Document:

Exhibit 10.14

                                   ENZON, INC.

                            2001 INCENTIVE STOCK PLAN

Section 1. Purpose

      The purpose of the Plan is to promote the interests of the Company and its
shareholders by aiding the Company in attracting and retaining employees,
officers, consultants, independent contractors and Non-Employee Directors
capable of contributing to the future success of the Company, to offer such
persons incentives to put forth maximum efforts for the success of the Company's
business and to afford such persons an opportunity to acquire a proprietary
interest in the Company.

Section 2. Definitions

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

      1. "Affiliate" shall mean (i) any entity that, directly or indirectly
      through one or more intermediaries, is controlled by the Company and (ii)
      any entity in which the Company has a significant equity interest, in each
      case as determined by the Committee.

      2. "Award" shall mean any Option, Stock Appreciation Right, Restricted
      Stock, Restricted Stock Unit, Performance Award, Dividend Equivalent,
      Other Stock Grant or Other Stock-Based Award granted under the Plan.

      3. "Award Agreement" shall mean any written agreement, contract or other
      instrument or document evidencing any Award granted under the Plan. Each
      Award Agreement shall be subject to the applicable terms and conditions of
      the Plan and any other terms and conditions (not inconsistent with the
      Plan) determined by the Committee.

      4. "Board" shall mean the Board of Directors of the Company.

      5. "Code" shall mean the Internal Revenue Code of 1986, as amended from
      time to time, and any regulations promulgated thereunder.

      6. "Committee" shall mean a committee of Directors designated by the Board
      to administer the Plan. The Committee shall be comprised of not less than
      such number of Directors as shall be required to permit Awards granted
      under the Plan by the Committee to qualify under Rule 16b-3, and each
      member of the Committee shall be a "Non-Employee Director" within the
      meaning of Rule 16b-3 and an "outside director" within the meaning of
      Section 162(m) of the Code. The Company expects to have the Plan
      administered in accordance with the requirements for the award of
      "qualified performance-based compensation" within the meaning of Section
      162(m) of the Code.

      7. "Company" shall mean Enzon, Inc., a Delaware corporation, and any
      successor corporation.

      8. "Director" shall mean a member of the Board, including Non-Employee
      Directors.

      9. "Dividend Equivalent" shall mean any right granted under Section 6(E)
      of the Plan.

      10. "Eligible Person" shall mean any employee, officer, consultant,
      independent contractor or Director (including any Non-Employee Director)
      providing services to the Company or any Affiliate whom the Committee
      determines to be an Eligible Person.

      11. "Exchange Act" shall mean the Securities Exchange Act of 1934, as
      amended.

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      12. "Fair Market Value" shall mean, with respect to any property
      (including, without limitation, any Shares or other securities), the fair
      market value of such property determined by such methods or procedures as
      shall be established from time to time by the Committee. Notwithstanding
      the foregoing, unless otherwise determined by the Committee, the Fair
      Market Value of a Share as of a given date shall be, if the Shares are
      then traded on the Nasdaq National Market, the last reported sale price of
      one Share as reported on the Nasdaq National Market on such date or, if
      the Nasdaq National Market is not open for trading on such date, on the
      most recent preceding date when it is open for trading.

      13. "Family Members" shall be those persons related to a Participant as
      determined by the Committee.

      14. "Incentive Stock Option" shall mean an option granted under Section 6A
      of the Plan that is intended to meet the requirements of Section 422 of
      the Code or any successor provision.

      15. "Non-Employee Director" shall have the meaning ascribed in Rule 16b-3
      promulgated under the Exchange Act or any successor provision.

      16. "Non-Qualified Stock Option" shall mean an option granted under
      Section 6A of the Plan that is not intended to be an Incentive Stock
      Option.

      17. "Option" shall mean an Incentive Stock Option or a Non-Qualified Stock
      Option.

      18. "Other Stock Grant" shall mean any right granted under Section 6F of
      the Plan.

      19. "Other Stock-Based Award" shall mean any right granted under Section
      6G of the Plan.

