<PAGE>   1
                                                                    EXHIBIT 10.2



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                              INVESTMENT AGREEMENT

                         dated as of September 22, 1998

                                 by and between

                                    BAYER AG
                a corporation of the Federal Republic of Germany

                                       and

                        MILLENNIUM PHARMACEUTICALS, INC.,
                             a Delaware corporation


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<PAGE>   2


                                TABLE OF CONTENTS

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ARTICLE I

         PURCHASE AND SALE OF SHARES.................................................     1
         Section 1.1    PURCHASE AND SALE............................................     1
         Section 1.2    CLOSING DATE.................................................     1
         Section 1.3    TRANSACTIONS AT THE CLOSING..................................     2

ARTICLE II

         REPRESENTATIONS AND WARRANTIES..............................................     2
         Section 2.1    REPRESENTATIONS AND WARRANTIES OF THE COMPANY................     2
         Section 2.2    REPRESENTATIONS AND WARRANTIES OF PURCHASER..................    11

ARTICLE III

         EQUITY PURCHASES FROM THE COMPANY............................................   12
         Section 3.1    SUBSCRIPTION RIGHTS...........................................   12
         Section 3.2    ISSUANCE AND DELIVERY OF NEW SECURITIES AND VOTING STOCK......   13
         Section 3.3    TERMINATION OF ARTICLE III....................................   13

ARTICLE IV

         LIMITATIONS ON PURCHASES OF ADDITIONAL EQUITY SECURITIES.....................   13
         Section 4.1    PURCHASES OF EQUITY SECURITIES................................   13

ARTICLE V

         TRANSFER OF COMMON STOCK.....................................................   16
         Section 5.1    LIMITATIONS ON TRANSFER.......................................   16

ARTICLE VI

         COVENANTS AND ADDITIONAL AGREEMENTS..........................................   18
         Section 6.1    ORDINARY COURSE...............................................   18
         Section 6.2    ACCESS AND INFORMATION........................................   18
         Section 6.3    FURTHER ACTIONS...............................................   18
         Section 6.4    FURTHER ASSURANCES............................................   19
         Section 6.5    BOARD ATTENDANCE RIGHTS AND RIGHTS TO INFORMATION ABOUT 
                        BOARD AND THE COMPANY.........................................   19
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<TABLE>
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ARTICLE VII

         CONDITIONS PRECEDENT.........................................................   19
         Section 7.1    EACH PARTY'S OBLIGATIONS......................................   20
         Section 7.2    CONDITIONS TO THE OBLIGATIONS OF THE COMPANY..................   20
         Section 7.3    CONDITIONS TO THE OBLIGATIONS OF PURCHASER....................   20

ARTICLE VIII

         TERMINATION..................................................................   21
         Section 8.1    TERMINATION...................................................   21
         Section 8.2    EFFECT OF TERMINATION.........................................   23

ARTICLE IX

         INDEMNIFICATION..............................................................   23
         Section 9.1    INDEMNIFICATION OF PURCHASER..................................   23
         Section 9.2    INDEMNIFICATION PROCEDURES....................................   23
         Section 9.3    SURVIVAL OF  REPRESENTATIONS AND WARRANTIES...................   24

ARTICLE X 

         INTERPRETATION; DEFINITIONS..................................................   24
         Section 10.1   INTERPRETATION................................................   24
         Section 10.2   DEFINITIONS...................................................   25

ARTICLE XI
         
         MISCELLANEOUS................................................................   29
         Section 11.1   SEVERABILITY..................................................   30
         Section 11.2   SPECIFIC ENFORCEMENT..........................................   30
         Section 11.3   ENTIRE AGREEMENT..............................................   30
         Section 11.4   COUNTERPARTS..................................................   30
         Section 11.5   NOTICES.......................................................   30
         Section 11.6   AMENDMENTS....................................................   32
         Section 11.7   COOPERATION...................................................   32
         Section 11.8   SUCCESSORS AND ASSIGNS........................................   32
         Section 11.9   EXPENSES AND REMEDIES.........................................   32
         Section 11.10  TRANSFER OF SHARES............................................   32
         Section 11.11  GOVERNING LAW.................................................   33
         Section 11.12  PUBLICITY.....................................................   33
         Section 11.13  NO THIRD PARTY BENEFICIARIES..................................   33
         Section 11.14  CONSENT TO JURISDICTION.......................................   33

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<PAGE>   4

                              INVESTMENT AGREEMENT


         THIS INVESTMENT AGREEMENT (the "Agreement") is made as of September 22,
1998 by and between MILLENNIUM PHARMACEUTICALS, INC., a Delaware corporation
(the "Company"), and BAYER AG, a corporation organized under the laws of the
Federal Republic of Germany ("Purchaser").

         WHEREAS, the Company and Purchaser are parties to that certain
Agreement, dated as of the date hereof (the "Collaboration Agreement"), which
contains the terms and conditions on which the parties have agreed to
collaborate on a small molecule drug discovery program; and

         WHEREAS, in connection with the execution of the Collaboration
Agreement, Purchaser wishes to purchase from the Company, and the Company wishes
to sell to Purchaser, shares of the Company's Common Stock, $.001 par value per
share ("Common Stock"), on the terms and subject to the conditions set forth
herein;

         WHEREAS, in connection with such sale and purchase of shares of Common
Stock, the Company and Purchaser wish to enter into a registration rights
agreement (the "Registration Rights Agreement"), substantially in the form
attached hereto as Exhibit A.

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants set forth herein, the Company and Purchaser agree as follows:


                                    ARTICLE I

                           PURCHASE AND SALE OF SHARES

         Section 1.1 PURCHASE AND SALE. Subject to the terms and conditions of
this Agreement, the Company agrees to issue and sell to Purchaser and Purchaser
agrees to purchase from the Company 4,957,660 shares of the Company's Common
Stock (the "Shares"), at a purchase price of $19.485 per share (which purchase
price reflects the greater of (i) 115% of the average daily closing price of a
share of the Company's Common Stock over the period from March 1, 1998 to August
31, 1998 or (ii) 100% of the closing price of a share of the Company's Common
Stock on September 21, 1998) for an aggregate purchase price of Ninety Six
Million, Six Hundred Thousand Dollars ($96,600,000) (the "Purchase Price").

         Section 1.2 CLOSING DATE. The closing of the purchase and sale of the
Shares hereunder (the "Closing") shall be held at the offices of Hale and Dorr,
60 State Street, Boston, Massachusetts, at 10:00 a.m., Boston time, on the third
Business Day following the first date on which all the conditions to Closing set
forth in Article VII have been satisfied or

<PAGE>   5

waived, or at such other, place, time and date as the Company and Purchaser
shall agree. The Company shall give Purchaser three (3) Business Days prior
notice of the date the Closing is scheduled to occur. The date of the Closing is
hereinafter referred to as the "Closing Date."

         Section 1.3 TRANSACTIONS AT THE CLOSING. At the Closing, subject to the
terms and conditions of this Agreement, (a) the Company shall issue and sell to
Purchaser and Purchaser shall purchase the Shares; (b) the Company shall deliver
to Purchaser a certificate representing the Shares, registered in the name of
Purchaser against payment of the Purchase Price by wire transfer of immediately
available funds to an account or accounts previously designated by the Company
no less than five (5) Business Days prior to the Closing Date; and (c) the
Company and Purchaser shall enter into the Registration Rights Agreement.


                                   ARTICLE II

                         REPRESENTATIONS AND WARRANTIES

         Section 2.1 REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company
hereby represents and warrants to Purchaser as follows:

                  (a) CORPORATE ORGANIZATION. The Company is a corporation duly
         organized, validly existing and in good standing under the laws of the
         State of Delaware. Each Subsidiary is duly organized and validly
         existing and, if applicable, is in good standing, under the laws of the
         jurisdiction of its incorporation or organization. Each of the Company
         and its Subsidiaries is duly qualified or licensed and, if applicable,
         is in good standing as a foreign corporation, in each jurisdiction in
         which the properties owned, leased or operated, or the business
         conducted, by it require such qualification or licensing, except for
         any such failure so to qualify or be in good standing which,
         individually or in the aggregate, would not have a Material Adverse
         Effect on the Company and its Subsidiaries, taken as a whole. Each of
         the Subsidiaries has the requisite power and authority to carry on its
         business as it is now being conducted. The Company has heretofore made
         available to Purchaser complete and correct copies of the Certificate
         of Incorporation of the Company (the "Company Charter") and the By-laws
         of the Company (the "Company By-Laws") and the certificate of
         incorporation and by-laws, or the comparable organizational documents,
         of each of its Subsidiaries, each as amended to date and currently in
         full force and effect.

                  (b) Corporate Authority. The Company has the requisite
         corporate power and authority to execute, deliver and perform this
         Agreement and the Registration Rights Agreement and to consummate the
         transactions contemplated hereby and thereby. The execution, delivery
         and performance of this Agreement and the Registration Rights Agreement
         by the Company, the issuance and sale by the Company of the Shares and
         the performance by the Company of the other transactions 

                                       2

<PAGE>   6

         contemplated hereby and thereby have been duly authorized by the
         Company's Board of Directors, and no other corporate proceedings on the
         part of the Company are necessary to authorize this Agreement or the
         Registration Rights Agreement or for the Company to consummate the
         transactions so contemplated herein and therein. This Agreement is, and
         the Registration Rights Agreement, when executed or delivered will be,
         valid and binding agreements of the Company, enforceable against the
         Company in accordance with their respective terms, assuming that this
         Agreement and the Registration Rights Agreement are valid and binding
         agreements of Purchaser, subject as to enforcement of remedies to
         applicable bankruptcy, insolvency, reorganization, moratorium or
         similar laws affecting generally the enforcement of creditors' rights
         and subject to a court's discretionary authority with respect to the
         granting of a decree ordering specific performance or other equitable
         remedies.

                  (c) NO VIOLATIONS; CONSENTS AND APPROVALS. (i) Neither the
         execution, delivery or performance by the Company of this Agreement or
         the Registration Rights Agreement or the consummation by the Company of
         the transactions contemplated hereby or thereby (A) will result in a
         violation or breach of the Company Charter or the Company By-Laws or
         the charter or by-laws of any of the Company's Subsidiaries or (B) will
         result in a violation or breach of (or give rise to any right of
         termination, revocation, cancellation or acceleration under or
         increased payments under), or constitute a default (with or without due
         notice or lapse of time or both) under, or result in the creation of
         any lien, mortgage, charge, encumbrance or security interest of any
         kind (a "Lien") upon any of the properties or assets of the Company or
         any of its Subsidiaries under, (1) any of the terms, conditions or
         provisions of any note, bond, mortgage, indenture, contract, agreement,
         obligation, instrument, offer, commitment, understanding or other
         arrangement (each a "Contract") or of any license, waiver, exemption,
         order, franchise, permit or concession (each a "Permit") to which the
         Company or any of its Subsidiaries is a party or by which any of their
         properties or assets may be bound, or (2) subject to the governmental
         filings and other matters referred to in clause (ii) below, any
         judgment, order, decree, statute, law, regulation or rule applicable to
         the Company or any of its Subsidiaries, except, in the case of clause
         (B), for violations, breaches, defaults, rights of cancellation,
         termination, revocation or acceleration or Liens that would not,
         individually or in the aggregate, have a Material Adverse Effect on the
         Company and its Subsidiaries, taken as a whole.

