall have the right to terminate
this Agreement and its obligations hereunder for material breach by the other
Party, which breach remains uncured for [ * ] after written notice is provided
to the breaching Party, or in the case of an obligation to pay royalty payments
or other amounts owing under this Agreement, which breach remains uncured for [
* ] after written notice to the breaching Party.

      11.3 Licenses upon Expiration. In the event that the Agreement expires as
set forth in Section 11.1, the licenses granted under Section 4.3 shall
automatically become irrevocable, fully paid licenses to use, and/or sublicense
the use of the Licensed Know-How, to research, develop, make, have made, use,
sell, offer for sale, import and otherwise commercialize the Licensed Products
worldwide.

      11.4 Product Rights upon Termination.

            (a) If Raven terminates this Agreement pursuant to Section 11.2, or
if Raven terminates this Agreement with respect to a particular Licensed Product
in any country of the Territory as provided in Section 6.4(b) (a "Partial
Termination"), the licenses by Raven to ImmunoGen pursuant to Section 4.3 shall
terminate (in the case of a Partial Termination, solely with respect to such
Licensed Product in such country).

            (b) [ * ] of such [ * ] in and to the [ * ] (or in the case of [ *
]), including without limitation all [ * ] (or in the case of a [ * ])
(collectively, such rights the [ * ] and such [ * ] the [ * ] and such [ * ] of
finished [ * ] as Raven may, in its sole discretion, request.

            (c) Upon such termination, Raven shall [ * ] for [ * ] in the [ * ]
of any [ * ] by [ * ] a [ * ] of [ * ] covered by [ * ] until the [ * ] (as
defined below) applicable to such [ * ] have been [ * ] (or [ * ] , such [ * ]
and [ * ] applicable to the [ * ] for [ * ] . [ * ] shall [ * ] if [ * ] to [ *
], the [ * ] of [ * ] , including [ * ] any [ * ] obtained [ * ] and under [ * ]
pursuant to [ * ]; and [ * ] associated with [ * ] of such product; but shall be
[ * ]. As used herein, [ * ] with respect to [ * ] shall mean the [ * ] by [ * ]
and [ * ] to [ * ] in respect of [ * ].

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       29
<PAGE>

            (d) If Raven [ * ] of any [ * ] that would be [ * ] if sold by
ImmunoGen prior to termination, Raven [ * ] to ImmunoGen [ * ], in addition to
the royalty provided for in [ * ] of [ * ] at a [ * ] in the [ * ] of a [ * ],
in each [ * ].

            (e) The provisions of Sections 1.42 (Net Sales definition), 5.5
(solely with respect to the last paragraph) and 5.7 through 5.13 shall apply
mutatis mutandis to Raven's [ * ] obligation under this Section 11.4. Raven
shall be responsible for [ * ], including without [ * ] under any [ * ].

      11.5 Accrued Rights and Obligations; Survival.

            (a) Expiration or termination of this Agreement shall not affect any
accrued rights or obligations, including, without limitation, ImmunoGen's
responsibility to make any payment accruing prior to or in respect of Net Sales
made prior to the effective date of termination or expiration, and any
provisions applicable to the calculation or payment or royalties shall survive
until all royalties have been paid.

            (b) The provisions of Articles 1, 9, 10, 11 and 12 and Sections 4.1,
4.4, 4.6, 5.10, 5.11, 5.12, 7.2 (but only with respect to alleged infringement
prior to the effective date of termination), 7.3 (but only with respect to
alleged infringement prior to the effective date of termination), 7.4, of this
Agreement shall survive expiration or termination of this Agreement for any
reason (subject to any subsequent dates of termination referred to in such
individual Articles). Additionally, all Sections of this Agreement that by their
terms survive termination or expiration of this Agreement shall survive any such
termination or expiration.

                                   ARTICLE 12

                                  MISCELLANEOUS

      12.1 Dispute Resolution. The Parties recognize that disputes may from time
to time arise between the Parties during the term of this Agreement. It is the
objective of the Parties to establish procedures to facilitate the resolution of
disputes arising under this Agreement in an expedient manner by mutual
cooperation and without resort to litigation. To accomplish this objective, the
Parties agree to follow the procedures set forth in this Section 12.1 to resolve
any dispute arising under this Agreement. In the event of such a dispute between
the Parties, either Party, by written notice to the other Party, have such
dispute referred to the Parties' respective executive officers designated below
or their successors, for attempted resolution by good faith negotiations within
[ * ] after such notice is received. Said designated officers are as follows:

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       30
<PAGE>

               For Raven:     Chief Executive Officer
               For ImmunoGen: Chief Executive Officer

In the event the designated executive officers are not able to resolve such
dispute after such [ * ] period then the Parties shall resolve such dispute by
arbitration in accordance with the rules provided for in Sections 1282-1288 of
the California Code of Civil Procedure before a panel of three (3) arbitrators.
ImmunoGen and Raven shall each select one (1) arbitrator and the two (2)
arbitrators selected by the Parties shall select the third (3rd) arbitrator.
Unless otherwise agreed by ImmunoGen and Raven, the arbitration will be held in
Denver, Colorado.

