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                                                                   EXHIBIT 10.77




                            STOCK PURCHASE AGREEMENT

                            dated as of May 31, 2001


                                     between

                                  VALIGEN N.V.

                                       and

                          IMCLONE SYSTEMS INCORPORATED
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<TABLE>
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                                Table of Contents

                                                                                                Page

                                    ARTICLE 1
                                  THE PURCHASE
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Section 1.01.  Purchase And Sale; Research....................................................    1
Section 1.02.  Number Of Shares...............................................................    1
Section 1.03.  Purchase Price.................................................................    1
Section 1.04.  Purchase Price Adjustment......................................................    1
Section 1.05.  Deliveries.....................................................................    2
Section 1.06.  Conditions To The Obligations Of The Parties At Closing........................    2
Section 1.07.  Closing........................................................................    3

                                    ARTICLE 2
                  REPRESENTATIONS AND WARRANTIES OF THE COMPANY

Section 2.01.  Authority Of The Company.......................................................    4
Section 2.02.  Organization And Good Standing Of The Company And Subsidiaries.................    4
Section 2.03.  Capitalization; Ownership Of The Shares........................................    4
Section 2.04.  No Conflict....................................................................    5
Section 2.05.  Financial Statements...........................................................    6
Section 2.06.  Undisclosed Liabilities........................................................    6
Section 2.07.  Litigation.....................................................................    6
Section 2.08.  Compliance With Laws; Company Permits..........................................    6
Section 2.09.  Proprietary Rights.............................................................    7
Section 2.10.  Deleted .......................................................................    8
Section 2.11.  Contracts......................................................................    8
Section 2.12.  No Brokers.....................................................................    8

                                    ARTICLE 3
                   REPRESENTATIONS AND WARRANTIES OF PURCHASER

Section 3.01.  Organization, Standing And Corporate Power.....................................    8
Section 3.02.  Authority......................................................................    8
Section 3.03.  No Conflict....................................................................    8
Section 3.04.  Investment Intent; Ability To Bear Risk........................................    9
Section 3.05.  No Public Offering.............................................................    9
Section 3.06.  Exculpation And Acknowledgments................................................    9
Section 3.07.  Sufficient Funds...............................................................   10
Section 3.08.  No Brokers.....................................................................   10
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<TABLE>
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                                    ARTICLE 4
                                 INDEMNIFICATION

Section 4.01.  Survival Of Representations And Warranties.....................................   10
Section 4.02.  Indemnification By The Company.................................................   10
Section 4.03.  Indemnification By Purchaser...................................................   10
Section 4.04.  Losses Net Of Insurance........................................................   11
Section 4.05.  Exclusive Remedy...............................................................   11

                                    ARTICLE 5
                              COVENANTS OF INVESTOR

Section 5.01.  Acquisition Of Voting Securities...............................................   11
Section 5.02.  Sale Or Transfer Of Restricted Securities......................................   11
Section 5.03.  Rights Of First Refusal........................................................   12
Section 5.04.  Certain Actions................................................................   13
Section 5.05.  Voting Arrangements............................................................   13
Section 5.06.  Legend.........................................................................   13
Section 5.07.  Specific Performance...........................................................   14
Section 5.08.  Termination Of The Provisions Of Article 5.....................................   14

                                    ARTICLE 6
                            MISCELLANEOUS PROVISIONS

Section 6.01.  Expenses.......................................................................   15
Section 6.02.  Notices........................................................................   15
Section 6.03.  Certain Definitions............................................................   15
Section 6.04.  Amendment; Waiver..............................................................   17
Section 6.05.  Interpretation.................................................................   17
Section 6.06.  Counterparts...................................................................   17
Section 6.07.  Entire Agreement...............................................................   17
Section 6.08.  Governing Law; Dispute Resolution..............................................   17
Section 6.09.  Assignment.....................................................................   18
Section 6.10.  Severability...................................................................   18
Section 6.11.  No Third Party Beneficiaries...................................................   18

                                    Schedules

Schedule A          Certificate of Designation of Series B Convertible Preferred
                    Stock of ValiGen N.V.
Schedule B          Form of Investor Rights Agreement
Schedule C          Company Disclosure Schedule
Schedule D          Purchaser Disclosure Schedule
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                            STOCK PURCHASE AGREEMENT

      THIS STOCK PURCHASE AGREEMENT (the "AGREEMENT") is entered into as of May
_31, 2001, by and between ValiGen N.V., a limited liability company organized
under the laws of the Netherlands (the "COMPANY"), and ImClone Systems
Incorporated, a Delaware corporation (the "PURCHASER").

                                    RECITALS

      WHEREAS, upon and subject to the terms and conditions contained in this
Agreement, Purchaser desires to purchase from the Company, and the Company
desires to sell to Purchaser, the number of shares of Series B Convertible
Preferred Stock, par value Euro .01 per share (the "SERIES B STOCK") specified
in Section 1.02 below (the "SHARES") and having the rights, preferences,
privileges and restrictions set forth in the Articles of Association of the
Company and the Certificate of Designation for the Series B Stock in the form
attached hereto as Schedule A (respectively, the "ARTICLES" and the "CERTIFICATE
OF DESIGNATION");

      NOW, THEREFORE, in reliance on the truth and accuracy of the foregoing and
in consideration of the representations, warranties, covenants and agreements
contained in this Agreement and other good and valuable consideration, the
receipt and sufficiency of which is hereby acknowledged, the Company and
Purchaser agree as follows:

                                   ARTICLE 1
                                  THE PURCHASE

      Section 1.01. Purchase And Sale . Upon and subject to the terms and
conditions contained herein, the Company shall sell to Purchaser, and Purchaser
shall purchase from the Company, the Shares (the "TRANSACTION").