      20. "Participant" shall mean an Eligible Person designated to be granted
      an Award under the Plan.

      21. "Performance Award" shall mean any right granted under Section 6D of
      the Plan.

      22. "Person" shall mean any individual, corporation, partnership,
      association or trust.

      23. "Plan" shall mean the Enzon, Inc. 2001 Incentive Stock Plan, as
      amended from time to time, the provisions of which are set forth herein.

      24. "Plan Year" shall mean a consecutive 12-month period ending on
      December 31 of each year.

      25. "Reload Option" shall mean any Option granted under Section 6A(5) of
      the Plan.

      26. "Restricted Stock" shall mean any Shares granted under Section 6C of
      the Plan.

      27. "Restricted Stock Unit" shall mean any unit granted under Section 6C
      of the Plan evidencing the right to receive a Share (or a cash payment
      equal to the Fair Market Value of a Share) at some future date.

      28. "Rule 16b-3" shall mean Rule 16b-3 promulgated by the Securities and
      Exchange Commission under the Exchange Act, or any successor rule or
      regulation.

      29. "Share" or "Shares" shall mean shares of common stock, $0.01 par value
      per share, of the Company or such other securities or property as may
      become subject to Awards pursuant to an adjustment made under Section 4C
      of the Plan.

      30. "Stock Appreciation Right" shall mean any right granted under Section
      6B of the Plan.

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Section 3. Administration

      A. Power and Authority of the Committee. The Plan shall be administered by
the Committee. Subject to the express provisions of the Plan and to applicable
law, the Committee shall have full power and authority to: (i) designate
Participants; (ii) determine the type or types of Awards to be granted to each
Participant under the Plan; (iii) determine the number of Shares to be covered
by (or the method by which payments, or other rights are to be calculated in
connection with) each Award; (iv) determine the terms and conditions of any
Award or Award Agreement; (v) amend the terms and conditions of any Award or
Award Agreement and accelerate the exercisability of any Award or the lapse of
restrictions relating to any Award; (vi) determine whether, to what extent and
under what circumstances Awards may be exercised in cash, Shares, promissory
notes, other securities, other Awards or other property, or canceled, forfeited
or suspended; (vii) determine whether, to what extent and under what
circumstances cash, Shares, promissory notes, other securities, other Awards,
other property and other amounts payable with respect to an Award under the Plan
shall be deferred either automatically or at the election of the holder thereof
or the Committee; (viii) interpret and administer the Plan and any instrument or
agreement, including an Award Agreement, relating to the Plan; (ix) establish,
amend, suspend or waive such rules and regulations and appoint such agents as it
shall deem appropriate for the proper administration of the Plan; and (x) make
any other determination and take any other action that the Committee deems
necessary or desirable for the administration of the Plan. Unless otherwise
expressly provided in the Plan, all designations, determinations,
interpretations and other decisions under or with respect to the Plan or any
Award shall be within the sole discretion of the Committee, may be made at any
time and shall be final, conclusive and binding upon any Participant, any holder
or beneficiary of any Award and any employee of the Company or any Affiliate.

      B. Delegation. The Committee may delegate its powers and duties under the
Plan to one or more Directors or officers of the Company, or to a committee of
Directors or officers, subject to such terms, conditions and limitations as the
Committee may establish in its sole discretion, provided, however, that the
Committee shall not delegate its powers and duties under the Plan (i) with
regard to officers or directors of the Company or any Affiliate who are subject
to Section 16 of the Exchange Act or (ii) in such a manner as would cause the
Plan not to comply with the requirements of Section 162(m) of the Code.

      C. Power and Authority of the Board. Notwithstanding anything to the
contrary contained herein, the Board may, at any time and from time to time,
without any further action of the Committee, exercise the powers and duties of
the Committee under the Plan.

