<PAGE>   1
                                                                    EXHIBIT 10.8


          Confidential Materials omitted and filed separately with the
      Securities and Exchange Commission. Asterisks denote such omissions.



                            RIGHTS EXCHANGE AGREEMENT

         This Agreement is effective as of May 28, 1997 (the "Effective Date")
by and between Millennium Pharmaceuticals, Inc., a Delaware corporation having
its principal office at 640 Memorial Drive, Cambridge, Massachusetts 02139-4815
("MPI"), and Millennium BioTherapeutics, Inc., a Delaware corporation having its
principal place of business at 640 Memorial Drive, Cambridge, Massachusetts
02139-4815 ("MBI").

                                  INTRODUCTION

         1.  MPI is engaged in the discovery and development of pharmaceutical
products, and has entered into a number of collaboration agreements with
pharmaceutical companies relating to such discovery and development activities.

         2.  MPI has established MBI as a wholly-owned subsidiary and, pursuant
to a Technology Transfer and License Agreement dated as of May 28, 1997 (the
"Transfer Agreement"), has transferred and/or licensed certain technology to MBI
in exchange for the issuance of 9,000,000 shares of Series A Convertible
Preferred Stock, $.001 par value per share.

         3.  MBI has been established in order to pursue the identification,
discovery, research, development and commercialization of products in the MBI
Core Field (as defined below).

         4.  MPI has agreed to transfer and assign to MBI certain existing and
future rights that it may develop or acquire in the MBI Core Field to MBI, and
MBI wishes to accept such assignment.

         5.  MBI has agreed to transfer and assign to MPI certain existing and
future rights that it may develop or acquire in the MPI Core Field (as defined
below), and MPI wishes to accept such assignment.

         NOW THEREFORE, in consideration of the mutual covenants set forth
herein and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, MPI and MBI hereby agree as
follows:

SECTION 1.    Definitions

         As used herein, the following terms shall have the meanings set forth
below:

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<PAGE>   2
          Confidential Materials omitted and filed separately with the
      Securities and Exchange Commission. Asterisks denote such omissions.


         "Affiliate" means any corporation, company partnership, joint venture
and/or firm which controls, is controlled by or is under common control with a
Party. For the purposes of this definition, "control" shall mean, (a) in the
case of corporate entities, direct or indirect ownership of at least fifty
percent (50%) of the stock or shares having the right to vote for the election
of directors, and (b) in the case of non-corporate entities, direct or indirect
ownership of at least fifty percent (50%) of the equity interest with the power
to direct the management and policies of such non-corporate entities.

         "Agreement Term" means the period commencing on the Effective Date and
ending on the later of (a) May 31, 2002, and (b) the date on which MBI ceases to
be an Affiliate of MPI.

         "Antibody-based Product" means any drug or candidate drug which
comprises (a) a  ***************************************************************
********************************************************************************
********************************************************************************
************* or (b) *********************************** ***********************
******** of the type described in (a) (e.g., *****************************.

         "Antisense-based Product" shall mean any drug or drug candidate which
comprises **********************************************************************
***** and which is complementary to a ******************************************
**************** ************** and which, upon delivery by ********************
********************************************************************************
*********** of or by such target gene.

         "Collaborative Partner" means a party (other than MPI or MBI) to an MPI
Collaboration Agreement or an MBI Collaboration Agreement.

         "Confidential Information" means all trade secrets or confidential or
proprietary information designated as such in writing, whether by letter or by
the use of an appropriate proprietary stamp or legend, prior to or at the time
any such trade secret or confidential or proprietary information is disclosed to
the receiving Party. Notwithstanding the foregoing, information which is orally
or visually disclosed to the receiving Party, or is disclosed in writing without
an appropriate letter, proprietary stamp or legend, shall constitute
Confidential Information if (i) it would be apparent to a reasonable person,
familiar with the business and the industry of the disclosing Party, that such
information is of a confidential or proprietary nature the maintenance of which
is important to the disclosing Party or if (ii) the disclosing Party, within
thirty (30) days after such disclosure, delivers to the receiving Party a

                                        2
<PAGE>   3
          Confidential Materials omitted and filed separately with the
      Securities and Exchange Commission. Asterisks denote such omissions.


written document or documents describing such information and referencing the
place and date of such oral, visual or written disclosure and the names of the
employees or officers.