      Section 1.02. Number Of Shares. The number of shares of Series B Stock to
be purchased and sold pursuant to this Agreement shall be one hundred sixty
(160,000).

      Section 1.03. Purchase Price. The aggregate purchase price (the "PURCHASE
PRICE") for the Shares to be paid by Purchaser to the Company shall be two
million U.S. dollars ($2,000,000) equal to the product of the number of Shares
to be purchased and sold pursuant to this Agreement multiplied by $12.50 per
Share.

      Section 1.04. Purchase Price Adjustment. In the event that the Company
closes a private placement of its common stock on or before December 31, 2001 at
a gross sales price per share to the purchasers which is less than U.S. $12.50
per share (on a pre-split basis and without giving effect to any costs, expenses
and
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underwriting discounts or commissions of such private placement) (the "PRIVATE
PLACEMENT PRICE"), then, upon the closing of such initial private placement ,and
only such initial private placement , the Company shall issue to Purchaser,
without further consideration other than the Purchase Price paid by Purchaser
pursuant to this Agreement and the amount specified below in this Section 1.04,
such additional number of shares of Series B Stock equal to:

            (the Purchase Price divided by the Private Placement Price) x the
                     number of Shares minus the number of Shares (the
                     "Additional Shares");

      provided,however, that the Purchaser shall pay to the Company with respect
      to the Additional Shares an amount equal to the product of (a) the
      Additional Shares and (b) Euro 0.01 and such amount shall be paid to the
      Company at the time of the issuance of such Additional Shares.

         .

      Section 1.05. Deliveries. At Closing (defined below):

      (a)   The Company shall deliver to Purchaser:

            (i) (i)registered certificates evidencing the Shares;

            (ii) certificates of officers of the Company; and

            (iii) opinions of Davis Polk and Wardwell and Loyens Loeff, counsel
      to the Company, in form and substance reasonably satisfactory to the
      Purchaser .

      (b) Purchaser shall deliver to the Company:

            (i) the Purchase Price.

      Section 1.06. Conditions To The Obligations Of The Parties At Closing.

      (a) The obligation of the Company to issue and sell the Shares being
purchased by Purchaser at the Closing is, at its option, subject to the
satisfaction, on or before the Closing Date, of the following conditions:

            (i) Representations and Warranties to be True. The representations
      and warranties contained in Article 3 shall be true in all material
      respects on and as of the Closing Date with the same effect as though such
      representations and warranties had been made on and as of such date.


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            (ii) Waivers and Consents. The Board of Managers shall have waived
      the preemptive rights that shareholders of the Company have as a matter of
      Dutch law.. The Company shall have obtained all other material consents
      and waivers necessary or advisable or otherwise provided any required
      notice to any shareholder that benefits from contractual preemptive rights
      to issue and deliver the Shares and all such consents and waivers shall be
      in full force and effect.

            (iii) Investor Rights Agreement. The parties shall have entered into
      an Investor Rights Agreement in the form set forth in Schedule B attached
      hereto.

            (iv) Authorization. Company's board of directors shall have duly
      authorized the Transaction.

            (v) Governmental Approvals. All required governmental approvals
      (including any approval required under the Hart-Scott-Rodino Act) shall
      have been obtained.

         .

      (b) The obligation of Purchaser to purchase and pay for the Shares being
purchased by Purchaser at the Closing is, at its option, subject to the
satisfaction, on or before the Closing Date, of the following conditions:

            (i) Representations and Warranties to be True. The representations
      and warranties contained in Article 2 shall be true in all material
      respects on and as of the Closing Date with the same effect as though such
      representations and warranties had been made on and as of such date.

            (ii) Investor Rights Agreement. The parties shall have entered into
      an Investor Rights Agreement in the form set out in Schedule B attached
      hereto.

            (iii) Authorization. Purchaser's board of directors shall have duly
      authorized the Transaction.

            (iv) Governmental Approvals. All required governmental approvals
      (including any approval required under the Hart-Scott-Rodino Act) shall
      have been obtained.



            (v)Opinions of Counsel. Purchaser shall have received opinions of
      United States and Netherlands counsel of the Company in form and substance
      reasonably acceptable to the Purchaser.


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      Section 1.07. Closing. The closing of the purchase and sale of the Shares
contemplated hereby (the "CLOSING") shall be held on May 31, 2001 (the "CLOSING
DATE") at 10:00 A.M. (New York Time) at the offices of Purchaser at the address
specified in Section 6.02, unless such date is extended by the Company upon
written notice to Purchaser, but in no event shall such extension be to a date
later than 30 days immediately following the date of this Agreement.

                                   ARTICLE 2
                  REPRESENTATIONS AND WARRANTIES OF THE COMPANY

      Except as set forth in the disclosure schedule of the Company attached
hereto as Schedule C (the "COMPANY DISCLOSURE SCHEDULE"), the Company hereby
represents and warrants to Purchaser as follows:

      Section 2.01. Authority Of The Company. The Company has all requisite
authority to enter into this Agreement and to consummate the Transaction. The
execution and delivery of this Agreement by the Company and the consummation by
the Company of the Transaction have been duly authorized by all necessary
corporate action on the part of the Company. This Agreement has been duly
executed and delivered by the Company and constitutes a valid and binding
obligation of the Company, enforceable in accordance with its terms.

      Section 2.02. Organization And Good Standing Of The Company And
Subsidiaries.

      (a) The Company is a limited liability company, duly organized, validly
existing and in good standing under the laws of the Netherlands, and has the
requisite corporate power and authority to carry on its business as now being
conducted. The Company is duly qualified or licensed to do business and is in
good standing in each jurisdiction in which the nature of its business or the
ownership or leasing of its properties makes such qualification or licensing
necessary, except where the failure to be so qualified would not have a Material
Adverse Effect (as defined below) on the Company.