                  (ii) Except for filings as may be required under, and other
         applicable requirements of, the Hart-Scott-Rodino Antitrust
         Improvements Act of 1976, as amended (the "HSR Act"), no consent,
         approval, order or authorization of, or registration, declaration or
         filing with, any government or any court, administrative agency or
         commission or other governmental authority or agency, federal, state,
         local or foreign (a "Governmental Entity"), is required with respect to
         the Company in connection with the execution, delivery or performance
         by the Company of this Agreement or the consummation by the Company of
         the transactions contemplated hereby (except where the failure to
         obtain such consents, approvals, orders or 


                                       3



         
<PAGE>   7
         authorizations, or to make such filings would not, individually or in
         the aggregate, have a Material Adverse Effect on the Company and its
         Subsidiaries, taken as a whole).

                  (d) CAPITAL STOCK. The authorized capital stock of the Company
         consists of (i) 100,000,000 shares of Common Stock, of which an
         aggregate 29,550,646 shares of Common Stock were issued and outstanding
         as of the close of business on August 31, 1998, and (ii) 5,000,000
         shares of preferred stock, $.001 par value per share, of which none
         were issued and outstanding as of the close of business on August 31,
         1998. As of the close of business on August 31, 1998, there were
         outstanding under the Company's stock option and incentive plans
         (collectively, the "Company Stock Plans"), options to acquire an
         aggregate of 6,215,608 shares of Common Stock (subject to adjustment on
         the terms set forth therein). All of the outstanding shares of Common
         Stock have been duly authorized and validly issued, and are fully paid
         and nonassessable. Except as set forth on Schedule 2.1(d), there are no
         preemptive or similar rights on the part of any holders of any class of
         securities of the Company or of any of its Subsidiaries. Except for the
         Common Stock, the Company has outstanding no bonds, debentures, notes
         or other obligations or securities the holders of which have the right
         to vote (or are convertible or exchangeable into or exercisable for
         securities having the right to vote) with the stockholders of the
         Company on any matter. Except as set forth above or on Schedule 2.1(d),
         as of the date of this Agreement, there are no securities convertible
         into or exchangeable for, or options, warrants, calls, subscriptions,
         rights, contracts, commitments, arrangements or understandings of any
         kind to which the Company or any of its Subsidiaries is a party or by
         which any of them is bound obligating the Company or any of its
         Subsidiaries contingently or otherwise to issue, deliver or sell, or
         cause to be issued, delivered or sold, additional shares of capital
         stock or other voting securities of the Company or of any of its
         Subsidiaries. Except as set forth on Schedule 2.1(d), there are no
         outstanding agreements of the Company or any of its Subsidiaries to
         repurchase, redeem or otherwise acquire any shares of capital stock of
         the Company or any of its Subsidiaries.

                  (e) SUBSIDIARIES.  (i) Schedule 2.1(e) contains a complete and
         correct description of the shares of stock or other equity interests
         that are authorized, or issued and outstanding, of each of the
         Company's Subsidiaries. The Company has no equity interests with a
         value of $100,000 or more in any Person other than its Subsidiaries,
         and there are no commitments on the part of the Company or any
         Subsidiary to contribute additional capital in respect of any equity
         interest in any Person. Each of the outstanding shares of capital stock
         of each of the Subsidiaries has been duly authorized and validly
         issued, and is fully paid and nonassessable. Except as set forth on
         Schedule 2.1(e), all of the outstanding shares of capital stock of each
         Subsidiary are owned, either directly or indirectly, by the Company
         free and clear of all Liens.


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<PAGE>   8


                                    (ii) Schedule 2.1(e)(ii) contains a complete
         and correct list of all Subsidiaries of the Company.

                  (f) SEC FILINGS. The Company has timely filed all reports,
         schedules, forms, statements and other documents required to be filed
         by it with the SEC under the Securities Act and the Exchange Act since
         May 1996 (the "Company SEC Documents"). As of its filing date, each
         Company SEC Document filed, as amended or supplemented, if applicable,
         (i) complied in all material respects with the applicable requirements
         of the Securities Act or the Exchange Act, as applicable, and the rules
         and regulations thereunder and (ii) did not, at the time it was filed,
         contain any untrue statement of a material fact or omit to state any
         material fact required to be stated therein or necessary to make the
         statements therein in light of the circumstances under which they were
         made, not misleading.

                  (g) ABSENCE OF CERTAIN EVENTS AND CHANGES. Except as disclosed
         in the Company SEC Documents filed with the SEC and publicly available
         prior to the date hereof, or as otherwise contemplated or permitted by
         this Agreement, and except for any items referred to in Schedule
         2.1(g), since June 30, 1998, the Company and its Subsidiaries have
         conducted their respective businesses in the ordinary course consistent
         with past practice and there has not been any event, change or
         development which, individually or in the aggregate, would have a
         Material Adverse Effect on the Company and its Subsidiaries, taken as a
         whole.

                  (h) COMPLIANCE WITH APPLICABLE LAW. Except as disclosed in the
         Company SEC Documents, each of the Company and its Subsidiaries is in
         compliance with all statutes, laws, regulations, rules, judgments,
         orders and decrees of all Governmental Entities applicable to it that
         relate to its respective business, and neither the Company nor any of
         its Subsidiaries has received any notice alleging noncompliance except,
         with reference to all the foregoing, where the failure to be in
         compliance would not, individually or in the aggregate, have a Material
         Adverse Effect on the Company and its Subsidiaries, taken as a whole.
         This Section 2.1(h) does not relate to employee benefits matters (for
         which Section 2.1(l) is applicable, environmental matters, (for which
         Section 2.1(m) is applicable) or tax matters (for which Section 2.1(k)
         is applicable). Each of the Company and its Subsidiaries has all
         Permits that are required in order to permit it to carry on its
         business as it is presently conducted, except where the failure to have
         such Permits or rights would not, individually or in the aggregate,
         have a Material Adverse Effect on the Company and its Subsidiaries,
         taken as a whole. All such Permits are in full force and effect and the
         Company and its Subsidiaries are in compliance with the terms of such
         Permits, except where the failure to be in full force and effect or in
         compliance would not, individually or in the aggregate, have a Material
         Adverse Effect on the Company and its Subsidiaries, taken as a whole.

                  (i) LITIGATION. Except as disclosed in the Company SEC
         Documents filed 

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         with the SEC and publicly available prior to the date hereof or
         referred to in Schedule 2.1(i), there are no civil, criminal or
         administrative actions, suits, or proceedings pending or, to the
         Knowledge of the Company, threatened, against the Company or any of its
         Subsidiaries that, individually or in the aggregate, are likely to have
         a Material Adverse Effect on the Company and its Subsidiaries, taken as
         a whole. Except as disclosed in the Company SEC Documents filed with
         the SEC and publicly available prior to the date hereof, there are no
         outstanding judgments, orders, decrees, or injunctions of any
         Governmental Entity against the Company or any of its Subsidiaries
         that, insofar as can reasonably be foreseen, individually or in the
         aggregate, in the future would have a Material Adverse Effect on the
         Company and its Subsidiaries, taken as a whole.

                  (j) CONTRACTS. (i) The Company has filed as exhibits to the
         Company SEC Documents all material agreements required to be filed
         under the rules and regulations of the SEC (the "Material Contracts").

                  (ii) All Material Contracts are legal, valid, binding, in full
         force and effect and enforceable against each party thereto, except to
         the extent that any failure to be enforceable, individually or in the
         aggregate, would not reasonably be expected to have or result in a
         Material Adverse Effect on the Company and its Subsidiaries, taken as a
         whole, PROVIDED that no representation is made as to the enforceability
         of any non-competition provision in any employment agreement. There
         does not exist under any Material Contract any violation, breach or
         event of default, or event or condition that, after notice or lapse of
         time or both, would constitute a violation, breach or event of default
         thereunder, on the part of any of the Company or its Subsidiaries or,
         to the Knowledge of the Company or any of its Subsidiaries, any other
         Person, other than such violations, breaches or events of default as
         would not, individually or in the aggregate, have a Material Adverse
         Effect on the Company and its Subsidiaries, taken as a whole. The
         enforceability of all Material Contracts will not be adversely affected
         in any manner by the execution, delivery or performance of this
         Agreement or the Registration Rights Agreement or the consummation of
         the transactions contemplated hereby or thereby, and no Material
         Contract contains any change in control or other terms or conditions
         that will become applicable or inapplicable as a result of the
         consummation of such transactions. Except as set forth on Schedule
         2.1(j), neither the Company nor the Subsidiaries are a party to any
         contract prohibiting or materially restricting the ability of the
         Company or any of its Subsidiaries to conduct its business, to engage
         in any business or operate in any geographical area or compete with any
         person.

                  (k) TAXES. (i) Except as set forth on Schedule 2.1(k), (A) all
         Tax Returns required to be filed by or on behalf of each of the Company
         and its Subsidiaries have been filed except to the extent that a
         failure to file, individually or in the aggregate, would not have a
         Material Adverse Effect on the Company and its Subsidiaries, taken as a
         whole; (B) all such filed Tax Returns are complete and accurate in all
         respects,

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<PAGE>   10


         other than any incompleteness or any inaccuracy that would not,
         individually or in the aggregate, have a Material Adverse Effect on the
         Company and its Subsidiaries, taken as a whole, and all Taxes shown to
         be due on such Tax Returns have been paid; (C) no written claim (other
         than a claim that has been finally settled) has been made by a taxing
         authority that any of the Company or its Subsidiaries is subject to an
         obligation to file Tax Returns or to pay or collect Taxes imposed by
         any jurisdiction in which either the Company or its Subsidiaries does
         not file Tax Returns or pay or collect Taxes, other than any such claim
         that would not have a Material Adverse Effect on the Company and its
         Subsidiaries, taken as a whole, or for which adequate reserves have
         been provided on the balance sheets contained in the Company SEC
         Documents filed with the SEC and publicly available prior to the date
         hereof; (D) there is no deficiency with respect to any Taxes which
         would, individually or in the aggregate, have a Material Adverse Effect
         on the Company and its Subsidiaries, taken as a whole, other than any
         such deficiency for which adequate reserves have been provided on the
         balance sheet contained in the financial statements in the SEC Company
         Documents filed with the SEC and publicly available prior to the date
         hereof; and (E) all material assessments for Taxes due with respect to
         completed and settled examinations or concluded litigation have been
         paid which, individually or in the aggregate, exceed $100,000. As used
         in this Agreement, "Taxes" shall include all federal, state, local and
         foreign income, franchise, property, sales, excise and other taxes,
         tariffs or governmental charges of any nature whatsoever, including
         interest and penalties, and additions thereto; and "Tax Returns" shall
         mean all federal state, local and foreign tax returns, declarations,
         statements, reports, schedules, forms and information returns relating
         to Taxes.