         "Force Majeure" shall mean any act of God, any accident, explosion,
fire, storm, earthquake, flood, drought, peril of the sea, riot, embargo, war or
foreign, federal, state or municipal order of general application seizure,
requisition or allocation, any failure or delay of transportation, shortage of
or inability to obtain supplies, equipment fuel or labor or any other
circumstances or event beyond the reasonable control of the party relying upon
such circumstance or event.

         "Gene Therapy-based Product" shall mean *******************************
********************************************************************************
******** and which, upon delivery ******************** ************ which is ***
*******************************.  A Gene Therapy-based Product shall not include
an Antisense-based Product.

         "Lilly Collaboration Agreement" means the Collaboration Agreement dated
as of May 28, 1997 between MBI and Eli Lilly and Company.

         "MBI Collaboration Agreements" means collaboration agreements,
strategic alliances, license agreements and similar arrangements entered into by
MBI during the Agreement Term, pursuant to which, and to the extent that, MBI
retains or obtains development or commercialization rights in the MPI Core
Field, including, but not limited to, the Lilly Collaboration Agreement.

         "MBI Core Field" means the discovery, development and commercialization
of *****************************************************************************
********************************************************************************
*********************.  The MBI Core Field shall *******************************
********************************************************************************
********************************************************************************
**********************************************************************.

         "MBI Retained Rights" means all research, development and
commercialization rights, if any, retained by MBI in the MPI Core Field pursuant
to the MBI Collaboration Agreements.

         "MPI Collaboration Agreements" means (a) the MPI Existing Collaboration
Agreements, and (b) MPI Future Collaboration Agreements.


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<PAGE>   4
          Confidential Materials omitted and filed separately with the
      Securities and Exchange Commission. Asterisks denote such omissions.


         "MPI Core Field" shall mean the discovery, development and
commercialization of all products and services outside of the MBI Core Field.

         "MPI Existing Collaboration Agreements" means the agreements listed in
Exhibit A to this Agreement.

         "MPI Future Collaboration Agreements" means collaboration agreements,
strategic alliances, license agreements and similar arrangements entered into by
MPI from time to time during the Agreement Term pursuant to which, and to the
extent that, MPI retains or obtains development or commercialization rights in
the MBI Core Field.

         "MPI Retained Rights" shall mean all research, development and
commercialization rights, if any, retained by MPI in the MBI Core Field pursuant
to the MPI Collaboration Agreements.

         "Party" means MPI or MBI; "Parties" means MPI and MBI.

         "Peptidomimetic" means a ************** which is designed and developed
using **************************************************************************
**********************.

         "Protein-based Product" means any drug or drug candidate that comprises
a ******************************************************************************
********************************************************************************
********************************************************************************
********************************************************************************
********************************************************************************
********************************************************************************
***********. Protein-based Products shall not include a Peptidomimetic.

SECTION 2.    Core Field Opportunities

         (a)  MBI Core Field. MBI and MPI agree that MBI has been organized in
order to engage in the identification, discovery, research, development and
commercialization of products in the MBI Core Field. Except as provided in
subsection (c) below, MPI hereby agrees that it shall transfer, assign and/or
license to MBI or otherwise make available to MBI, all product development
opportunities and other technology rights developed or acquired or otherwise
controlled by MPI during the Agreement Term to the extent applicable to the MBI
Core Field, including, but not limited to, the transfer and assignment of MPI
Retained Rights pursuant to Section 3 below. Except as provided in subsection
(c), MPI shall not engage in the


                                        4
<PAGE>   5
    Confidential Materials have been omitted and filed separately with the
     Securities and Exchange Commission. Asterisks denote such omissions.

development or commercialization of products in the MBI Core Field during the
Agreement Term without the prior written consent of MBI.

         (b)  MPI Core Field. MPI and MBI agree that MPI is engaged in the
identification, discovery, research, development and commercialization of
products in the MPI Core Field. Except as provided in subsection (c) below, MBI
hereby agrees that it shall transfer, assign and/or license to MPI, or to
otherwise make available to MPI, all product development opportunities and other
technology rights developed or acquired or otherwise controlled by MBI during
the Agreement Term to the extent applicable to the MPI Core Field, including,
but not limited to, the transfer and assignment of MBI Retained Rights pursuant
to Section 4 below. Except as provided in subsection (c), MBI shall not engage
in the development or commercialization of products in the MPI Core Field during
the Agreement Term without the prior written consent of MPI.