      (b) Each Subsidiary (as defined below) is a corporation or limited
liability company, duly incorporated, validly existing and in good standing
under the laws of its jurisdiction of incorporation, has all corporate powers
and all governmental licenses, authorizations, permits, consents and approvals
required to carry on its business as now conducted, is duly qualified to do
business as a foreign corporation and is in good standing in each jurisdiction
where such qualification is necessary, except where the failure to be so
qualified would not have a Material Adverse Effect on the Company.

      (c) Except for directors qualifying shares, all of the outstanding capital
stock or other voting securities of each Subsidiary is owned by the Company,
directly or indirectly, free and clear of any Lien and free of any other
limitation or


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restriction (including any restriction on the right to vote, sell
or otherwise dispose of such capital stock or other voting securities).

      Section 2.03. Capitalization; Ownership Of The Shares. The authorized
capital stock of the Company consists of 32,000,000 shares of common stock, par
value Euro .01 per share ("COMMON STOCK"), 6,000,000 preferred Shares Class A,
nominal value Euro .01 per share ("Class A Shares"), 6,000,000 preferred Shares
Class B, nominal value Euro .01 and 6,000,000 preferred Shares Class C, nominal
value Euro .01. Of these authorized amounts, 2,718,983 Class A Shares are issued
and outstanding, 8,978,263 shares of Common Stock are issued and outstanding,
and 4,096,227 shares of Common Stock are reserved for issuance pursuant to
outstanding options and warrants. The Shares shall be at Closing, and upon
issuance in accordance with the terms of the Articles and the Certificate of
Designation, the shares of Common Stock issuable upon conversion of the Shares
(collectively, the "CONVERSION SHARES") shall be and all outstanding Shares of
Common Stock are, and the shares issuable upon the exercise of outstanding
options and warrants will be, validly issued, fully paid, and nonassessable,
issued in full compliance with all applicable securities laws and without
violation of any preemptive rights, with the rights, preferences, privileges and
restrictions described in the Articles, and free and clear of any pledges,
options, claims, liens, charges, encumbrances, and security interests of any
kind or nature (collectively, "LIENS") other than those created by Purchaser or
as otherwise specified in Section 2.03 of the Company Disclosure Schedule. The
Company shall have at Closing the full right, power and authority to sell,
transfer and deliver the Shares and, upon issuance in accordance with the terms
of the Articles, the Conversion Shares, to Purchaser without obtaining the
consent or approval of any person or governmental authority, except for any
consents which have previously been obtained. Upon delivery of the certificate
evidencing the Shares to Purchaser and Purchaser's payment of the Purchase Price
in accordance with this Agreement, Purchaser will acquire the Shares free and
clear of adverse claims, other than those created by Purchaser. Section 2.03 of
the Company Disclosure Schedule contains a list of all outstanding
subscriptions, options, warrants, convertible securities or other rights or
agreements to which the Company is bound relating to the issuance or sale of
shares of the Company's capital stock or any other equity interest in the
Company or obligating the Company to issue or sell any shares of the Company's
capital stock or other equity interest in the Company.

      Section 2.04. No Conflict. Except with respect to the Stock Purchase
Agreement dated as of May 31, 2000 between the Company and various purchasers
(the conflicting terms of which shall be amended or waived prior to Closing),
the execution, delivery and performance of this Agreement and the consummation
of the Transaction do not conflict with, or result in any breach or violation
of, or default (with or without notice or lapse of time or both) under, or give
rise to a right of termination, cancellation or acceleration of any obligation
under, or result in the creation of any Lien upon the Shares or any of the
assets of


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the Company under, (i) the Articles or the articles of association or
certificate of incorporation of any Subsidiary, (ii) any note, bond, mortgage,
indenture, lease or other agreement, instrument, permit, concession, franchise
or license applicable to the Company or any Subsidiary, or (iii) any judgment,
order, decree, statute, law, ordinance, rule, or regulation applicable to the
Company or any Subsidiary. No consent, approval, order, or authorization of, or
registration, declaration or filing with, or notice to, any government or any
court, administrative entity or commission or other governmental authority or
agency (a "GOVERNMENTAL ENTITY") is required by or with respect to the Company
or any Subsidiary in connection with the execution, delivery or performance of
this Agreement by the Company or the consummation by the Company of the
Transaction.

      Section 2.05. Financial Statements. The financial statements (the
"FINANCIAL STATEMENTS") listed in Section 2.5 of the Company Disclosure Schedule
have been prepared in conformity with generally accepted accounting principles
applied on a consistent basis throughout the periods reflected, except as may be
indicated therein. The Financial Statements fairly present in all material
respects the financial position of the Company of the date thereof and the
results of operations, stockholder's equity, and cash flows of the Company for
the periods then ended, subject to normal year-end adjustments and any other
adjustments described therein. The pro forma financial statement (the "PRO FORMA
STATEMENT") of the Company listed in Section 2.5 of the Company Disclosure
Schedule fairly presents in all material respects the consolidated financial
position of the Company as of December 31, 2000, after giving pro forma effect
to the to the transactions discussed in the notes to the Pro Forma Statement
(assuming that such transactions occurred on the dates set forth in the Pro
Forma Financial Statements) in conformity with generally accepted accounting
principles.

      Section 2.06. Undisclosed Liabilities. The Company and the Subsidiaries
have no liabilities or obligations (whether accrued, absolute, contingent or
otherwise) of the type required to be reflected on a balance sheet prepared in
accordance with generally accepted accounting principles, except for liabilities
or obligations (i) reflected or reserved against in the Financial Statements or
the Pro Forma Statements, or (ii) incurred since the date thereof in the
ordinary course of business, and which have not, individually or in the
aggregate, had a Material Adverse Effect on the Company.