                  (ii) Except as set forth on Schedule 2.1(k), each of the
         Company and its Subsidiaries has duly and timely withheld all Taxes
         required to be withheld in connection with its business and assets, and
         such withheld Taxes have been either duly and timely paid to the proper
         governmental authorities or properly set aside in accounts for such
         purpose, except to the extent that any failure to do so would not have
         a Material Adverse Effect on the Company and its Subsidiaries, taken as
         a whole.

                  (iii) Except as set forth on Schedule 2.1(k), (A) all taxable
         periods of each of the Company and its Subsidiaries ending before
         December 31, 1993 are closed or no longer subject to audit; (B) none of
         the Company or any of its Subsidiaries is currently under any audit by
         any taxing authority as to which such taxing authority has asserted in
         writing any claim which, if adversely determined, could have a Material
         Adverse Effect on the Company and its Subsidiaries, taken as a whole;
         and (C) no waiver of the statute of limitations is in effect with
         respect to any taxable year of the Company or any of its Subsidiaries.

                  (iv) Except as set forth on Schedule 2.1(k), (A) none of the
         Company or its Subsidiaries is a party to or bound by or has any
         obligation under any Tax allocation,


                                       7
<PAGE>   11


         sharing, indemnification or similar agreement or arrangement with any
         Person which might result in a Material Adverse Effect to the Company
         or Subsidiary which entered into such agreement or arrangement; and (B)
         none of the Company or its Subsidiaries is or has been at any time a
         member of any group of companies filing a consolidated, combined or
         unitary income tax return other than any such group the common parent
         of which is the Company.

                  (l) EMPLOYEE BENEFIT PLANS AND RELATED MATTERS; ERISA. (i)
         Employee Benefit Plans. Each Employee Benefit Plan that provides for
         equity-based compensation or that has associated costs that are
         expected to be material to the Company or its Subsidiaries in the
         aggregate and that is expected to provide for contributions to be made
         by the Company or its Subsidiaries or their Employees after the date
         hereof or to permit the accrual of additional benefits by any Employee
         of the Company or its Subsidiaries after the date hereof is either
         listed on Schedule 2.1(l) or has been filed with the SEC as a material
         contract (collectively, the "Plans").

                  (ii) QUALIFICATION. Except to the extent that failure to meet
         the requirements of Section 401(a) of the Code would not result in any
         material liability as to which adequate reserves have not been
         established, each Employee Benefit Plan intended to be qualified under
         section 401(a) of the Code, and the trust (if any) forming a part
         thereof, (A) has received a favorable determination letter from the IRS
         as to its qualification under the Code and to the effect that each such
         trust is exempt from taxation under section 501(a) of the Code, and
         nothing has occurred since the date of such determination letter that
         could adversely affect such qualification or tax-exempt status or (B) a
         timely application for such a favorable determination letter was filed
         and the Company has no reason to believe that such a favorable
         determination letter will not be granted.

                  (iii) COMPLIANCE; LIABILITY. (A) No liability has been or is
         reasonably expected to be incurred under or pursuant to Title I or IV
         of ERISA or the penalty, excise Tax or joint and several liability
         provisions of the Code relating to employee benefit plans that is or
         would be material to the Company and its Subsidiaries, taken as a
         whole.

                           (B) Each of the Employee Benefit Plans has been
                  operated and administered in all respects in compliance with
                  its terms, all applicable laws and all applicable collective
                  bargaining agreements, except for any failure so to comply
                  that, individually and in the aggregate, could not reasonably
                  be expected to result in a material liability or obligation on
                  the part of the Company and its Subsidiaries in the aggregate.
                  There are no pending or threatened claims by or on behalf of
                  any of the Employee Benefit Plans, by any Employee or
                  otherwise involving any such Employee Benefit Plan or the
                  assets of any Employee Benefit Plan (other than routine claims
                  for benefits or actions seeking qualified domestic relations
                  orders or qualified medical child 


                                       8

<PAGE>   12


                  support orders, all of which have been fully reserved for on 
                  the regularly prepared balance sheets of the Company or its 
                  Subsidiaries, as applicable) which would reasonably be 
                  expected to result in any material liability to the Company 
                  and its Subsidiaries in the aggregate.

                           (C) Except to the extent that it would not give rise
                  to a material liability or obligation on the part of the
                  Company and it Subsidiaries in the aggregate, no Employee is
                  or will become entitled to post-employment benefits of any
                  kind by reason of employment with the Company or its
                  Subsidiaries, including, without limitation, death or medical
                  benefits (whether or not insured), other than (x) coverage
                  mandated by section 4980B of the Code or other applicable
                  laws, (y) retirement benefits payable under any Plan qualified
                  under section 401 (a) of the Code or (z) accrued deferred
                  compensation. The consummation of the transactions hereunder
                  and under the Registration Rights Agreement will not result in
                  an increase in the amount of compensation or benefits or the
                  acceleration of the vesting or timing of payment of any
                  compensation or benefits payable to or in respect of any
                  Employee by any of the Company or its Subsidiaries.

                  (iv) EMPLOYEES, LABOR MATTERS, ETC. Neither the Company nor
         any of its Subsidiaries is a party to or bound by any collective
         bargaining agreement, and there are no labor unions or other
         organizations representing, purporting to represent or attempting to
         represent any employees employed by the Company or any of its
         Subsidiaries. Since June 30, 1998, there has not occurred or been
         threatened any strike, slowdown, picketing, work stoppage, concerted
         refusal to work overtime or other similar labor activity with respect
         to any employees of the Company or any of its Subsidiaries. Except as
         set forth on Schedule 2.1(l), there are no labor disputes currently
         subject to any grievance procedure, arbitration or litigation and there
         is no petition pending or threatened with respect to any employee of
         the Company or its Subsidiaries. The Company and each of its
         Subsidiaries has complied with all applicable laws pertaining to the
         employment or termination of employment of their respective employees,
         including, without limitation, all such laws relating to labor
         relations, equal employment opportunities, fair employment practices,
         prohibited discrimination or distinction and other similar employment
         activities, except for any failure so to comply that, individually and
         in the aggregate, could not result in a Material Adverse Effect on the
         Company and its Subsidiaries, taken as a whole.

                  (m) ENVIRONMENTAL MATTERS. Except as disclosed in the Company
         SEC Documents filed with the SEC and publicly available prior to the
         date hereof and except for such matters that, individually or in the
         aggregate, would not have a Material Adverse Effect on the Company and
         its Subsidiaries, taken as a whole, (i) the Company and its
         Subsidiaries are in compliance with all applicable Environmental Laws
         (as defined below), (ii) the Company and its Subsidiaries have all
         Permits required under Environmental Laws for the operation of their
         respective businesses as 


                                       9
<PAGE>   13


         presently conducted ("Environmental Permits"), (iii) neither the
         Company nor its Subsidiaries has received notice from any Governmental
         Entity asserting that either the Company or any of its Subsidiaries may
         be in violation of, or liable under, any Environmental Law, and (iv)
         there are no actions, proceedings or claims pending (or, to the
         Knowledge of the Company or any of its Subsidiaries, threatened)
         seeking to impose any liability on Environmental Permits or Hazardous
         Substances (as defined below).

                  For purposes of this Agreement, "Environmental Law" means any
         federal, state, local or foreign law, statute, regulation or decree
         relating to (x) the protection of the environment or (y) the use,
         storage, treatment, generation, transportation, processing, handling,
         release or disposal of Hazardous Substances in each case as in effect
         on the date hereof. "Hazardous Substance" means any waste, substance,
         material, pollutant or contaminant listed, defined, designated or
         classified as hazardous, toxic or radioactive, or otherwise regulated,
         under any Environmental Law.

                  (n) DELAWARE LAW. The Company has taken all action necessary
         to ensure that the provisions of Section 203 of the Delaware General
         Corporation law (the "DGCL") will not be applicable to Purchaser or its
         Affiliates as a result of the transactions contemplated by this
         Agreement.

                  (o) STATUS OF SHARES. The Shares being issued at the Closing
         have been duly authorized by all necessary corporate action on the part
         of the Company, and at Closing such Shares will have been validly
         issued and, assuming payment therefor has been made, will be fully paid
         and nonassessable, and the issuance of such Shares will not be subject
         to preemptive rights of any other stockholder of the Company. The
         Shares will be eligible for listing on the Nasdaq Stock Market.

                  (p) INTELLECTUAL PROPERTY. The Intellectual Property that is
         owned by the Company or any of its Subsidiaries is owned free from any
         Liens (other than Permitted Liens), except where the failure to be free
         from liens would not have a Material Adverse Effect on the Company and
         its Subsidiaries, taken as a whole. All material Intellectual Property
         Licenses are in full force and effect in accordance with their terms,
         and are free and clear of any Liens (other than Permitted Liens),
         except where the failure to be free from Liens or to be in full force
         and effect would not have a Material Adverse Effect on the Company and
         its Subsidiaries, taken as a whole. To the Knowledge of the Company,
         the conduct of the business of the Company and its Subsidiaries does
         not infringe or conflict with the rights of any third party in respect
         of any Intellectual Property, except where such conduct would not
         materially affect the ability of the Company and its Subsidiaries to
         conduct their business as presently conducted. To the Knowledge of the
         Company, none of the Company Intellectual Property is being infringed
         by any third party except where such infringement would not have a
         Material Adverse Effect on Company and its Subsidiaries, taken as a
         whole. There is no claim or demand of any Person pertaining to, or any
         proceeding 


                                       10
<PAGE>   14



         which is pending or, to the Knowledge of the Company, threatened, that
         challenges the rights of the Company or any of its Subsidiaries in
         respect of any Company Intellectual Property, or that claims that any
         default exists under any Intellectual Property License, except where
         such claim, demand or proceeding would not materially affect the
         ability of the Company and its Subsidiaries to conduct their business
         as presently conducted. For purposes of this Agreement, "Company
         Intellectual Property" means the Intellectual Property that is owned by
         the Company and its Subsidiaries and the Intellectual Property subject
         to written or oral licenses, agreements or arrangements pursuant to
         which its use by the Company or any of its Subsidiaries is permitted by
         any Person.

                  (q) BROKERS OR FINDERS. No agent, broker, investment banker or
         other firm is or will be entitled to any broker's or finder's fee or
         any other commission or similar fee in connection with any of the
         transactions contemplated by this Agreement.

         Section 2.2  REPRESENTATIONS AND WARRANTIES OF PURCHASER.

         Purchaser hereby represents and warrants to the Company as follows:

         (a) ORGANIZATION. Purchaser is a corporation duly organized and validly
existing and in good standing under the laws of the Federal Republic of Germany,
with all requisite power and authority to own, lease and operate its properties
and to conduct its business as now being conducted.