      12.2 Governing Law. This Agreement is made in accordance with and shall be
governed and construed under the laws of the State of California, excluding its
choice of law principles.

      12.3 No Agency. Neither Party is, nor will be deemed to be, an employee,
agent or legal representative of the other Party for any purpose. Neither Party
will be entitled to enter into any contracts in the name of, or on behalf of the
other Party, nor will a Party be entitled to pledge the credit of the other
Party in any way or hold itself out as having authority to do so. This Agreement
is an arm's-length agreement between the Parties and shall not constitute or be
construed as a joint venture.

      12.4 Assignment.

            (a) Affiliates. ImmunoGen may assign any of its rights or
obligations under this Agreement in any country to any of its Affiliates;
provided, however, that such assignment shall not relieve ImmunoGen of its
responsibilities for performance of its obligations under this Agreement.

            (b) Merger, Acquisition or Sale of Assets. Subject to the terms
hereof, either Party may assign its rights or obligations under this Agreement
to a non-Affiliate only in connection with a merger or similar reorganization;
the sale of all or substantially all of its assets or the sale of all or
substantially all of its pharmaceutical and/or healthcare assets; or otherwise
with the prior written consent of the other Party. This Agreement shall survive
any such merger or reorganization of either Party with or into, or such sale of
assets to, another party and no consent for such merger, reorganization or sale
shall be required hereunder.

            (c) Binding Upon Successors and Assigns. This Agreement shall be
binding upon and inure to the benefit of the successors and permitted assigns of
the Parties. Any assignment of this Agreement not made in accordance with this
Agreement shall be void.

      12.5 Amendment. No amendment or modification hereof shall be valid or
binding upon the Parties unless made in writing and signed by both Parties.

      12.6 Covenant of Further Assurances. Each Party shall duly execute and
deliver, or cause to be duly executed and delivered, such further instruments
and do and cause to be done such further acts and things, including the filing
of such assignments, agreements, documents

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       31
<PAGE>

and instruments, as may be necessary or as the other Party may reasonably
request in connection with this Agreement or to carry out more effectively the
provisions and purposes, or to better assure and confirm unto such other Party
its rights under this Agreement.

      12.7 Notices. Any notice or other communication required or permitted to
be given to either Party hereto shall be in writing unless otherwise specified
and shall be deemed to have been properly given and to be effective upon
receiving written confirmation of receipt signed by an employee of the receiving
Party, if sent by a nationally recognized overnight delivery service or
registered or certified mail, to the other Party at the following address:

      In the case of Raven:

                Raven Biotechnologies, Inc.
                305 Old County Road
                San Carlos, CA 94070
                Attention: Chief Executive Officer

      With a copy to:

                Cooley Godward LLP
                One Freedom Square
                Reston Town Center
                11951 Freedom Drive
                Reston, VA 20190
                Attention: Matthias Alder, Esq.

      In the case of ImmunoGen:

                ImmunoGen, Inc.
                128 Sidney Street
                Cambridge, MA 02139
                Attention: Chief Executive Officer

      With a copy to:

                Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
                One Financial Center
                Boston, MA 02140
                Attention: Jeffrey M. Wiesen, Esq.

Either Party may change its address for communications by a notice to the other
Party in accordance with this Section 12.7.

      12.8 Rights in Bankruptcy. All rights and licenses granted under or
pursuant to this Agreement 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 of the

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       32
<PAGE>

U.S. Bankruptcy Code. The Parties agree that, as a licensee of such rights under
this Agreement, the other Party (the "Licensee") shall retain and may fully
exercise all of their rights and elections under the U.S. Bankruptcy Code,
however nothing herein shall be deemed to constitute a present exercise of such
rights and elections. The Parties further agree that, in the event of the
commencement of a bankruptcy proceeding by or against the other Party (the
"Bankrupt Party") under the U.S. Bankruptcy Code, the Licensee 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 the
Licensee (a) upon any such commencement of a bankruptcy proceeding upon their
written request therefore, unless the Bankrupt Party elects to continue to
perform all of its obligations under this Agreement, or (b) if not delivered
under (a) above, upon rejection of this Agreement by or on behalf of the
Bankrupt Party upon written request therefor by the Licensee.

      12.9 Force Majeure. Any delay in performance by any Party under this
Agreement shall not be considered a breach of this Agreement if and to the
extent caused by occurrences beyond the reasonable control of the Party
affected, including but not limited to acts of God, embargoes, governmental
restrictions, strikes or other concerted acts of workers, fire, flood,
earthquakes, explosions, riots, wars, civil disorder, rebellion or sabotage. The
Party suffering such occurrence shall immediately notify the other Party and any
time for performance hereunder shall be extended by the actual time of delay
caused by the occurrence.

      12.10 Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be deemed an original but all of which
together shall constitute a single instrument.

      12.11 Captions. All section titles or captions contained in this
Agreement, in any Exhibit referred to herein and the table of contents, if any,
to this Agreement are for convenience only, shall not be deemed a part of this
Agreement and shall not affect the meaning or interpretation of this Agreement.