      Section 2.07. Litigation. There is no suit, action, judicial or arbitral
proceeding or investigation pending with respect to which the Company has
received service of process or other formal notice, or to the knowledge of the
Company threatened, against or affecting the Company or any Subsidiary that (i)
seeks to delay or prevent the consummation of, or would materially adversely
affect the Company's ability to consummate, the Transaction, or (ii)
individually or in the aggregate with any related such suit, action, proceeding
or investigation, involves a claim against the Company in an amount, or which
involves a claim of unspecified amount that would, if determined adversely to
the Company, result in


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a Material Adverse Change with respect to the Company, nor is the Company or any
Subsidiary in default under any material judgment, decree, injunction, or order
of any Governmental Entity or arbitrator to which the Company or any of its
assets is bound.

      Section 2.08. Compliance With Laws; Company Permits. To the knowledge of
the Company, (i) neither the Company nor any Subsidiary is in violation of any
applicable law, rule, or regulation or any order of any Governmental Entity, and
no investigation by any Governmental Entity concerning any such possible
violation is pending or threatened, except for such violations which would not,
individually or in the aggregate with any related such violation, reasonably be
expected to result in a Material Adverse Effect, and (ii) the Company and the
Subsidiaries hold or are a party to all licenses, permits, registrations,
franchises, variances, exemptions, and other authorizations, consents, and
approvals necessary to conduct its business as presently conducted, except for
such failures to hold or be a party to such authorizations, consents and
approvals which would not, individually or in the aggregate, reasonably be
expected to result in a Material Adverse Effect on the Company.

      Section 2.09. Proprietary Rights.

      (a) The Company and the Subsidiaries own or have the right to use, free
and clear of all liens and restrictions, or, to the knowledge of the Company,
any third party claims, all material know-how, formulae, processes, inventions,
designs, programs, patents, patent applications, trademarks, trade names,
copyrights and other proprietary rights (collectively, "PROPRIETARY RIGHTS")
used in the conduct of the Company's business as now conducted or proposed to be
conducted in its business plan without, to the knowledge of the Company,
infringing upon the right or claimed right of any person under or with respect
to the foregoing. The Company has not received any communications alleging that
the Company has violated or, by conducting its business as proposed, would
violate any of the patents, trade names, copyrights or trade proprietary rights
of any other person or entity. The Company and the Subsidiaries are not in
default under any material term of any license or other agreement relating to
such Proprietary Rights and, to the Company's knowledge, each party other than
the Company and the Subsidiaries to each such license or other agreement is not
in default thereunder. All such licenses and agreements are valid, enforceable
and in full force and effect.

      (b) The Company and the Subsidiaries have taken security measures to
protect the secrecy, confidentiality, and value of all the Proprietary Rights,
which measures are reasonable and customary in the industry in which the Company
and the Subsidiaries conduct business. Each of the Company's employees,
consultants, and other persons who, either alone or in concert with others,
developed, invented, discovered, derived, programmed or designed any of the
Proprietary Rights, or who has knowledge of or access to information about any
Proprietary Rights, has entered into a written agreement (i) providing that the


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Proprietary Rights and other information are proprietary to the Company and are
not to be divulged or misused and (ii) transferring to the Company, without any
further consideration being given therefor by the Company, all of such
employee's, consultant's, or other person's rights, title and interest in and to
such Proprietary Rights with respect to such Proprietary Rights and information.
The Company is not aware that any of its employees, consultants or prospective
employees who have signed such agreements is in violation thereof, nor is it
aware that any former employee or consultant has made any claim of ownership in
or rights with respect to any of the Proprietary Rights.

      Section 2.10. [Deleted]

      Section 2.11. Contracts. Neither the Company nor the Subsidiaries is in
material breach of any contract the breach of which is reasonably likely,
individually or in the aggregate, to have a Material Adverse Effect on the
Company.

      Section 2.12. No Brokers. No broker, investment banker, financial advisor
or other person, is entitled to any broker's, finder's, financial advisors or
similar fee or compensation in connection with the Transaction based upon
arrangements made by or on behalf of the Company.

                                   ARTICLE 3
                   REPRESENTATIONS AND WARRANTIES OF PURCHASER

      Except as set forth on the disclosure schedule of Purchaser attached
hereto as Schedule D (the "PURCHASER DISCLOSURE SCHEDULE"), Purchaser represents
and warrants to the Company as follows:

      Section 3.01. Organization, Standing And Corporate Power. Purchaser is a
corporation duly organized, validly existing and in good standing under the laws
of the state of its organization and has the requisite corporate power and
authority to carry on its business as now being conducted. Purchaser is duly
qualified or licensed to do business and is in good standing in each
jurisdiction in which the nature of its business and operations makes such
qualification or licensing necessary, except where the failure to be so
qualified would not have a Material Adverse Effect on the Company.

      Section 3.02. Authority. Purchaser has all requisite corporate power and
authority to enter into this Agreement and to consummate the Transaction. The
execution and delivery of this Agreement by Purchaser and the consummation by
Purchaser of the Transaction have been duly authorized by all necessary
corporate action on the part of Purchaser. This Agreement has been duly executed
and delivered by Purchaser. This Agreement constitutes a valid and binding
obligation of Purchaser, enforceable in accordance with its terms.