         (b) AUTHORITY. Purchaser has the requisite corporate power and
authority to execute, deliver and perform its obligations under this Agreement
and to consummate the transactions contemplated hereby and thereby. All
necessary corporate action required to have been taken by or on behalf of
Purchaser by applicable law or otherwise to authorize the approval, execution,
delivery and performance by Purchaser of this Agreement and the Registration
Rights Agreement and the consummation by it of the transactions contemplated
hereby and thereby have been duly authorized, and no other proceedings on its
part are or will be necessary to authorize this Agreement or the Registration
Rights Agreement or for it to consummate such transactions. This Agreement is,
and the Registration Rights Agreement, when executed and delivered will be,
valid and binding agreements of Purchaser, enforceable against Purchaser in
accordance with their respective terms, assuming that this Agreement and the
Registration Rights Agreement are valid and binding agreements of the Company,
subject as to enforcement of remedies to applicable bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting generally the enforcement
of creditors' rights and subject to a court's discretionary authority with
respect to the granting of a decree ordering specific performance or other
equitable remedies.

         (c) CONFLICTING AGREEMENTS AND OTHER MATTERS. Neither the execution and
delivery of this Agreement or the Registration Rights Agreement nor the
performance by Purchaser of its obligations hereunder or thereunder will
conflict with, result in a breach of the terms, 

                                       11
<PAGE>   15

conditions or provisions of, constitute a default under, result in the creation
of any Lien upon any of the properties or assets of Purchaser pursuant to, or
require any consent, approval or other action by or any notice to or filing with
any Government Entity pursuant to, the organizational documents or agreements of
Purchaser or any agreement, instrument, order, judgment, decree, statute, law,
rule or regulation by which Purchaser is bound (assuming that the Company shall
have made or obtained all consents, approvals, orders, authorizations or filings
referred to in Section 2.1(c)(ii)), except for filings after the Closing under
Section 13(d) of the Exchange Act and filings under the HSR Act.

         (d) ACQUISITION FOR INVESTMENT. (i) Purchaser is acquiring the Shares
for its own account for the purpose of investment and not with a view to or for
sale in connection with any distribution thereof, and Purchaser has no present
intention to effect, or any present or contemplated plan, agreement,
undertaking, arrangement, obligation, indebtedness, or commitment providing for,
any distribution of Shares, (ii) Purchaser is an "accredited investor" as
defined in Rule 501(a) under the Securities Act, (iii) Purchaser has carefully
reviewed the representations concerning the Company contained in this Agreement
and has made detailed inquiry concerning the Company, its business and its
personnel, and (iv) Purchaser has sufficient knowledge and experience in finance
and business that it is capable of evaluating the risks and merits of its
investment in the Company and is able financially to bear the risks thereof.

         (e) BROKERS OR FINDERS. Except as set forth on Schedule 2.2(e), no
agent, broker, investment banker or other firm is or will be entitled to any
broker's or finder's fee or any other commission or similar fee from Purchaser
in connection with any of the transactions contemplated by this Agreement


                                   ARTICLE III

                        EQUITY PURCHASES FROM THE COMPANY


         Section 3.1 SUBSCRIPTION RIGHTS. So long as Purchaser has not sold more
than 1,000,000 Shares (as adjusted to reflect any stock splits, stock dividends
and similar recapitalizations) (other than sales to Affiliates), if the Company
proposes the issuance of New Securities (other than any New Securities issued
(i) to officers, employees, directors, consultants or advisors of the Company or
any of its Subsidiaries pursuant to any employee stock offering, plan or
arrangement, (ii) in connection with any acquisition of another corporation by
the Company by merger, purchase of all or substantially all of such
corporation's assets or other reorganization (iii) in connection with
Pharmaceutical Alliances, (iv) in connection with Research and Development
Funding Transactions, (v) in connection with equipment leasing or equipment
financing arrangements, to the Person who leased or financed such equipment and
(vi) to Purchaser or its Affiliates) then, prior to each such issuance of New
Securities, the Company shall offer to Purchaser a Pro Rata Share of such New
Securities. Any offer of New Securities made to Purchaser under this Section 3.1
shall 

                                       12

<PAGE>   16


be made by notice in writing (the "Subscription Notice"). The Subscription
Notice shall set forth (i) the number of New Securities proposed to be issued to
Persons other than Purchaser and the terms of such New Securities, (ii) the
consideration (or manner of determining the consideration), if any, for which
such New Securities are proposed to be issued and the terms of payment, (iii)
the number of New Securities offered to Purchaser in compliance with the
provisions of this Section 3.1 and (iv) the proposed date of issuance of such
New Securities. Not later than 20 Business Days after its receipt of a
Subscription Notice, Purchaser shall notify the Company in writing whether it
elects to purchase all or any portion of the New Securities offered to Purchaser
pursuant to the Subscription Notice. If Purchaser shall elect to purchase any
such New Securities, the New Securities which it shall have elected to purchase
shall be issued and sold to Purchaser by the Company at the same time and on the
same terms and conditions as the New Securities are issued and sold to any other
Person. If, for any reason, the issuance of such New Securities is not
consummated, Purchaser's right to its Pro Rata Share of such issuance shall
lapse, subject to Purchaser's ongoing subscription right with respect to
issuances of New Securities at later dates or times.

         Section 3.2 ISSUANCE AND DELIVERY OF NEW SECURITIES AND VOTING STOCK.
The Company represents and covenants to Purchaser that (i) upon issuance, all
the shares of New Securities sold to Purchaser pursuant to this Article III
shall be duly authorized, validly issued, fully paid and nonassessable and shall
be approved (if outstanding securities of the Company of the same type are at
the time already approved and the New Securities are registered under the
Securities Act) for listing on the Nasdaq Stock Market or for quotation or
listing on the principal trading market for the securities of the Company at the
time of issuance, (ii) upon delivery of such shares, they shall be free and
clear of all claims and Liens of any nature and shall not be subject to any
preemptive right of any stockholder of the Company and (iii) in connection with
any such issuance, the Company shall take such actions as are specified in
Section 2.1(o) with respect to such shares. Each share issued or delivered by
the Company hereunder shall bear the legends set forth in Section 11.10.

         Section 3.3 TERMINATION OF ARTICLE III. This Article III shall
terminate upon the earlier of (i) a sale of all or substantially all of the
assets or business of the Company, by merger, sale of assets or otherwise, or
(ii) termination of the Program Term.


                                   ARTICLE IV

            LIMITATIONS ON PURCHASES OF ADDITIONAL EQUITY SECURITIES

         Section 4.1 PURCHASES OF EQUITY SECURITIES. (a) Prior to the third
anniversary of the Closing, except as permitted by Section 4.1(b), (c) or (d),
Purchaser and its Affiliates will not (and will not assist or encourage other
to) directly or indirectly in any manner:

                     (i) acquire, or agree to acquire, directly or indirectly, 
alone or in concert with others, by purchase, gift or otherwise, any direct or
indirect beneficial ownership 


                                       13

<PAGE>   17


(within the meaning of Rule 13d-3 under the Exchange Act) or interest in any
securities or direct or indirect rights, warrants or options to acquire, or
securities convertible into or exchangeable for, any securities of the Company;

                     (ii) make, or in any way participate in, directly or 
indirectly, alone or in concert with others, any "solicitation" of "proxies" to
vote (as such terms are used in the proxy rules of the SEC promulgated pursuant
to Section 14 of the Exchange Act); provided, however, that the prohibition in
this subparagraph (ii) shall not apply to solicitations exempted from the proxy
solicitation rules by Rule 14a-2 under the Exchange Act as such Rule is in
effect as of the date hereof.

                     (iii) form, join or in any way participate in a
"group" within the meaning of Section 13(d)(3) of the Exchange Act with respect
to any voting securities of the Company; or

                     (iv)  acquire or agree to acquire, directly or indirectly,
alone or in concert with others, by purchase, exchange or otherwise, (i) any of
the assets, tangible or intangible, of the Company or (ii) direct or indirect
rights, warrants or options to acquire any assets of the Company, except for
such assets as are then being offered for sale by the Company;

                     (v)   enter into any arrangement or understanding with 
others to do any of the actions restricted or prohibited under clauses (i), (ii)
or (iii) of this Section 4.1(a); or

                     (vi)  otherwise act in concert with others, to seek to 
offer to the Company or any of its stockholders any business combination,
restructuring, recapitalization or similar transaction to or with the Company or
otherwise seek in concert with others, to control, change or influence the
management, board of directors or policies of the Company or nominate any person
as a director of the Company who is not nominated by the then incumbent
directors, or propose any matter to be voted upon by the stockholders of the
Company; provided, however, that nothing in this subparagraph (vi) shall prevent
Purchaser or any Affiliate of Purchaser, either acting alone or in concert with
each other, from taking any action..

                  (b) Nothing herein shall prevent Purchaser from purchasing any
securities of the Company pursuant to the terms of this Agreement (including
through exercise of its rights under Section 3.1 hereof) and Purchaser shall not
be treated as having breached any covenant in this Agreement solely as a result
of such purchase.

                  (c) Nothing herein shall prevent Purchaser from purchasing
additional Equity Securities of the Company if (i) prior thereto Purchaser has
not sold any Shares (other than to an Affiliate of Purchaser) and (ii) after
such purchase Purchaser and its Affiliates would own fourteen and three-tenths
percent (14.3%) or less of the Total Voting Power of all Voting 

                                       14
<PAGE>   18


Securities of the Company then outstanding.

                  (d) This Section 4.1 shall terminate and Purchaser and its
Affiliates shall have the right to acquire any securities of the Company without
regard to the limitation on share ownership set forth in Section 4.1 in the
event that:

                  (i) the Company has entered into (A) a merger agreement in
which the holders of the Company's Voting Securities would cease to hold a
majority of the voting securities of the surviving corporation, (B) an agreement
to sell all or substantially all its assets, or (C) an agreement to be acquired,
business combination, consolidation or any such similar transaction, in each
case with any Person other than a wholly-owned subsidiary of the Company;
PROVIDED, HOWEVER, the limitation shall continue if (i) the merger agreement is
with a majority-owned subsidiary of the Company and the Company is to be the
surviving corporation in the merger, or (ii) the merger agreement or other
agreements referred to in sections (A), (B), or (C) is subsequently terminated
or the transactions contemplated thereunder are not consummated; or

                  (ii) a tender or exchange offer (other than a tender or
exchange offer that the Company's Board of Directors has recommended be
rejected) is made by any Person or 13D Group (as hereinafter defined) (other
than an Affiliate of, or any Person acting in concert with, Purchaser) to
acquire Voting Securities which, if added to the Voting Securities (if any)
already owned by such Person or 13D Group, would result, if consummated in
accordance with its terms, in the Beneficial Ownership by such Person or 13D
Group of more than 30% of the Total Voting Power of all Voting Securities of the
Company then outstanding, PROVIDED THAT the limitation shall be reinstated if
such tender or exchange offer is withdrawn or terminated without such Person or
13D Group acquiring such 30% ownership level; or

         (iii) it is publicly disclosed or Purchaser otherwise learns that
Voting Securities representing more than 35% of the Total Voting Power of all
Voting Securities of the Company then outstanding are Beneficially Owned by any
Person or 13D Group (other than an Affiliate of, or any person acting in concert
with, Purchaser); or

         (iv) a proxy contest (or similar incident) is made by any Person of 13D
Group (other than an Affiliate of, or any Person acting in concert with,
Purchaser) to elect individuals who at the beginning of any calendar year did
not constitute the majority of the members of the Board of Directors of the
Company then in office and the Purchaser, upon the advice of legal counsel and
financial advisors, reasonably believes in good faith, that such proxy contest
will result in the election of individuals who will constitute a majority of
members of the Board of Directors of the Company, but who did not, at the
beginning of the calendar year, constitute the majority of the members of the
Board of Directors of the Company then in office, PROVIDED THAT the limitation
shall be reinstated if such proxy contest or similar incident is terminated or
withdrawn without affecting the change in the Board of Directors referred to
above.