         (c)  Collaborative Agreements. MPI and MBI each recognize that the
other party intends to enter into collaboration agreements, strategic alliances,
license agreements and similar arrangements with Collaborative Partners (a
"Collaboration Agreement"). MPI and MBI each agree to use reasonable efforts in
negotiating such arrangements to limit the licenses and rights granted to a
Collaborative Partner to the MPI Core Field and MBI Core Field, respectively.
However, (i) in the event that ***, ** *********** * ************* *********,
******** ** **** ***** that the ******* ** include in such ********* ****** ***
******** ** *** *** **** ***** ** ********** ********* pursuant to the
Collaboration Agreement would have a ******** ****** ** ****** *** ***********
** *** ************* ******* ** ***** **** * ************* ********* ** ***
********* ***** *******, **** *** ***, ***** ********* *** ** ******* ***
********** ** ** ********* ***** **** *** *** ***** ******** to the ********
************* *********, ***** **** **** ************* ********* and the 
provisions of subsection (a) shall *** ***** ** *** ******** *** ****** **
*******, provided that the ***** ** **** ******** *** ****** ******* *** ***
**** ***** ** ********** ** * **** **** ********* ********** ** *** *** ****
*****; and (ii) in the ***** **** ***, ** *********** * ************* *********,
******** ** **** ***** that the ******* ** ******* ** **** ********* ****** ***
******** ** *** *** **** ***** ** ********** ********* ******** ** ***
************* ********* would have a ******** ****** ** ****** *** ***********
** *** ************* ******* ** ***** **** *** ************* ********* ** ***
********* ***** *******, **** *** ***, ***** ********* *** ** ******* ***
********** ** ** ********* ***** **** *** *** ***** ******** to the ********
************* *********, ***** **** **** ************* ********* and the 
provisions of subsection (b) shall *** ***** ** *** ******** *** ****** **
*******, provided that the ***** ** **** ******** *** ****** ** ********** ** *
**** **** ********* ********** ** *** *** **** *****.


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<PAGE>   6
SECTION 3.    Transfer of MPI Retained Rights

         (a)  MPI Existing Collaboration Agreements. MPI hereby transfers and
assigns to MBI all MPI Retained Rights under the MPI Existing Collaboration
Agreements, subject to receipt of approvals and consents to such transfer and
assignment from the applicable Collaborative Partners (to the extent required
under the applicable MPI Existing Collaboration Agreements). MPI agrees to use
its best efforts to obtain such approvals and consents as soon as reasonably
practicable following the Effective Date. By accepting the transfer and
assignment of MPI Retained Rights under each MPI Existing Collaboration
Agreement, MBI agrees to perform all obligations, and to undertake all
responsibilities, financial or otherwise, relating to the exploitation of the
MPI Retained Rights.

         (b)  Other MPI Collaboration Agreements. The Parties anticipate that
from time to time after the Effective Date and during the Agreement Term, MPI
will enter into MPI Future Collaboration Agreements with Collaborative Partners
pursuant to which MPI Retained Rights may exist. MPI hereby agrees to transfer
and assign to MBI all MPI Retained Rights under each such MPI Future
Collaboration Agreements, if any, and shall use its best efforts to include in
each such MPI Future Collaboration Agreement provisions enabling MPI to effect
such transfer and assignment, or to obtain such approval or consent thereafter.
By accepting the transfer and assignment of MPI Retained Rights under each MPI
Future Collaboration Agreement, MBI agrees to perform all obligations, and to
undertake all responsibilities, financial or otherwise, relating to the
exploitation of the MPI Retained Rights.

         (c)  Failure to Obtain Approval and Consent. In the event that MPI is
unable to obtain any approval or consent required from a Collaborative Partner
in connection with the transfer and assignment of MPI Retained Rights under any
MPI Existing or Future Collaboration Agreement (referred to as "Nonassigned MPI
Retained Rights"), MPI shall, in consultation with and at the direction of MBI,
take such reasonable actions as shall be necessary to confer the benefits of any
Nonassigned MPI Retained Rights to MBI, including undertaking research or
development work, granting sublicenses to relevant patent rights and know how to
MBI or its designees and taking such other reasonable actions as may be agreed
upon by the Parties, in all cases at the expense of MBI.