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      Section 3.03. No Conflict. The execution, delivery and performance of this
Agreement and the consummation of the Transaction will not conflict with, or
result in any breach or violation of, or default (with or without notice or
lapse of time or both) under, or give rise to a right of termination,
cancellation or acceleration of any obligation under, or result in the creation
of any Lien upon any of the assets of Purchaser under, (i) the charter documents
of Purchaser, (ii) any note, bond, mortgage, indenture, lease or other
agreement, instrument, permit, concession franchise or license applicable to
Purchaser or its assets, or (iii) subject to the governmental filings and other
matters referred to in the following sentence, any judgment, order, decree,
statute, law, ordinance, rule, or regulation applicable to Purchaser or its
assets. No consent, approval, order or authorization of, or registration,
declaration or filing with, or notice to, any Governmental Entity or any other
person is required by or with respect to Purchaser in connection with the
execution, delivery or performance of this Agreement by Purchaser or the
consummation by Purchaser of the Transaction.

      Section 3.04. Investment Intent; Ability To Bear Risk. Purchaser is
acquiring the Shares and the underlying Conversion Shares solely for investment
for its own account, not as a nominee or agent, and not with a view to the
resale or distribution of any part thereof, and Purchaser has no present
intention of selling, granting any participation in, or otherwise distributing
the Shares or the underlying Conversion Shares. Purchaser is an "accredited
investor" within the meaning of Rule 501 (a) under the Securities Act. Purchaser
understands that the Shares and the underlying Conversion Shares may not be
resold except in compliance with the registration requirements of the Securities
Act of 1933, as amended, unless an exemption therefrom is available. Purchaser
understands that no public market now exists for any of the securities issued by
the Company and that the Company has made no assurances that a public market
will ever exist for the Company's securities. Purchaser is familiar with the
business in which the Company conducts its operations, and has the requisite
business and financial knowledge and experience to evaluate the merits and risks
of its acquisition of the Shares and the underlying Conversion Shares.

      Section 3.05. No Public Offering. The Purchaser understands that the
Shares are being offered in a transaction not involving any public offering
within any jurisdiction and the Shares and the Conversion Shares have not been
registered under the applicable securities laws of any jurisdiction, and the
Shares and, with appropriate modification, any Conversion Shares, will bear a
restricted transfer legend unless the Company determines otherwise in compliance
with applicable law.

      Section 3.06. Exculpation And Acknowledgments. Purchaser acknowledges that
it has made all investigations and conducted all due diligence it deems
necessary with respect to the Company and its investment in the Shares and the
underlying Conversion Shares and that neither any shareholder of the Company nor
the Company, nor any other person acting on behalf of any of them, has made any
representation or warranty, express or implied, as to the condition,


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<PAGE>   13
merchantability, suitability or fitness for any particular purpose of any of the
assets owned or used by the Company, or made any representation or warranty
regarding the Company or any other matter relating to the Company except as
expressly set forth in this Agreement (including the Company Disclosure
Schedule). Purchaser acknowledges that all forward-looking information received
from or made available by or on behalf of the Company, including projected
statements of revenues, cash flow and other business and financial condition and
results, contain numerous uncertainties inherent in attempting to make
estimates, projections and other forecasts, are not warranties of future
performance or financial results, and involve numerous assumptions about risks
and events that are difficult to predict and many of which are beyond the
control of the Company, their financial advisers, and other agents. Purchaser
further acknowledges that it is familiar with such uncertainties and
assumptions, that it takes full responsibility for making its own independent
evaluation of possible future revenues, income, cash flow, and other and
financial condition and results, that it understands and has taken into account
the likelihood that actual future results will differ, perhaps materially, from
those reflected in any such forward-looking statements, and that Purchaser shall
have no claim against the Company, its affiliates, its representatives, or other
agents with respect to any such forward-looking information.

      Section 3.07. Sufficient Funds. Purchaser possesses liquid capital or
sources therefor committed in writing sufficient to enable Purchaser to
consummate the Transaction and timely perform all of its obligations hereunder.

      Section 3.08. No Brokers. No broker, investment banker, financial advisor
or other person is entitled to or may be paid any broker's, finder's, financial
advisor's or similar fee or commission in connection with the Transaction based
upon arrangements made by or on behalf of Purchaser or its affiliates.

                                   ARTICLE 4
                                 INDEMNIFICATION

      Section 4.01. Survival Of Representations And Warranties. Solely for
purposes of Section 4.02 and Section 4.03, the representations and warranties
contained in this Agreement shall survive until 18 months after the date of this
Agreement.

      Section 4.02. Indemnification By The Company. The Company shall indemnify
and hold harmless Purchaser and its affiliates against any losses, liabilities,
damages, claims, costs, expenses, interest, awards, and judgments, including
reasonable attorneys' fees and expenses and costs of investigation, but
excluding consequential damages, punitive damages, lost profits, diminution in
value, damage to reputation or the like (collectively, other than such excluded
items, "LOSSES") actually suffered or incurred by any such indemnified party


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<PAGE>   14
arising out of or resulting from (i) the breach of any representation and
warranty made by the Company in this Agreement, or (ii) the breach of any
covenant or agreement by the Company contained in this Agreement.

      Section 4.03. Indemnification By Purchaser. Purchaser shall indemnify and
hold harmless the Company against any Losses actually suffered or incurred by it
arising out of or resulting from (i) the breach of any representation and
warranty made by Purchaser in this Agreement, or (ii) the breach of any covenant
or agreement by Purchaser contained in this Agreement.

      Section 4.04. Losses Net Of Insurance. The amount of any Losses under this
Article 4 shall be determined net of any related amounts recovered or
recoverable by the indemnified party under insurance policies, indemnities or
other reimbursement arrangements with respect to such Losses.

      Section 4.05. Exclusive Remedy. . The indemnification provided in this
Article, subject to the limitations set forth herein, shall be the exclusive
remedy available to an Indemnified Party for any breach of a representation,
warranty, covenant, or other agreement contained in this Agreement.