                                       15
<PAGE>   19


         (e) As used herein, the term "13D Group" shall mean any group of
Persons formed for the purpose of acquiring, holding, voting or disposing of
Voting Securities which would be required under Section 13(d) of the Exchange
Act and the rules and regulations thereunder (as now in effect and based on
present legal interpretations thereof) to file a statement on Schedule 13D with
the SEC as a "person" within the meaning of Section 13(d)(3) of the Exchange
Act. Ownership of Voting Securities under Section 4.1(d) above and Section
5.1(d) below shall be determined in accordance with Rule 13d-3 of the Exchange
Act as currently in effect.


                                    ARTICLE V

                            TRANSFER OF COMMON STOCK

         Section 5.1 LIMITATIONS ON TRANSFER. (a) Prior to the second 
anniversary of the Closing, Purchaser will not, directly or indirectly, sell, 
transfer or otherwise dispose of any Shares (except to any Affiliate of 
Purchaser).

         (b) After the second anniversary of the Closing and prior to the
expiration of the Program Term, Purchaser will not, directly or indirectly,
sell, transfer or otherwise dispose of, in any one calendar year, more than
2,500,000 Shares (as adjusted to reflect any stock splits, stock dividends and
similar recapitalizations) (except to an Affiliate of Purchaser).

         (c) (i) After the second anniversary of the Closing and prior to the
expiration of the Program Term, if Purchaser proposes to sell any Shares other
than pursuant to a registration statement under the Securities Act, the Company
shall have a right of first negotiation with respect to the acquisition of those
Shares proposed to be sold. Said right of first negotiation must be asserted by
the Company within fifteen (15) Business Days of its receipt of written notice
from Purchaser of its intention to sell any Shares other than pursuant to a
registration statement under the Securities Act. Such notice shall specify the
general terms and conditions on which Purchaser intends to sell such Shares. If
the Company and Purchaser are unable to agree on the terms and conditions of the
Company's acquisition of such Shares within fifteen (15) Business Days of
Purchaser's notification to the Company of its intention to sell Shares,
Purchaser may sell such Shares at any time within the next 120 days on terms and
conditions no more favorable, taken as a whole, than those originally proposed
to the Company. (ii) After the second anniversary of the Closing and prior to
the expiration of the Program Term, if Purchaser proposes to sell any Shares
pursuant to a registration statement under the Securities Act, the Company shall
have a right of first negotiation with respect to the acquisition of those
Shares proposed to be sold. Said right of first negotiation must be asserted by
the Company within fifteen (15) Business Days of its receipt of written notice
from Purchaser of its intention to sell any Shares pursuant to a registration
statement under the Securities Act. Such notice shall specify the price for such
Shares, which shall be equal to the average closing price of the Common Stock
over the 30-day period ending on the date of such notice. If the Company does
not agree to purchase such Shares at such price within 

                                       16
<PAGE>   20


fifteen (15) Business Days of Purchaser's notification to the Company of its
intention to sell Shares, Purchaser may sell such Shares at any time within the
next 180 days pursuant to a registration statement without restriction as to the
price at which such Shares may be sold.

                  (d) The limitation on share transfer set forth in this Section
5.1 shall terminate and Purchaser and its Affiliates shall have the right,
directly or indirectly, to sell, transfer or otherwise dispose of any Shares
without regard to any limitation on share transfer set forth in Section 5.1 in
the event that:

                  (i) the Company has entered into (A) a merger agreement in
which the holders of the Voting Securities would cease to hold a majority of the
voting securities of the surviving corporation, (B) an agreement to sell all or
substantially all its assets, or (C) an agreement to be acquired, business
combination, consolidation or any such similar transaction, in each case with
any Person other than a wholly-owned subsidiary of the Company; PROVIDED,
HOWEVER, the limitation shall (1) continue if (x) the merger agreement is with a
majority-owned subsidiary of the Company and the Company is to be the surviving
corporation in the merger or (y) the majority of the directors of the Company,
who have held that position for at least nine (9) months prior to the entering
into of the merger agreement continue as the directors of the surviving company
after the merger or (2) be reinstated if such merger agreement or other
agreements referred to in sections (A), (B) or (C) is subsequently terminated or
the transactions contemplated thereunder are not consummated;

                  (ii) a tender or exchange offer (other than a tender or
exchange offer that the Company's Board of Directors has recommended be
rejected) is made by any Person or 13D Group (other than an Affiliate of, or any
Person acting in concert with, Purchaser) to acquire Voting Securities which, if
added to the Voting Securities (if any) already owned by such Person or 13D
Group, would result, if consummated in accordance with its terms, in the
Beneficial Ownership by such Person or 13D Group of more than 50% of the Total
Voting Power of all Voting Securities of the Company then outstanding, PROVIDED
THAT the limitation shall be reinstated if such tender or exchange offer is
withdrawn or terminated without such Person or 13D Group acquiring such 50%
ownership level; or

                  (iii) a tender or exchange offer, which the Company's Board of
Directors has not approved or recommended, is made by any Person or 13D Group
(other than an Affiliate of, or any Person acting in concert with, Purchaser) to
acquire Voting Securities which, if added to the Voting Securities (if any)
already owned by such Person or 13D Group, would result, if consummated in
accordance with its terms, in the Beneficial Ownership by such Person or 13D
Group of more than 50% of the Total Voting Power of all Voting Securities of the
Company then outstanding and the Purchaser, upon the advice of legal counsel and
financial advisors, reasonably believes in good faith, taking into account the
conditions of the offer, that such tender or exchange offer will result in
Voting Securities being purchased, PROVIDED THAT the limitation shall be
reinstated if such tender or exchange offer is withdrawn or terminated without
such Person or 13D Group acquiring such 50% ownership level.

                                       17

<PAGE>   21



                                   ARTICLE VI

                       COVENANTS AND ADDITIONAL AGREEMENTS

         Section 6.1 ORDINARY COURSE. During the period from the date of this
Agreement and continuing until the Closing, the Company agrees as to itself and
its Subsidiaries that, except as set forth in Schedule 6.1, or to the extent
that Purchaser otherwise consents in writing, the Company and its Subsidiaries
shall conduct their respective businesses in the ordinary course in
substantially the same manner as presently conducted.

         Section 6.2 ACCESS AND INFORMATION. So long as this Agreement remains
in effect, prior to the Closing, the Company will (and will cause each of its
Subsidiaries and each of their respective accountants, counsel, consultants,
officers, directors, employees, agents and representatives of or to any of the
Subsidiaries, to) give Purchaser and its Representatives, reasonable access
during reasonable business hours to all of their respective properties, assets,
books, contracts, reports and records relating to the Company and its
Subsidiaries, and furnish to them all such documents, records and information
with respect to the properties, assets and business of the Company and its
Subsidiaries, as Purchaser shall from time to time reasonably request. The
Company will keep Purchaser generally informed as to the business of the Company
and its Subsidiaries.

         Section 6.3 FURTHER ACTIONS. (a) Each of the Company and Purchaser
shall use reasonable best efforts to take or cause to be taken all actions, and
to do or cause to be done all other things, necessary, proper or advisable in
order to fulfill and perform its obligations in respect of this Agreement and
the Registration Rights Agreement, or otherwise to consummate and make effective
the transactions contemplated hereby and thereby.

         (b) Each of the Company and Purchaser shall, as promptly as
practicable, (i) make, or cause to be made, all filings and submissions
(including but not limited to under the HSR Act and foreign antitrust filings)
required under any law applicable to it or any of its Subsidiaries, and give
such reasonable undertakings as may be required in connection therewith, and
(ii) use all reasonable efforts to obtain or make, or cause to be obtained or
made, all Permits necessary to be obtained or made by it or any of its
Subsidiaries, in each case in connection with this Agreement and the
Registration Rights Agreement, the sale and transfer of the Shares pursuant
hereto and the consummation of the other transactions contemplated hereby or
thereby.

         (c) Each of the Company and Purchaser shall coordinate and cooperate
with the other party in exchanging such information and supplying such
reasonable assistance as may be reasonably requested by such other party in
connection with the filings and other actions contemplated by this Agreement and
the Registration Rights Agreement.


                                       18
<PAGE>   22


         (d) At all times prior to the Closing Date, the Company and Purchaser
shall promptly notify each other in writing of any fact, condition, event or
occurrence that could reasonably be expected to result in the failure of any of
the conditions contained in Article VII to be satisfied, promptly upon becoming
aware of the same.

         Section 6.4 FURTHER ASSURANCES. Following the Closing Date, the Company
shall, from time to time, execute and deliver such additional instruments,
documents, conveyances or assurances and take such other actions as shall be
necessary, or otherwise reasonably be requested by Purchaser, to confirm and
assure the rights and obligations provided for in this Agreement and the
Registration Rights Agreement and render effective the consummation of the
transactions contemplated hereby and thereby, or otherwise to carry out the
intent and purposes of this Agreement.

         Section 6.5 BOARD ATTENDANCE RIGHTS AND RIGHTS TO INFORMATION ABOUT
BOARD AND THE COMPANY. (a) The Company shall permit a Representative (which
Representative shall be designated by Purchaser) to attend two (2) regular
meetings of the Company's Board of Directors each year during the Program Term
(one such meeting to be held between January and June of each year, and the
second such meeting to be held between July and December of each year);
PROVIDED, HOWEVER, that, at the request of the Board of Directors, the
Representative shall excuse himself or herself from such portions of the Board
of Directors meeting that he or she is attending if the Board of Directors and
its legal counsel believe it to be necessary so to conduct the Board=s business
(i) in order to avoid conflict with the Company's obligations to third parties
(such as, but not limited to, discussions of proprietary aspects of
collaborations with third parties), (ii) discussions of disputes with Purchaser
and/or its Affiliates, and (iii) any other matters in which the Board of
Directors and such legal counsel reasonably conclude that the exercise of the
fiduciary duties of the Board of Directors requires such Representative to
excuse himself or herself.