         (d)  Direct Agreement. At MPI's request, MBI shall use reasonable
efforts to enter into an agreement directly with MPI and the Collaborative
Partner with respect to the MPI Retained Rights under a MPI Collaboration
Agreement, on such terms as shall be mutually agreeable.



                                        6
<PAGE>   7
         (e)  No Additional Compensation. MBI shall not be required to account
to MPI with respect to its exploitation of the MPI Retained Rights, and shall
not be obligated to make any royalty or other payments to MPI relating thereto.

SECTION 4.    Transfer of MBI Retained Rights

         (a)  MBI Collaboration Agreements. The Parties anticipate that from
time to time from and after the Effective Date and during the Agreement Term MBI
will enter into MBI Collaboration Agreements with Collaborative Partners
pursuant to which MBI Retained Rights may exist. MBI hereby agrees to transfer
and assign to MPI all MBI Retained Rights under each such MBI Collaboration
Agreement, if any, and shall use its best efforts to include in each such MBI
Collaboration Agreement provisions enabling MBI to effect such transfer and
assignment, or to obtain such approval or consent thereafter. By accepting the
transfer and assignment of MBI Retained Rights under each MBI Collaboration
Agreement, MPI agrees to perform all obligations, and to undertake all
responsibilities, financial or otherwise, relating to the exploitation of the
MBI Retained Rights.

         (b)  Failure to Obtain Approval and Consent. In the event that MBI is
unable to obtain any approval or consent required from a Collaborative Partner
in connection with the transfer and assignment of MBI Retained Rights under any
MBI Collaboration Agreement (referred to as "Nonassigned MBI Retained Rights"),
MBI shall, in consultation with and at the direction of MPI, take such
reasonable actions as shall be necessary to confer the benefits of any
Nonassigned MBI Retained Rights to MPI, including undertaking research and
development work, granting sublicenses to relevant patent rights and know-how to
MBI or its designees and taking such other reasonable actions as may be agreed
upon by the Parties, in all cases at the expense of MPI.

         (c)  Direct Agreement. At MBI's request, MPI shall use reasonable
efforts to enter into an agreement directly with MBI and the Collaborative
Partner with respect to the MBI Retained Rights under a MBI Collaboration
Agreement, on such terms as shall be mutually agreeable.

         (d)  No Additional Compensation. MPI shall not be required to account
to MBI with respect to its exploitation of the MBI Retained Rights and shall not
be obligated to make any royalty or other payments to MBI relating thereto.

SECTION 5.    Confidential Information and Permitted Disclosures.

         5.1 Confidential Information. Any party receiving Confidential
Information shall maintain the confidential and proprietary status of such
Confidential Information, keep such Confidential Information and each part
thereof within its possession or under its control sufficient to prevent any
activity with respect to the


                                        7
<PAGE>   8
Confidential Information that is not specifically authorized by this Agreement,
use commercially reasonable efforts to prevent the disclosure of any
Confidential Information to any other Person, and use commercially reasonable
efforts to ensure that such Confidential Information is used only for those
purposes specifically authorized herein; provided, however, that such
restriction shall not apply to any Confidential Information which is (a)
independently developed by the receiving party, (b) in the public domain at the
time of its receipt or thereafter becomes part of the public domain through no
fault of the receiving party, (c) received without an obligation of
confidentiality from a third party having the right to disclose such
information, (d) released from the restrictions of this Section 5.1 by the
express written consent of the disclosing party, (e) disclosed to any permitted
assignee, permitted sublicensee or permitted subcontractor of either MPI or MBI
hereunder (if such assignee, sublicense or subcontractor is subject to the
provisions of this Section 5.1 or comparable provisions of such other
documents), or (f) required by law, statute, rule or court order to be disclosed
(the disclosing party shall, however, use commercially reasonable efforts to
obtain confidential treatment of any such disclosure).