                                   ARTICLE 5
                              COVENANTS OF INVESTOR

      Section 5.01. Acquisition Of Voting Securities. Investor will not, and
will not permit its Affiliates to, at any time and from time to time, purchase
or otherwise acquire, or agree or offer to purchase or otherwise acquire,
beneficial ownership of any securities of the Company entitled, in the ordinary
course, to vote in the election of directors of the Company ("VOTING
SECURITIES"), if after giving effect thereto Investor and its Affiliates as a
group would beneficially own Voting Securities representing more than twenty
percent (20%) of the aggregate number of votes which may be cast by holders of
outstanding Voting Securities. Voting Securities shall include warrants which,
if exercised, would entitle the holder to purchase securities of the Company
entitled, in the ordinary course, to vote in the election of directors of the
Company.

      Section 5.02. Sale Or Transfer Of Restricted Securities. Investor will
not, and will not permit its Affiliates to, sell, pledge, encumber or otherwise
transfer, or agree to sell, pledge, encumber or otherwise transfer, directly or
indirectly, any Voting Securities, except as provided in any of clauses (a) -
(g) of this Section 5.02:

      (a) to an Affiliate of Investor; provided that such Affiliate agrees in
writing to be bound by the terms of this Agreement;

      (b) to any Person who, after giving effect to such sale, pledge,
encumbrance or transfer, would beneficially own Voting Securities representing


                                       11
<PAGE>   15
in the aggregate twelve percent (12%) or less of Total Voting Power and such
sale, pledge, encumbrance or transfer has been made in accordance with the
Securities Act; provided that Investor shall be entitled to rely on the
representations and warranties of such Person in satisfying the limitation of
this Section 5.02 (b) on the percentage of Total Voting Power that such Person
may have;

      (c) pursuant to a registered offering under the Securities Act,;

      (d) pursuant to Rule 144 of the General Rules and Regulations of the
Securities Act;

      (e) pursuant to a tender or exchange offer made by the Company or
recommended by the Company's board of directors to the Company's stockholders;

      (f) as a pledge to a financial institution reasonably acceptable to the
Company; or

      (g) with the prior written consent of the Company.

      Section 5.03. Rights Of First Offer.

      (a) Investor will not, and will not permit any of its Affiliates to, sell,
pledge, encumber or otherwise transfer any Voting Securities in accordance with
Section 5.02(b) or Section 5.02(d) without first giving the Company prior notice
thereof (an "OFFER NOTICE") and the opportunity (as hereinafter provided) to
purchase such securities (the "OFFERED SECURITIES") at a cash price equal to the
sum of the amount of any cash plus the fair market value of any other
consideration offered by the prospective purchaser or other transferee (the
"OFFER PRICE"). The Offer Notice shall constitute an offer (the "OFFER") by the
Investor to sell all, but not less than all, of the Offered Securities at the
Offer Price.

      (b) The Offer may be accepted within 60 days of receipt by the Company of
the Offer Notice and, if accepted, the Offered Securities shall be purchased
within 60 days after such acceptance. If the Offer is not accepted or the
Offered Securities are not purchased as contemplated above, such Investor may
sell the Offered Securities to such prospective purchaser or transferee at a
price not less than the Offer Price. If the sale to such prospective purchaser
or transferee is not consummated as contemplated above within 45 days after the
expiration of the 60-day offer period or earlier irrevocable rejection of such
Offer or failure to purchase the Offered Securities after acceptance of the
Offer, no sale may be made by the Investor without again complying with this
Section 5.03.

      (c) If the consideration offered by the prospective purchaser or
transferee includes non-cash consideration, the Company and Investor shall in
good faith seek to agree upon the value of such non-cash consideration. If


                                       12
<PAGE>   16
Investor and the Company fail to agree on such value within 30 days following
receipt by the Company of the Offer Notice, then the items in dispute shall be
referred to a nationally recognized investment banking firm selected jointly by
Investor and the Company. Investor and the Company shall share all expenses of
such investment banking firm. The value of any securities offered as
consideration shall be the fair market value of such securities determined on a
fully distributed basis, and the value of any property other than securities
shall be the fair market value of such property. If a determination under this
Section 5.03(c) is required, any date for acceptance of an Offer provided for in
Section 5.03(b) hereof shall be postponed until the second Business Day after
the date of such determination. All determinations made pursuant to this Section
5.03(c) shall be final and binding on the Investor and the Company.

      (d) The Company's rights under this Section 5.03 may be assigned, in whole
or in part, to any third party, provided that the Company remains liable for its
obligations hereunder.

      Section 5.04. Certain Actions. Investor will not, and will not permit its
Affiliates to:

      (a) make, or take any action to solicit, initiate or encourage, an
Acquisition Proposal;

      (b) "solicit", or become a "participant" in any "solicitation" of, any
"proxy" (as such terms are defined in Regulation 14A under the Exchange Act)
from any holder of Voting Securities in connection with any vote on any matter,
or agree or announce its intention to vote with any Person undertaking a
"solicitation";

      (c) 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;

      (d) grant any proxies with respect to any Voting Securities to any Person
(other than as recommended by the Board of Directors of the Company) or deposit
any Voting Securities in a voting trust or enter into any other arrangement or
agreement with respect to the voting thereof; or

      (e) propose any amendment to this Agreement that is or may be required to
be publicly disclosed.

      Section 5.05. Voting Arrangements. Investor shall vote or cause to be
voted all Voting Securities owned by the Investor Group for nominees to the
Board of Directors of the Company who have been recommended by the Company's
Board of Directors. Investor shall cause all Voting Securities owned by the
Investor to be represented, in person or by proxy, at all meetings of holders of
Voting Securities of which Investor has actual notice, so that such


                                       13
<PAGE>   17
Voting Securities may be counted for the purpose of determining the presence of
a quorum at such meetings.