         (b) The Company shall provide to the Purchaser copies of agendas for
all Board meetings prior to such meetings and the minutes for all Board
meetings, including meetings of committees of the Board, as soon as practicable
after the conclusion of any such meeting.

         (c) Once during each calendar quarter, at the request of Purchaser, a
Representative (which Representative shall be designated by Purchaser) will be
entitled to receive a confidential briefing from officers of the Company
concerning the Company's business and strategy, including a report on the status
of the Discovery Program and the Company's other collaborative programs (to the
extent not precluded by the terms and conditions under any such collaboration
program) and the Company's consolidated corporate strategic overview.


                                   ARTICLE VII

                              CONDITIONS PRECEDENT


                                       19
<PAGE>   23


         Section 7.1 EACH PARTY'S OBLIGATIONS. The obligations of the Company
and Purchaser to consummate the transactions contemplated to occur at the
Closing shall be subject to the satisfaction prior to the Closing of each of the
following conditions, each of which may be waived only if it is legally
permissible to do so:

         (a) HSR AND OTHER APPROVALS. Any applicable waiting period under the
HSR Act relating to the transactions contemplated hereby shall have expired or
been terminated, and all other material authorizations, consents, orders or
approvals of, or regulations, declarations or filings with, or expirations of
applicable waiting periods imposed by, any Governmental Entity (including,
without limitation, any foreign antitrust filing) necessary for the consummation
of the transactions contemplated hereby, shall have been obtained or filed or
shall have occurred.

         (b) NO LITIGATION, INJUNCTIONS, OR RESTRAINTS. No statute, rule,
regulation, executive order, decree, temporary restraining order, preliminary or
permanent injunction or other order enacted, entered. promulgated, enforced or
issued by any Governmental Entity or other legal restraint or prohibition
preventing the consummation of the transactions contemplated by this Agreement
and the Registration Rights Agreement shall be in effect.

         (c) COLLABORATION AGREEMENT. The Collaboration Agreement shall have
become effective in accordance with the terms and conditions thereof.

         (d) NASDAQ LISTING. The Shares shall have been approved for listing on
the Nasdaq Stock Market, subject only to official notice of issuance.

         Section 7.2 CONDITIONS TO THE OBLIGATIONS OF THE COMPANY. The
obligations of the Company to consummate the transactions contemplated to occur
at the Closing shall be subject to the satisfaction or waiver thereof prior to
the Closing of each of the following conditions:

         (a) REPRESENTATIONS AND WARRANTIES. The representations and warranties
of Purchaser that are qualified as to materiality shall be true and correct, and
those that are not so qualified shall be true and correct in all material
respects, as of the date of this Agreement and as of the time of the Closing as
though made at and as of such time, except to the extent such representations
and warranties expressly relate to an earlier date (in which case such
representations and warranties that are qualified as to materiality shall be
true and correct, and those that are not so qualified shall be true and correct
in all material respects, on and as of such earlier date) and the Company shall
have received a certificate signed by an authorized officer of Purchaser to such
effect.

         (b) REGISTRATION RIGHTS AGREEMENT. Purchaser shall have executed and
delivered the Registration Rights Agreement.

         Section 7.3 CONDITIONS TO THE OBLIGATIONS OF PURCHASER. The obligations
of 

                                       20
<PAGE>   24


Purchaser to consummate the transactions contemplated to occur at the Closing
shall be subject to the satisfaction or waiver thereof prior to the Closing of
each of the following conditions:

         (a) REPRESENTATIONS AND WARRANTIES. The representations and warranties
of the Company set forth in this Agreement that are qualified as to materiality
shall be true and correct, and those that are not so qualified shall be true and
correct in all material respects, as of the date of this Agreement and as of the
time of the Closing as though made at and as of such time, except to the extent
such representations and warranties expressly relate to an earlier date (in
which case such representations and warranties that are qualified as to
materiality shall be true and correct, and those that are not so qualified shall
be true and correct in all material respects, on and as of such earlier date),
and Purchaser shall have received a certificate signed by the chief executive
officer and chief financial officer of the Company to such effect.

         (b) OPINION OF THE COMPANY'S COUNSEL. Purchaser shall have received the
opinion dated as of the Closing of Hale and Dorr, counsel to the Company, in
form and substance reasonably satisfactory to Purchaser.

         (c) REGISTRATION RIGHTS AGREEMENT. The Company shall have executed and
delivered the Registration Rights Agreement.

         (d) PERFORMANCE OF OBLIGATIONS OF THE COMPANY. The Company shall have
performed or complied in all material respects with all obligations and
covenants required to be performed or complied with by the Company under this
Agreement and the Purchaser shall have received a certificate signed by the
chief executive officer and chief financial officer of the Company to such
effect.

         (e) CORPORATE PROCEEDINGS. All corporate proceedings of the Company in
connection with the transactions contemplated by this Agreement and the
Registration Rights Agreement, and all documents and instruments incident
thereto, shall be satisfactory in form and substance to Purchaser and its
counsel, and Purchaser and its counsel shall have received all such documents
and instruments, or copies thereof, certified or requested, as may be reasonably
requested.


                                  ARTICLE VIII

                                   TERMINATION

         Section 8.1  TERMINATION.  This Agreement may be terminated at any time
prior to the Closing:

                  (a)  by mutual written consent of Purchaser and the Company;

                                       21
<PAGE>   25

                  (b)  by Purchaser or the Company;

                      (i) if the Closing shall not have occurred prior to
                  December 31, 1998, PROVIDED, THAT the right to terminate this
                  Agreement pursuant to this clause (i) shall not be available
                  to any party whose failure to fulfill any obligation under
                  this Agreement results in the failure of the Closing to occur;

                      (ii) if there shall be any statute, law, regulation or
                  rule that makes consummating the transactions contemplated
                  hereby illegal or if any court or other Governmental Entity of
                  competent jurisdiction shall have issued a judgment, order,
                  decree or ruling, or shall have taken such other action
                  restraining, enjoining or otherwise prohibiting the
                  consummation of the transactions contemplated hereby and such
                  judgment, order, decree or ruling shall have become final and
                  non-appealable; or

                      (iii) if the Collaboration Agreement shall have
                  terminated;

                  (c)  by Purchaser:

                      (i) if the Company shall have failed to perform in any
                  material respect any of its obligations hereunder or shall
                  have breached in any respect any representation or warranty
                  contained herein qualified by materiality or shall have
                  breached in any material respect any representation or
                  warranty not so qualified, and the Company has failed to
                  perform such obligation or cure such breach, within 30 days of
                  its receipt of written notice thereof from Purchaser, and such
                  failure to perform shall not have been waived in accordance
                  with the terms of this Agreement; or

                      (ii) if any of the conditions set forth in Section 7.1 or
                  7.3 shall become impossible to fulfill (other than as a result
                  of any breach by Purchaser of the terms of this Agreement) and
                  shall not have been waived in accordance with the terms of
                  this Agreement;

                  (d)  by the Company:

                      (i) if Purchaser shall have failed to perform in any
                  material respect any of its obligations hereunder or shall
                  have breached in any respect any representation or warranty
                  contained herein qualified by materiality or shall have
                  breached any material respect any representation or warranty
                  not so qualified, and Purchaser has failed to perform such
                  obligation or cure such breach, within 30 days of its receipt
                  of written notice thereof from the Company, and such failure
                  to perform shall not have been waived in accordance with the
                  terms of this Agreement; or

                                       22

<PAGE>   26

                      (ii) if any of the conditions set forth in Section 7.1 or
                  7.2 shall become impossible to fulfill (other than as a result
                  of any breach by the Company of the terms of this Agreement)
                  and shall not have been waived in accordance with the terms of
                  this Agreement;

         Section 8.2 EFFECT OF TERMINATION. In the event of termination of this
Agreement by either the Company or Purchaser as provided in Section 8.1, this
Agreement shall forthwith become void and have no effect, without any liability
or obligation on the part of Purchaser or the Company, other than the provisions
of this Section 8.2, Section 11.9 and Article IX and except to the extent that
such termination results from the willful and material breach by a party of any
of its representations, warranties, covenants or agreements set forth in this
Agreement.


                                   ARTICLE IX

                                 INDEMNIFICATION

         Section 9.1 INDEMNIFICATION OF PURCHASER. The Company covenants and
agrees to defend, indemnify and hold harmless each of Purchaser, its Affiliates
(other than the Company and any of its Subsidiaries), and their respective
officers, directors, partners, employees, agents, advisers and representatives
(collectively, the "Purchaser Indemnities") from and against, and pay or
reimburse the Purchaser Indemnitees for, any and all claims, demands,
liabilities, obligations, losses, costs, expenses, fines or damages (whether
absolute, accrued, conditional or otherwise and whether or not resulting from
third party claims), including interest and penalties with respect thereto and
all expenses incurred in the investigation or defense of any of the same or in
asserting, preserving or enforcing any of their respective rights hereunder
(collectively, "Losses"), resulting from or based on (or allegedly resulting
from or based on) any breach by the Company of any representation, warranty,
covenant or obligation of the Company hereunder. The Losses described in this
Section 9.1 are herein referred to as "Purchaser Indemnifiable Losses". The
Company shall reimburse the Purchaser Indemnitees for any legal or other
expenses incurred by such Purchaser Indemnitees in connection with investigating
or defending any such Purchaser Indemnifiable Losses as such expenses are
incurred.

         Section 9.2 INDEMNIFICATION PROCEDURES. Promptly after receipt by a
Purchaser Indemnitee of notice of the commencement of any action or the written
assertion of any claim, such Purchaser Indemnitee shall, if a claim in respect
thereof is to be made against the Company, as the case may be (the "Indemnifying
Person"), notify the Indemnifying Person in writing of the commencement or the
written assertion thereof. Failure by a Purchaser Indemnitee to so notify the
Indemnifying Person shall relieve the Indemnifying Person from the obligation to
indemnify such Purchaser Indemnitee only to the extent that the Indemnifying
Person suffers actual and material prejudice as a result of such failure but in
no event shall such failure to notify the Indemnifying Person (i) constitute
prejudice suffered by 

                                       23
<PAGE>   27


the Indemnifying Person if it has otherwise received notice of the actions
giving rise to such obligation to indemnify or (ii) relieve it from any
liability or obligation that it may otherwise have to such Purchaser Indemnitee.
In case any such action or claim shall be brought or asserted against any
Purchaser Indemnitee and it shall notify the Indemnifying Person of the
commencement or assertion thereof, the Indemnifying Person shall be entitled to
participate therein but the defense of such action or claim shall be conducted
by counsel to the Purchaser Indemnitee, PROVIDED, HOWEVER, that the Indemnifying
Person shall not, in connection with any one such action or proceeding or
separate but substantially similar actions or proceedings arising out of the
same general allegations, be liable for the fees and expenses of more than one
separate firm of attorneys at any time for all Purchaser Indemnitees, except to
the extent that local counsel, in addition to regular counsel, is required in
order to effectively defend against such action or proceeding and PROVIDED
FURTHER that a Purchaser Indemnitee shall not enter into any settlement of any
such claim without the prior consent of the Company, such consent not to be
unreasonably withheld or delayed. In no event shall the Company be liable under
this Article IX, and the Company's obligation to defend, indemnify and hold
harmless the Purchaser Indemnitee shall not apply to: (a) any special,
incidental or consequential damages resulting from or based upon any breach by
the Company of any representation, warranty, covenant or obligation of the
Company hereunder, (b) any Purchaser Indemnifiable Losses until the aggregate
amount of such Losses exceeds $5,000,000, and (c) any Purchaser Indemnifiable
Losses in excess of $50,000,000. The remedies set forth in this Article 9 are
cumulative and shall not be construed to restrict or otherwise affect any other
remedies that may be available to a Purchaser Indemnitee or a Party under any
other agreement, pursuant to statutory or common law or equity. Notwithstanding
anything to the contrary in this Agreement, any claim for indemnification under
this Article IX must be brought prior to the second anniversary of the Closing
Date, except for claims relating to the representations and warranties in
Sections 2.1(d), 2.1(k) and 2.1 (m) which can be brought any time prior to the
expiration of the applicable statute of limitations.