         5.2 Employee Obligations. MPI and MBI each agree that it shall provide
Confidential Information received from the other Party only to its employees,
consultants and advisors who have a need to know and have an obligation to treat
such information and materials as confidential. Without limiting the generality
of the foregoing, MPI and MBI each shall use commercially reasonable efforts to
obtain confidentiality agreements from its respective employees and agents,
similar in scope to Section 5.1, to protect the Confidential Information.
Notwithstanding anything to the contrary herein, MPI and MBI shall each be
deemed to have satisfied its obligations under Section 5.1 if it protects the
Confidential Information of the other party with the same degree of care that it
uses to protect its own similar Confidential Information.

         5.3 Third Party Obligations. Each Party acknowledges that the other
Party may from time to time have agreements with other persons which impose
obligations or restrictions on such other Party with respect to the disclosure
or use of inventions or information relating to the subject matter of such
agreements ("Third Party Agreements"). Without limiting the generality of the
foregoing, MPI acknowledges that MBI is subject to certain obligations and
restrictions under the Lilly Collaboration Agreement. Each Party agrees to be
bound by all such obligations and restrictions which are made known to it under
Third Party Agreements (excluding the Lilly Collaboration Agreement) relating to
Confidential Information, and MPI agrees to be bound by all such obligations and
restrictions under the Lilly Collaboration Agreement relating to Confidential
Information.

         5.4 Permitted Disclosures. Notwithstanding the provisions of Section
5.1 hereof, MPI and MBI may, to the extent necessary, disclose and use
Confidential


                                        8
<PAGE>   9
Information, consistent with the rights of MPI and MBI otherwise granted
hereunder (a) for the purpose of engaging in research and development,
conducting clinical testing and marketing programs, or securing institutional or
government approval to clinically test or market any product, (b) for the
purpose of sharing clinical trial results and data with third parties conducting
clinical trials on products arising from MPI Retained Rights or MBI Retained
Rights, as applicable, (c) for the purpose of securing patent protection for an
invention arising from the exploitation of MPI Retained Rights or MBI Retained
Rights, as applicable, or (d) for the purpose of obtaining private investment.

         5.5 Term The obligations set forth in this Section 5 shall survive for
a period of five (5) years from the termination or expiration of this Agreement.


SECTION 6.    Disclaimer of Warranty; Consequential Damages.

         6.1 Disclaimer of Warranty. Nothing in this Agreement shall be
construed as a representation made or warranty given by either party hereto that
the practice by the other party hereto of any rights transferred hereunder, or
that the exploitation of any MPI Retained Rights or MBI Retained Rights, will
not infringe the patent or proprietary rights of any other Person. In addition,
MPI and MBI acknowledge that THE MPI RETAINED RIGHTS AND MBI RETAINED RIGHTS ARE
TRANSFERRED, AS THE CASE MAY BE, TO MBI AND MPI, RESPECTIVELY, AS IS, AND MPI
AND MBI EXPRESSLY DISCLAIM AND HEREBY WAIVE, RELEASE AND RENOUNCE ANY WARRANTY,
EXPRESS OR IMPLIED, WITH RESPECT TO SUCH TECHNOLOGY, INCLUDING, WITHOUT
LIMITATION, ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

         6.2 Consequential Damages. EXCEPT AS OTHERWISE SET FORTH HEREIN,
NEITHER PARTY TO THIS AGREEMENT SHALL BE ENTITLED TO RECOVER FROM THE OTHER ANY
SPECIAL, INCIDENTAL, CONSEQUENTIAL OR PUNITIVE DAMAGES.

SECTION 7.    Indemnification

         7.1 Indemnification by MPI. MPI agrees to defend MBI, at MPI's cost and
expense, and will indemnify and hold MBI and its directors, officers, employees
and agents (the "MBI Indemnified Parties") harmless from and against, any
losses, costs, damages, fees or expenses arising out of any claim relating to
personal injury from the exploitation by MPI or its licensees of the MBI
Retained Rights or other rights transferred by MBI to MPI pursuant to Section 2,
including the development, manufacture, use, sale or other disposition of
products or services resulting therefrom. In the event of any such claim against
the MBI Indemnified Parties by any party, MBI shall promptly notify MPI in
writing of the claim and MPI shall


                                        9
<PAGE>   10
manage and control, at its sole expense, the defense of the claim and its
settlement. The MBI Indemnified Parties shall cooperate with MPI and may, at
their option and expense, be represented in any such action or proceeding. MPI
shall not be liable for any litigation costs or expenses incurred by the MBI
Indemnified Parties without MPI's prior written authorization. In addition, MPI
shall not be responsible for the indemnification of any MBI Indemnified Party
arising from any negligent or intentional acts by such party.