      Section 5.06. Legend.

      (a) All certificates representing Voting Securities subject to this
Agreement shall bear the following legend:

      "THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO STANDSTILL
PROVISIONS CONTAINED WITHIN THE STOCK PURCHASE AGREEMENT DATED MAY 31, 2001 (A
COPY OF WHICH IS ON FILE WITH THE SECRETARY OF THE COMPANY) WHICH PROVIDES,
AMONG OTHER THINGS, FOR CERTAIN RIGHTS OF PURCHASE OF SUCH SHARES BY THE COMPANY
AND CERTAIN RESTRICTIONS ON TRANSFER THEREOF. ANY SALE OR OTHER TRANSFER NOT IN
COMPLIANCE WITH SAID AGREEMENT SHALL BE VOID."

      (b) Investor agrees to the entry of stop transfer orders with the transfer
agent and registrar of the stock subject to this Agreement against transfer of
legended stock held by the Investor except in compliance with the requirements
of this Agreement.

      Section 5.07. Specific Performance. Investor agrees that any breach by it
of any material provision of this Agreement would irreparably injure the Company
and that money damages would be an inadequate remedy therefor. Accordingly,
Investor agrees that the Company shall be entitled to one or more injunctions
enjoining any such breach and requiring specific performance of this Agreement
and consents to the entry thereof, in addition to any other remedy to which the
Company is entitled at law or in equity.

      Section 5.08. Termination Of The Provisions Of Article 5. The provisions
of this Article 5 shall terminate upon the occurrence of any of the following:

      (a) the written agreement of the Company and Investor to terminate
provisions of this Article 5;


      (b) the earlier of (i) two years after the initial public offering of the
Company's Common Stock, (ii) the fifth anniversary of the date of this Agreement
and (iii) the date that the Investor shall beneficially own Voting Securities
representing less than 4% of Total Voting Power;

      (c)any Person shall have acquired Voting Securities representing more than
fifty percent (50%) of Total Voting Power; or

      (d)the dissolution, liquidation or winding up of the Company.


                                       14
<PAGE>   18
                                   ARTICLE 6
                            MISCELLANEOUS PROVISIONS

      Section 6.01. Expenses. All costs and expenses incurred in connection with
this Agreement and the Transaction shall be paid by the party incurring such
expenses.

      Section 6.02. Notices. All notices, requests, claims, demands and other
communications under this Agreement shall be in writing and shall be deemed
given if delivered personally or delivered by internationally recognized
overnight courier (providing proof of delivery) or sent by facsimile
transmission to the parties at the following addresses (or at such other address
for a party as shall be specified by like notice):

                  (a)   if to the Company, to:

                        ValiGen N.V.
                        c/o ValiGene S.A.
                        300 Pheasant Run
                        Newtown, PA  18940
                        Attention:  Douglas Watson, President
                        Telephone: 215-504-4444

                  (b)   if to Purchaser, to:

                        ImClone Systems Incorporated
                        180 Varrick Street, 6th Floor
                        New York, NY  10014
                        Attention:  Samuel D. Waksal, President
                        Telephone:  212-645-1405

All such communications shall be deemed to have been duly given: (A) in the case
of a communication delivered by hand, when personally delivered; (B) in the case
of a communication sent by facsimile, upon transmission subject to telephone
confirmation of receipt; and (C) in the case of a communication sent by Federal
Express or other recognized overnight courier service, on the date delivered at
the designated address.

      Section 6.03. Certain Definitions. For purposes of this Agreement:

      "ACQUISITION PROPOSAL" means any offer or proposal for, or any indication
of interest in, a merger or other business combination involving the Company or
any subsidiary of the Company or the acquisition of any equity interest in, or a
substantial portion of the assets of, the Company or any subsidiary of the
Company.


                                       15
<PAGE>   19
      "AFFILIATE" means, with respect to any Person, any Person directly or
indirectly controlling, controlled by, or under common control with, such other
Person. For the purposes of this definition, "control" when used with respect to
any Person, means the possession, directly or indirectly, of the power to direct
or cause the direction of the management and policies of such Person, whether
through the ownership of voting securities, by contract or otherwise; the terms
"controlling" and "controlled" have meanings correlative to the foregoing.

      "BENEFICIAL OWNERSHIP" and "beneficially own" shall be determined in
accordance with Rules 13d-3 and 13d-5 under the Exchange Act.

      "BUSINESS DAY" means any day except a Saturday, Sunday or other day on
which commercial banks in New York are authorized by law to close.

      "COMMISSION" shall mean the U.S. Securities and Exchange Commission or any
other federal agency at the time administering the Securities Act, and any
equivalent agency in any jurisdiction outside the United States.

      "COMMON STOCK" shall mean the ordinary shares, nominal value Euro .01 per
share, of the Company.

      "EXCHANGE ACT" means the Securities Exchange Act of 1934, as amended, and
the rules and regulations promulgated thereunder.

      "INVESTOR GROUP" means Investor and its Affiliates.

      "MATERIAL ADVERSE CHANGE" or "MATERIAL ADVERSE EFFECT", when used in
reference to the Company or Purchaser, means any change or effect that,
individually or in the aggregate with other changes or effects of such type, is
materially adverse to the business, operations, assets, properties, condition,
(financial or other) or results of operations of such person and its
subsidiaries taken as a whole; provided, however, that a decline in general
economic conditions or matters generally affecting companies engaged in the
business of the Company shall not be deemed to be a "Material Adverse Change" or
to have a "Material Adverse Effect."