         Section 9.3 SURVIVAL OF REPRESENTATIONS AND WARRANTIES. The
representations and warranties of the Company contained in this Agreement shall
expire for all purposes on the second anniversary of the Closing Date, except
for the representations and warranties contained in Sections 2.1(d), 2.1(k) and
2.1(m) which shall expire for all purposes upon expiration of the applicable
statute of limitations.

                                    ARTICLE X

                           INTERPRETATION; DEFINITIONS

         Section 10.1 INTERPRETATION.  As used in this Agreement, unless the 
context otherwise requires:

         (a) any reference to the Company and its Subsidiaries means the Company
         and each of its Subsidiaries;

                                       24

<PAGE>   28


         (b)  words of any gender include all genders;

         (c) words using the singular or plural number also include the plural
         or singular number, respectively; and

         (d) the terms "hereof" "herein", and "hereby" and derivative or similar
         words refer to this entire Agreement.

         Section 10.2 DEFINITIONS. For purposes of this Agreement, the following
terms shall have the following meanings:

         "13D GROUP" is defined in Section 4.1(e).

         "AFFILIATE" shall have the meaning set forth in Rule 12b-2 under the
Exchange Act (as in effect on the date of this Agreement).

         "AGREEMENT" is defined in the recitals to this agreement.

         "BENEFICIALLY OWNED," "BENEFICIAL OWNERSHIP" or any like expression
with respect to any securities means having "beneficial ownership" of such
securities (as determined pursuant to Rule 13d-3 under the Exchange Act),
including pursuant to any agreement, arrangement or understanding, whether or
not in writing.

         "BUSINESS DAY" means any day on which banking institutions are open in
the City of Boston.

         "CLOSING" is defined in Section 1.2.

         "CLOSING DATE" is defined in Section 1.2.

         "CODE" means the Internal Revenue Code of 1986, as amended, and the
regulations promulgated thereunder, as amended.

         "COLLABORATION AGREEMENT" is defined in the recitals to this Agreement.

         "COMMON STOCK" is defined in the recitals to this Agreement.

         "COMPANY" is defined in the recitals to this Agreement.

         "COMPANY INTELLECTUAL PROPERTY" is defined in Section 2.1(p).

         "COMPANY STOCK PLANS" is defined in Section 2.1(d).

         "COMPANY SEC DOCUMENTS" is defined in Section 2.1(f).

                                       25
<PAGE>   29


         "CONTRACT" is defined in Section 2.1(c).

         "DISCOVERY PROGRAM" shall have the meaning set forth in the
Collaboration Agreement.

         "EMPLOYEE" means any employee or former employee of the Company or any
of its Subsidiaries or any beneficiary or dependent of any such employee or
former employee.

         "EMPLOYEE BENEFIT PLANS" means all defined contribution, defined
benefit, welfare benefit, bonus, incentive compensation, stock option, stock
purchase, stock appreciation right, stock bonus, incentive, deferred
compensation, insurance, medical, dental, vision, life, death benefit, fringe
benefit or other employee benefit plans, programs, policies or arrangements,
including without limitation, any employment, consulting, offer, secondment,
severance or other termination agreement, whether or not an employee benefit
plan within the meaning of section 3(3) of ERISA, maintained by the Company or
any of its Subsidiaries.

         "ENVIRONMENTAL LAWS" is defined in Section 2.1(m).

         "ENVIRONMENTAL PERMITS" is defined in Section 2.1(m).

         "EQUITY SECURITY" means (i) any Common Stock or other Voting
Securities, (ii) any securities of the Company convertible into or exchangeable
for Common Stock or other Voting Securities or (iii) any options, rights or
warrants (or any similar securities) issued by the Company to acquire Common
Stock or other Voting Securities.

         "ERISA" means the Employee Retirement Income Security Act of 1974, as
amended.

         "EXCHANGE ACT" means the Securities Exchange Act of 1934, as amended,
or any successor federal statute and the rules and regulations of the SEC
promulgated thereunder, all as the same shall be in effect from time to time.

         "GAAP" means United States generally accepted accounting principles.

         "GOVERNMENTAL ENTITY" is defined in Section 2.1(c).

         "HAZARDOUS SUBSTANCES" is defined in Section 2.1(m).

         "HSR ACT" is defined in Section 2.1(c).

         "INDEMNIFYING PERSON" is defined in Section 9.2.

         "INTELLECTUAL PROPERTY" means trademarks, trade names, trade dress,
service marks, copyrights, domain names, and similar rights (including
registrations and applications to register or renew the registration of any of
the foregoing), patents and patent applications, trade secrets, ideas,
inventions, improvements, practices, processes, formulas, designs, know-

                                       26
<PAGE>   30


how, confidential business and technical information, computer software,
firmware, data and documentation, licenses of or agreements relating to any of
the foregoing, rights of privacy and publicity, moral rights, and any other
similar intellectual property rights and tangible embodiments of any of the
foregoing (in any medium including electronic media.)

         "INTELLECTUAL PROPERTY LICENSE" means any license, permit,
authorization, approval, Contract or consent granted, issued by or with any
Person relating to the use of Intellectual Property.

         "IRS" means the Internal Revenue Service.

         "KNOWLEDGE OF THE COMPANY," "KNOWLEDGE OF THE COMPANY OR ANY OF ITS
SUBSIDIARIES" or any like expression means to the actual knowledge of the
persons listed on Schedule 10.2.

         "LIEN" is defined in Section 2.1(c).

         "LOSSES" is defined in Section 9.1.

         "MATERIAL ADVERSE EFFECT" on or with respect to an entity (or group of
entities taken as a whole) means any state of facts, event, change or effect
that has had, or would reasonably be expected to have, a material adverse effect
on (a) the business, properties, results of operations or financial condition of
such entity (or, if with respect thereto, of such group of entities taken as a
whole), other than any state of facts, event, change or effect attributable to
changes in general economic or market conditions affecting the biotechnology and
pharmaceutical industries, or (b) the ability of such entity (or group of
entities) to consummate the transactions contemplated under this Agreement or
the Registration Rights Agreement.

         "MATERIAL CONTRACT" is defined in Section 2.1(j).

         "NEW SECURITY" means any Equity Security issued by the Company after
the Closing; provided that "New Security" shall not include (i) any securities
issuable upon conversion of any convertible Equity Security, (ii) any securities
issuable upon exercise of any option, warrant or other similar Equity Security,
or (iii) any securities issuable in connection with any stock split, stock
dividend or recapitalization of the Company where such securities are issued to
all stockholders of the Company on a pro rata basis.

         "PERMIT" is defined in Section 2.1(c).

         "PERMITTED LIENS" means those Liens (A) securing debt that is reflected
on the balance sheets or the notes thereto contained in the Company SEC
Documents filed with the SEC and publicly available prior to the date hereof,
(B) referred to in Schedule 2.1(j), (c) for Taxes not yet due or payable or
being contested in good faith and for which adequate reserves have 

                                       27

<PAGE>   31


been established in accordance with GAAP, (D) that constitute mechanics',
carriers', workmens' or like liens, liens arising under original purchase price
conditional sales contracts and equipment leases with third parties entered into
in the ordinary course, (E) Liens incurred or deposits made in the ordinary
course of business consistent with past practice in connection with workers'
compensation, unemployment insurance and social security, retirement and other
legislation and (F) easements, covenants, declarations, rights or way,
encumbrances, or similar restrictions in connection with real property owned by
the Company of any of its Subsidiaries that do not materially impair the use of
such real property by the Company and any of its Subsidiaries, and in the case
of Liens described in clauses (B), (C), (D), (E) or (F) that, individually or in
the aggregate, would not have a material Adverse Effect on the Company and its
Subsidiaries, taken as a whole.

         "PERSON" means any individual, partnership, joint venture, corporation,
limited liability company, trust, unincorporated organization, government or
department or agency of a government or other entity.

         "PHARMACEUTICAL ALLIANCES" means a research, development,
commercialization and/or similar agreement or arrangement between the Company
and a pharmaceutical, biopharmaceutical, biotechnology or similar entity in
which Equity Securities are sold to such entity or an Affiliate of such entity.

         "PLANS" is defined in Section 2.1(l).

         "PRO RATA SHARE" means the fraction of an entire issuance of New
Securities, the numerator of which shall be the number of shares of Common Stock
and other Voting Securities owned by Purchaser and its Affiliates (other than
the Company and its Subsidiaries) on a fully-diluted basis immediately prior to
such issuance of such New Securities and the denominator of which shall be the
aggregate number of shares of Common Stock and other Voting Securities
outstanding on a fully-diluted basis (giving effect to the conversion of all
then-outstanding options, warrants and other convertible securities) immediately
prior to such issuance of such New Securities.

         "PROGRAM TERM" shall have the meaning set forth in the Collaboration
Agreement.

         "PURCHASE PRICE" is defined in Section 1.1.

         "PURCHASER" is defined in the recitals to this Agreement.

         "PURCHASER INDEMNIFIABLE LOSSES" is defined in Section 9.1.

         "PURCHASER INDEMNITEES" is defined in Section 9.1.

                                       28

<PAGE>   32


         "REGISTRATION RIGHTS AGREEMENT" is defined in the recitals to this
Agreement.

         "REPRESENTATIVE" means an agent or employee of Purchaser, or of an
independent public accounting firm, law firm, or other consulting company or
advisor of Purchaser.

         "RESEARCH AND DEVELOPMENT FUNDING TRANSACTION" means a transaction to
raise funding for the research and development of one or more specific products,
products in one or more disease areas and/or one or more specific technologies,
in each case structured as an "off balance sheet" financing. Examples of
Research and Development Funding Transactions are R&D partnerships, SWORDs and
SPARCs.