         7.2 Indemnification by MBI. MBI agrees to defend MPI, at MBI's cost and
expense, and will indemnify and hold MPI and its directors, officers, employees
and agents (the "MPI Indemnified Parties") harmless from and against any losses,
costs, damages, fees or expenses arising out of any claim relating to personal
injury from the exploitation by MBI or its licensees of the MPI Retained Rights
or other rights transferred by MPI to MBI pursuant to Section 2, including the
development, manufacture, use, sale or other disposition of products or services
resulting therefrom. In the event of any such claim against the MPI Indemnified
Parties by any party, MPI shall promptly notify MBI in writing of the claim and
MBI shall manage and control, at its sole expense, the defense of the claim and
its settlement. The MPI Indemnified Parties shall cooperate with MBI and may, at
their option and expense, be represented in any such action or proceeding. MBI
shall not be liable for any litigation costs or expenses incurred by the MPI
Indemnified Parties without MBI's prior written authorization. In addition, MBI
shall not be responsible for the indemnification of any MPI Indemnified Party
arising from any negligent or intentional acts by such party.

SECTION 8.    General Provisions

         8.1 No Implied Waivers; Rights Cumulative. No failure on the part of
MPI or MBI to exercise and no delay in exercising any right, power, remedy or
privilege under this Agreement, or provided by statute or at law or in equity or
otherwise, shall impair, prejudice or constitute a waiver of any such right,
power, remedy or privilege or be construed as a waiver of any breach of this
Agreement or as an acquiescence therein, nor shall any single or partial
exercise of any such right, power, remedy or privilege preclude any other or
further exercise thereof or the exercise of any other right, power, remedy or
privilege.

         8.2 Force Majeure. MPI and MBI shall each be excused for any failure or
delay in performing any of its respective obligations under this Agreement, if
such failure or delay is caused by Force Majeure.

         8.3 Cooperation. Each party agrees to cause each of its employees and
agents to take all actions and to execute, acknowledge and deliver all
instruments or agreements reasonably requested by the other party, and necessary
for the perfection, maintenance, enforcement or defense of that party's rights
as set forth herein.


                                       10
<PAGE>   11
         8.4 Notices. All notices, requests and other communications to MPI or
MBI hereunder shall be in writing (including telecopy or similar electronic
transmissions), shall refer specifically to this Agreement and shall be
personally delivered or sent by telecopy or other electronic facsimile
transmission or by certified mail, return receipt requested, postage prepaid, in
each case to the respective address specified below (or to such other address as
may be specified in writing to the other party hereto):

            Millennium Pharmaceuticals, Inc.
            640 Memorial Drive
            Cambridge, MA  02139-4815
            Attention:  Chief Business Officer (with a copy to Legal Department)


            Millennium BioTherapeutics, Inc.
            640 Memorial Drive
            Cambridge, MA  02139-4815
            Attention:  Vice President and General Manager (with a copy to Legal
                         Department)

Any notice or communication given in conformity with this Section 8.4 shall be
deemed to be effective when received by the addressee, if delivered by hand,
telecopy or other electronic facsimile transmission, and three (3) days after
mailing, if mailed.

         8.5 Further Assurances. Each of MPI and MBI agrees to 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, without
limitation, the filing of such additional assignments, agreements, documents and
instruments, that may be necessary or as the other party hereto may at any time
and from time to time reasonably request in connection with this Agreement or to
carry out more effectively the provisions and purposes of, or to better assure
and confirm unto such other party its rights and remedies under, this Agreement.

         8.6 Successors and Assigns. The terms and provisions of this Agreement
shall inure to the benefit of, and be binding upon, MPI, MBI, and their
respective successors and assigns; provided, however, that neither MPI nor MBI
may assign or otherwise transfer any of its rights and interests, nor delegate
any of its respective obligations hereunder, including, without limitation,
pursuant to a merger or consolidation, without the prior written consent of the
other party hereto, which consent shall not be unreasonably withheld. Any
attempt to assign or delegate any portion of this Agreement in violation of this
Section 8.6 shall be null and void. Subject to the foregoing, any reference to
MPI and MBI hereunder shall be deemed to include the successors thereto and
assigns thereof.