      "PERSON" means an individual, corporation, limited liability company,
partnership, joint venture, association, trust, unincorporated organization or
other entity;

      "PREFERRED STOCK" shall mean the Series B preferred shares, nominal value
Euro .01 per share, of the Company.

      "SECURITIES ACT" shall mean the U.S. Securities Act of 1933, as amended,
or any similar U.S. federal statute and the rules and regulations of the
Commission thereunder, and any similar statute of any non-U.S. jurisdiction, all
as the same shall be in effect at the time.


                                       16
<PAGE>   20
      "SUBSIDIARY" shall mean any entity of which securities or other ownership
interests having ordinary voting power to elect a majority of the board of
directors or other persons performing similar functions are at the time directly
or indirectly owned by the Company.

      "TOTAL VOTING POWER" means the aggregate number of votes which may be cast
by holders of outstanding Voting Securities.

      Section 6.04. Amendment; Waiver. This Agreement may be amended by an
instrument in writing signed on behalf of the Company and Purchaser. Either
party may (a) extend the time for the performance of any of the obligations or
other acts of the other party, or (b) waive compliance with any of the
agreements contained in this Agreement. The failure of any party to this
Agreement to assert any of its rights under this Agreement or otherwise shall
not constitute a waiver of such rights. No waiver of any of the provisions of
this Agreement shall be deemed or shall constitute a waiver of any other
provisions hereof (whether or not similar), nor shall such waiver constitute a
continuing waiver unless expressly provided.

      Section 6.05. Interpretation. A reference made in this Agreement to a
Section or Schedule shall be to a Section of, or an Schedule to, this Agreement
unless otherwise indicated. The table of contents and headings contained in this
Agreement are for reference purposes only and shall not affect the meaning or
interpretation of this Agreement. Whenever the words "include," "includes" or
"including" are used in this Agreement, they shall be deemed to be followed by
the words "without limitation" unless otherwise stated to the contrary.

      Section 6.06. Counterparts. This Agreement may be executed in one or more
counterparts, each of which shall be deemed to be an original, but all of which
shall constitute one and the same agreement.

      Section 6.07. Entire Agreement. This Agreement, which includes the
Schedules attached hereto, constitutes the entire agreement, and supersedes all
prior agreements and understandings, both written and oral, between the parties
with respect to the subject matter of this Agreement.

      Section 6.08. Governing Law; Dispute Resolution.

      (a) Except to the extent that the corporation law of the Netherlands shall
be applicable with respect to matters relating to the internal corporate affairs
of the Company, this Agreement and (unless otherwise provided) all amendments,
supplements, waivers and consents relating thereto or hereto shall be governed
by and construed in accordance with the laws of the State of New York.

      (b) The parties agree that any such litigation shall only be brought in
the United States District Court for the Southern District of New York (and, if
and only if jurisdiction is not available in the United States District Court
for the


                                       17
<PAGE>   21
Southern District of New York, in a New York State court sitting in New
York, New York), and each of the parties hereby consents to the jurisdiction of
such courts (and of the appropriate appellate courts therefrom) in any such
litigation and irrevocably waives, to the fullest extent permitted by law, any
objection which it may now or hereafter have to the laying of the venue of any
such litigation in any such court or that any such litigation which is brought
in any such court has been brought in an inconvenient forum. Each party agrees
that service of notice on such party as provided in Section 6.02 shall be deemed
effective service of process on such party.

      (c) EACH OF THE PARTIES HERETO HEREBY IRREVOCABLY WAIVES ANY AND ALL RIGHT
TO TRIAL BY JURY IN ANY LEGAL PROCEEDING ARISING OUT OF OR RELATED TO THIS
AGREEMENT, THE TRANSATION AGREEMENTS OR ANY OF THE TRANSACTIONS CONTEMPLATED
THEREBY.

      Section 6.09. Assignment. Neither this Agreement nor any of the rights,
interests or obligations under this Agreement shall be assigned, in whole or in
part, by operation of law or otherwise, by either of the parties without the
prior written consent of the other party. Subject to the preceding sentence,
this Agreement will be binding upon, inure to the benefit of, and be enforceable
by, the parties and their respective successors and assigns, enforceable in
accordance with its terms, except as such validity, binding nature and
enforceability may be affected by applicable bankruptcy, insolvency,
reorganization, moratorium or other laws of general application relating to or
affecting enforcement of creditors' rights.

      Section 6.10. Severability. Whenever possible, each provision or portion
of any provision of this Agreement will be interpreted in such manner as to be
effective and valid under applicable law, but if any provision or portion of any
provision of this Agreement is held to be invalid, illegal or unenforceable in
any respect under any applicable law or rule in any jurisdiction, such
invalidity, illegality or unenforceability will not affect any other provision
or portion of any provision in such jurisdiction, and this Agreement will be
reformed, construed and enforced in such jurisdiction as if such invalid,
illegal or unenforceable provision or portion of any provision had never been
contained herein, so long as the economic and legal substance of the
transactions contemplated hereby are not affected in a manner materially adverse
to either party hereto.

      Section 6.11. No Third Party Beneficiaries. This Agreement is not intended
to confer any rights or remedies upon any person other than the parties hereto.


                                       18
<PAGE>   22
      IN WITNESS WHEREOF, the undersigned have each caused this Agreement to be
signed by their respective officers, thereunto duly authorized, as of the date
first written above.

                               VALIGEN N.V.

                               By: /s/ Douglas Watson
                                   --------------------------------------------
                                   Name:    Douglas Watson
                                   Title:   President

                               IMCLONE SYSTEMS INCORPORATED

                               By: /s/ Samuel D. Waksal
                                   --------------------------------------------
                                   Name:    Samuel D. Waksal
                                   Title:   President