         "SEC" means the Securities and Exchange Commission.

         "SECURITIES ACT" means the Securities Act of 1933, as amended, or any
successor federal statute, and the rules and regulations of the SEC promulgated
thereunder, all as the same shall be in effect from time to time.

         "SHARES" is defined in Section 1.1.

         "SUBSIDIARY" means, as to any Person, any corporation at least a
majority of the shares of stock of which having general voting power under
ordinary circumstances to elect a majority of the Board of Directors of such
corporation (irrespective of whether or not at the time stock of any other class
or classes shall have or might have voting power by reason of the happening of
any contingency) is, at the time as of which the determination is being made,
owned by such Person, or one or more of its Subsidiaries or by such Person and
one or more of its Subsidiaries.

         "SUBSCRIPTION NOTICE" is defined in Section 3.1.

         "TAXES" is defined in Section 2.1(k).

         "TAX RETURNS" is defined in Section 2.1(k).

         "TOTAL VOTING POWER" means at any time the total combined voting power
in the general election of directors of all the Voting Securities then
outstanding.

         "VOTING SECURITIES" means at any time shares of any class of capital
stock of the Company which are then entitled to vote generally in the election
of directors.


                                   ARTICLE XI

                                  MISCELLANEOUS

                                       29

<PAGE>   33


         Section 11.1 SEVERABILITY. If any term, provision, covenant or
restriction of this Agreement is held by a court of competent jurisdiction to be
invalid, void or unenforceable, the remainder of the terms, provisions,
covenants and restrictions of this Agreement shall remain in full force and
effect and shall in no way be affected, impaired or invalidated. It is hereby
stipulated and declared to be the intention of the parties that they would have
executed the remaining terms, provisions, covenants, and restrictions without
including any of such which may be hereafter declared invalid, void or
unenforceable.

         Section 11.2 SPECIFIC ENFORCEMENT. Purchaser, on the one hand, and the
Company, on the other, acknowledge and agree that irreparable damage would occur
in the event that any of the provisions of this Agreement were not performed in
accordance with their specific terms or were otherwise breached. It is
accordingly agreed that the parties shall be entitled to an injunction or
injunctions to prevent breaches of the provisions hereof in any court of the
United States or any state thereof having jurisdiction, this being in addition
to any other remedy to which they may be entitled at law or equity.

         Section 11.3 ENTIRE AGREEMENT. This Agreement (including the documents
set forth in the Exhibits and Schedules hereto) and the Collaboration Agreement
contain the entire understanding of the parties with respect to the transactions
contemplated hereby.

         Section 11.4 COUNTERPARTS. This Agreement may be executed in one or
more counterparts, all of which shall be considered one and the same agreement,
and shall become effective when one or more of the counterparts have been signed
by each party and delivered to the other parties, it being understood that all
parties need not sign the same counterpart.

         Section 11.5 NOTICES. All notices and other communications required or
permitted under this Agreement shall be effective upon receipt and shall be in
writing and may be delivered in person, by telecopy, overnight delivery service
or registered or certified United States mail, addressed to the Company or the
Purchaser, as the case may be, at their respective addresses set forth below:

If to the Company:


                      MILLENNIUM PHARMACEUTICALS, INC.
                      238 Main Street
                      Cambridge, Massachusetts  02139-4815
                      Attn: Chief Executive Officer
                      Telephone (617) 679-7000
                      Facsimile: (617) 621-0264


With copies to:

                                       30
<PAGE>   34


                           Attention: Legal Department

and to

                      Hale and Dorr LLP
                      60 State Street
                      Boston, Massachusetts 01209
                      Attn: Steven Singer
                      Telephone (617) 526-6000
                      Facsimile: (617) 526-5000


If to Purchaser:

                      BAYER AG
                      D 51368
                      Leverkeusen
                      Federal Republic of Germany
                      Attn: General Counsel
                      Telephone: 011 49 214 30 81803
                      Facsimile:  011 49 214 30 50848

With copies to:

                      Bayer Corporation, Inc.
                      400 Morgan Lane
                      West Haven, CT
                      Attn: Legal Department
                      Telephone: (203) 812 - 2401
                      Facsimile:  (203) 812 - 2795


and to:

                      Wilmer, Cutler & Pickering
                      2445 M Street
                      Washington, D.C.  20037
                      Attn: Richard W. Cass
                      Telephone (202) 663-6503
                      Facsimile: (202) 663-6363


All notices and other communications shall be effective upon the earlier of
actual receipt thereof by the person to whom notice is directed or (a) in the
case of notices and 

                                       31

<PAGE>   35

communications sent by personal delivery or telecopy, one business day after
such notice or communication arrives at the applicable address or was
successfully sent to the applicable telecopy number, (b) in the case of notices
and communications sent by overnight delivery service, at noon (local time) on
the second business day following the day such notice or communications was
delivered to such delivery service, and (c) in the case of notices and
communications sent by United States mail, seven days after such notice or
communication shall have been deposited in the United States mail. Any notice
delivered to a party hereunder shall be sent simultaneously, by the same means,
to such party's counsel as set forth above.

         Section 11.6 AMENDMENTS. This Agreement may be amended as to Purchaser
and their successors and assigns (determined as provided in Section 11.8), and
the Company may take any action herein prohibited, or omit to perform any act
required to be performed by it, if the Company shall obtain the written consent
of Purchaser. This Agreement may not be waived, changed, modified, or discharged
orally, but only by an agreement in writing signed by the party or parties
against whom enforcement of any waiver, change, modification or discharge is
sought or by parties with the right to consent to such waiver, change,
modification or discharge on behalf of such party.

         Section 11.7 COOPERATION. Purchaser and the Company agree to take, or
cause to be taken, all such further or other actions as shall reasonably be
necessary to make effective and consummate the transactions contemplated by this
Agreement, including, without limitation, making all required filings under the
HSR Act, if any.

         Section 11.8 SUCCESSORS AND ASSIGNS. All covenants and agreements
contained herein shall bind and inure to the benefit of the parties hereto and
their respective successors and assigns; PROVIDED, HOWEVER, that Purchaser may
assign its rights hereunder (including its right to purchase the Shares) to an
Affiliate of Purchaser, provided that such Affiliate agrees in writing to be
bound by the terms and conditions set forth herein, and the Company may not
assign any of its rights under this Agreement without the written consent of
Purchaser, which consent shall not be unreasonably withheld.

         Section 11.9 EXPENSES AND REMEDIES. Whether or not the Closing takes
place, all costs and expenses incurred in connection with this Agreement and the
transactions contemplated hereby shall be borne by the party incurring such
expense.

         Section 11.10 TRANSFER OF SHARES. Purchaser understands and agrees that
the Shares have not been registered under the Securities Act or the securities
laws of any state and that they may be sold or otherwise disposed of only in one
or more transactions registered under the Securities Act and, where applicable,
such laws or as to which an exemption from the registration requirements of the
Securities Act and, where applicable, such laws is available. Purchaser
acknowledges that except as provided in the Registration Rights Agreement,
Purchaser has no right to require the Company to register the Shares and
understands and agrees that each certificate representing the Shares (other
than, with respect to the first legend, 

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<PAGE>   36


Shares that are no longer subject to the provisions of Article V and other than,
with respect to the second legend, Shares which have been transferred in a
transaction registered under the Securities Act or exempt from the registration
requirements of the Securities Act pursuant to Rule 144 thereunder or any
similar rule or regulation) shall bear the following legends:

                      "THE TRANSFER OF THE SECURITIES REPRESENTED BY THIS 
                  CERTIFICATE IS RESTRICTED BY AN AGREEMENT ON FILE AT THE 
                  OFFICE OF THE CORPORATION."

                      "THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT
                  BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 OR THE
                  SECURITIES LAWS OF ANY STATE AND MAY NOT BE SOLD OR OTHERWISE
                  DISPOSED OF EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION
                  STATEMENT UNDER SUCH ACT AND APPLICABLE STATE SECURITIES LAWS
                  OR AN APPLICABLE EXEMPTION TO THE REGISTRATION REQUIREMENTS OF
                  SUCH ACT OR SUCH LAWS."

and Purchaser agrees to transfer the Shares only in accordance with the
provisions of such legends.

         Section 11.11 GOVERNING LAW. This Agreement shall be governed by and
construed and enforced in accordance with the laws of the State of Delaware
without regard to conflicts of law principles.

         Section 11.12 PUBLICITY. The Company and Purchaser will consult and
cooperate with each other before issuing, and provide each other the opportunity
to review and comment upon, any press release or otherwise making any public
statement with respect to the transactions contemplated by this Agreement.

         Section 11.13 NO THIRD PARTY BENEFICIARIES. Nothing contained in this
Agreement is intended to confer upon any Person other than the parties hereto
and their respective successors and permitted assigns, any benefit, right or
remedies under or by reason of this Agreement; PROVIDED, HOWEVER, that the
parties hereto hereby acknowledge and agree that the Purchaser Indemnities
(other than Purchaser) are third party beneficiaries of Article IX of this
Agreement.

         Section 11.14 CONSENT TO JURISDICTION. Each of the Company and 
Purchaser irrevocably submits to the personal exclusive jurisdiction of the
United States District Court for the District of Delaware for the purposes of
any suit, action or other proceeding arising out of this Agreement or any
transaction contemplated hereby (and, to the extent permitted under applicable
rules of procedure, agrees not to commence any action, suit or proceeding
relating hereto except in such court). Each of the Company and Purchaser further
agrees that service of any process, summons, notice or document hand delivered
or sent by registered mail to such party's respective address set forth in
Section 11.5 will be effective service of 

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<PAGE>   37


process for any action, suit or proceeding in Delaware with respect to any
matters to which it has submitted to jurisdiction as set forth in the
immediately preceding sentence. Each of the Company and Purchaser irrevocably
and unconditionally waives any objection to the laying of venue of any action,
suit or proceeding arising out of this Agreement or the transactions
contemplated hereby in the United States District court for the District of
Delaware, and hereby further irrevocably and unconditionally waives and agrees
not to plead or claim in such court that any such action, suit or proceeding
brought in such court has been brought in an inconvenient forum.

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<PAGE>   38


IN WITNESS WHEREOF, PURCHASER and the COMPANY have caused this Agreement to be
executed as of the day and year first above written.



                                      BAYER AG


                                      By: /s/
                                         ---------------------------------------
                                          General Manager Business Group
                                           Pharmaceuticals


                                      ------------------------------------------
                                      Authorized Signature



                                      By: /s/
                                         ---------------------------------------
                                          Senior Counsel


                                      ------------------------------------------
                                      Authorized Signature



                                      MILLENNIUM PHARMACEUTICALS, INC.


                                      By: /s/ Steven H. Holtzman
                                         ---------------------------------------


                                      Title: Chief Business Officer
                                            ------------------------------------



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