                                       11
<PAGE>   12
         8.7  Amendments. No amendment, modification, waiver, termination or
discharge of any provision of this Agreement, nor consent to any departure by
MPI or MBI therefrom, shall be effective unless the same shall be in writing
specifically identifying this Agreement and the provision intended to be
amended, modified, waived, terminated or discharged and signed by MPI and MBI,
and each such amendment, modification, waiver, termination or discharge shall be
effective only in the specific instance and for the specific purpose for which
given. No provision of this Agreement shall be varied, contradicted or explained
by any oral agreements course of dealing or performance or any other matter not
set forth in an agreement in writing and signed by MPI and MBI.

         8.8  Governing Law. This Agreement shall be governed by and construed
in accordance with the laws of the Commonwealth of Massachusetts.

         8.9  Severability. If any provision hereof should be held invalid,
illegal or unenforceable in any respect in any jurisdiction, then, to the
fullest extent permitted by law, (a) all other provisions hereof shall remain in
full force and effect in such jurisdiction and shall be liberally construed in
order to carry out the intentions of the parties hereto as nearly as may be
possible and (b) such invalidity, illegality or unenforceability shall not
affect the validity, legality or enforceability of such provision in any other
jurisdiction. To the extent permitted by applicable law, MPI and MBI hereby
waive any provision of law that would render any provision hereof prohibited or
unenforceable in any respect.

         8.10 Headings. Headings used herein are for convenience only and shall
not in any way affect the construction of, or be taken into consideration
interpreting, this Agreement.

         8.11 Execution in Counterparts. This Agreement may be executed in
counterparts, each of which counterparts, when so executed and delivered, shall
be deemed to be an original, and all of which counterparts, taken together,
shall constitute one and the same instrument.

         8.12 Entire Agreement. This Agreement together with the Technology
Transfer and License Agreement, the Research Services and Collaboration
Agreement, Bioinformatics License Agreement, Trademark License Agreement,
Administrative Services Agreement and Tax Sharing Agreement, each dated as of
the date hereof, constitute, on and as of the date hereof, the entire agreement
of MPI and MBI with respect to the subject matter hereof, and all prior or
contemporaneous understandings or agreements, whether written or oral, between
MPI and MBI with respect to such subject matter are hereby superseded in their
entirety.



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<PAGE>   13
         IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed under seal and delivered as of the date first above written.


MILLENNIUM BIOTHERAPEUTICS, INC.            MILLENNIUM
                                            PHARMACEUTICALS, INC.


By: /s/John Maraganore                      By: /s/Steven H. Holtzman
    -----------------------------------         ---------------------------
    John Maraganore, Vice President              Steven H. Holtzman,
       and General Manager                       Chief Business Officer


                                       13
<PAGE>   14
                                    EXHIBIT A


A.       MPI Collaboration Agreements

         1.       Research and License Agreement between MPI and
                  Hoffmann-LaRoche, Inc. dated March 25, 1994.

         2.       Collaborative Research Agreement between Pfizer, Inc. and Myco
                  Pharmaceuticals, Inc. (later renamed ChemGenics
                  Pharmaceuticals, Inc.) dated January 1, 1995.

         3.       License Option, License and Royalty Agreement between Pfizer,
                  Inc. and Myco Pharmaceuticals, Inc. (later renamed ChemGenics
                  Pharmaceuticals, Inc.) dated January 1, 1995.

         4.       Research and License Agreement between MPI and Eli Lilly and
                  Company dated October 2, 1995.

         5.       Research and License Agreement between MPI and Astra AB, dated
                  December 9, 1995.

         6.       Research and License Agreement between MPI and Eli Lilly and
                  Company dated March 31, 1996.

         7.       Research and License Agreement between MPI and American Home
                  Products Corporation, dated August 1, 1996.

         8.       Collaborative Research and License Agreement between American
                  Home Products represented by its Wyeth-Ayerst Laboratories
                  Division and ChemGenics Pharmaceuticals, Inc. dated November
                  1, 1996.


                                       14