      12.12 Severability. If any term, condition or provision of this Agreement
is held to be unenforceable for any reason, it shall, if possible, be
interpreted to achieve the intent of the Parties to this Agreement to the extent
possible rather than voided. In any event, all other terms, conditions and
provision of this Agreement shall be deemed valid and enforceable to the full
extent.

      12.13 Cumulative Rights. The rights, powers and remedies hereunder shall
be in addition to, and not in limitation of, all rights, powers and remedies
provided at law or in equity, or under any other agreement between the Parties.
All of such rights, powers and remedies shall be cumulative, and may be
exercised successively or cumulatively.

      12.14 Waiver. No waiver by either Party hereto on any breach or default of
any of the covenants or agreements herein set forth shall be deemed a waiver as
to any subsequent or similar breach or default.

      12.15 Entire Agreement. This Agreement, and any and all Exhibits referred
to herein embody the entire understanding of the Parties with respect to the
subject matter hereof and shall

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       33
<PAGE>

supersede all previous communications, representations or understandings, either
oral or written, between the Parties relating to the subject matter hereof.

      IN WITNESS WHEREOF, both Raven and ImmunoGen have executed this Agreement
by their respective officers hereunto duly authorized.

IMMUNOGEN, INC.                         RAVEN BIOTECHNOLOGIES, INC.


By:    ___________________________      By:    _________________________________

Name:  ___________________________      Name:  _________________________________

Title: ___________________________      Title: _________________________________

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       34
<PAGE>

                                    Exhibit A

                                  RESEARCH PLAN

ImmunoGen/Raven Discovery Research Plan

1     Overview

      The [ * ] of [ * ] is [ * ]. [ * ] summarizing the [ * ] subject to [ * ].
      The overall [ * ] will be [ * will consist of an [ * ] from [ * ] , but
      will be [ * ].

Research Activities

2.    [ * ]

      [ * ] has [ * ] that are [ * ] of relevance to [ * ]. Sufficient [ * ] for
      the [ * ].

3.    [ * ]

      [ * ] will [ * ] during the [ * ], and [ * ] that [ * ]. Upon [ * ] will
      do [ * ]. All [ * ] for [ * ] to the [ * ]. [ * ] will be [ * ] on their [
      * ].

4.    [ * ]

      [ * ] will [ * ] on a [ * ] to [ * ] that [ * ] on [ * ] as [ * ]. [ * ]
      will [ * ] that [ * ], but do not, [ * ]. These [ * ] will be [ * ] that
      will be [ * ] to [ * ].

5.    [ * ]

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       1
<PAGE>

      [ * ] will [ * ] the [ * ] the [ * ] on a [ * ]. [ * ] with [ * ], and [ *
      ], will be [ * ]. [ * ] may [ * ] the [ * ] to [ * ] of the [ * ]. [ * ]
      that [ * ] are [ * ] , and their [ * ].

      [ * ] of [ * ] of all [ * ] by [ * ] will be prepared by [ * ] [ * ].

6.    [ * ]

      [ * ] of the [ * ] will be [ * ] to be [ * ] and will be [ * ]. If there
      are [ * ] may [ * ] from the [ * ]. [ * ] will provide [ * ] for [ * ]
      will provide [ * ].

            1.    [ * ].

            2.    [ * ] will determine the [ * ]. ImmunoGen will [ * ]. In
                  addition to [ * ] and [ * ] the [ * ] of [ * ].

            3.    [ * ]. As [ ] will [ * ] to [ * ].

7.    [ * ]

      [ * ] the [ * ] in [ * ], will [ * ] to be [ * ] that it [ * ] into in [ *
      ]. [ * ] will provide [ * ] with [ * ] with [ * ].

8.    [ * ]

      [ * ] will perform [ * ] of the [ * ] on the [ * ]. [ * ] will [ * ] the [
      * ].

      [ * ] of [ * ] of [ * ] specified [ * ] will be [ * ] at the [ * ] during
      the [ * ].

9.    [ * ]

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       2
<PAGE>

      [ * ] will [ * ] for [ * ], and [ * ] of the [ * ]. This will [ * ].

      [ * ] shall submit [ * ] after the [ * ] of all [ * ].

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.


                                       3
<PAGE>

                                   Appendix 1

                                  MAB CRITERIA

[ * ]:

      [ * ]: [ * ] will be [ * ] for [ * ]. [ * ] this [ * ] will be any [ * ]
      that [ * ] with at [ * ].

      [ * ]: [ * ] will also be evaluated for [ * ]. [ * ] will be [ * ] that [
      * ] with [ * ].

[ * ]: [ * ] that have [ * ] will be [ * ] a [ * ]:

      [ * ]: [ * ] will be [ * ] for [ * ] on [ * ]. [ * ].

      [ * ]: [ * ] will be [ * ] for [ * ] on a [ * ] using [ * ] that are [ *
      ]. [ * ], subject to availability, [ * ]. [ * ] will have [ * ] by [ * ].

Portions of this Exhibit were omitted and have been filed separately with the
Secretary of the Commission pursuant to the Company's application requesting
confidential treatment under Rule 24b-2 of the Securities Exchange Act.