Section 4. Shares Available for Awards

      A. Shares Available. Subject to adjustment as provided in Section 4C of
the Plan, the aggregate number of Shares that may be issued under all Awards
under the Plan shall be 2,000,000; provided that, any Shares with respect to
which Awards may be issued, but are not issued, under the Plan in any Plan Year
shall be carried forward and shall be available to be covered by Awards issued
in any subsequent Plan Year in which Awards may be issued under the Plan. Shares
to be issued under the Plan may be either authorized but unissued Shares or
Shares acquired in the open market or otherwise. Any Shares that are used by a
Participant as full or partial payment to the Company of the purchase price
relating to an Award, or in connection with the satisfaction of tax obligations
relating to an Award, shall again be available for granting Awards (other than
Incentive Stock Options) under the Plan. In addition, if any Shares covered by
an Award or to which an Award relates are not purchased or are forfeited, or if
an Award otherwise terminates without delivery of any Shares, then the number of
Shares counted against the aggregate number of Shares available under the Plan
with respect to such Award, to the extent of any such forfeiture or termination,
shall again be available for granting Awards under the Plan. Notwithstanding the
foregoing, the number of Shares available for granting Incentive Stock Options
under the Plan shall not exceed 2,000,000 shares subject to adjustment as
provided in the Plan and subject to the provisions of Section 422 or 424 of the
Code or any successor provision.

      B. Accounting for Awards. For purposes of this Section 4, if an Award
entitles the holder thereof to receive or purchase Shares, the number of Shares
covered by such Award or to which such Award relates shall be

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counted on the date of grant of such Award against the aggregate number of
Shares available for granting Awards under the Plan.

      C. Adjustments. In the event that the Committee shall determine that any
dividend or other distribution (whether in the form of cash, Shares, other
securities or other property), recapitalization, stock split, reverse stock
split, reorganization, merger, consolidation, split-up, spin-off, combination,
issuance of warrants or other rights to purchase Shares or other securities of
the Company to all holders of common stock pro rata whether as a dividend or
otherwise or other similar corporate transaction or event affects the Shares
such that an adjustment is determined by the Committee to be appropriate in
order to prevent dilution or enlargement of the benefits or potential benefits
intended to be made available under the Plan, then the Committee shall, in such
manner as it may deem equitable, adjust any or all of (i) the number and type of
Shares (or other securities or other property) that thereafter may be made the
subject of Awards, (ii) the number and type of Shares (or other securities or
other property) subject to outstanding Awards and (iii) the purchase or exercise
price with respect to any Award; provided, however, that the number of Shares
covered by any Award or to which such Award relates shall always be a whole
number.

      D. Award Limitations Under the Plan. No Eligible Person may be granted any
Award or Awards under the Plan, the value of which Award or Awards is based
solely on an increase in the value of the Shares after the date of grant of such
Award or Awards, for more than 1,000,000 Shares (subject to adjustment as
provided for in Section 4(c) of the Plan), in the aggregate in any calendar
year. The foregoing annual limitation specifically includes the grant of any
Award or Awards representing "qualified performance-based compensation" within
the meaning of Section 162(m) of the Code.

Section 5. Eligibility.

      Any Eligible Person shall be eligible to be designated a Participant. In
determining which Eligible Persons shall receive an Award and the terms of any
Award, the Committee may take into account the nature of the services rendered
by the respective Eligible Persons, their present and potential contributions to
the success of the Company or such other factors as the Committee, in its
discretion, shall deem relevant. Notwithstanding the foregoing, an Incentive
Stock Option may only be granted to full or part-time employees (which term as
used herein includes, without limitation, officers and Directors who are also
employees), and an Incentive Stock Option shall not be granted to an employee of
an Affiliate unless such Affiliate is also a "subsidiary corporation" of the
Company within the meaning of Section 424(f) of the Code or any successor
provision.

Section 6. Awards

      A.    Options. The Committee is hereby authorized to grant Options to
            Eligible Persons with the following terms and conditions and with
            such additional terms and conditions not inconsistent with the
            provisions of the Plan as the Committee shall determine:

            1. Exercise Price. The purchase price per Share purchasable under an
            Option shall be determined by the Committee; provided, however, that
            such purchase price shall not be less than 100% of the Fair Market
            Value of a Share on the date of grant of such Option.

            2. Option Term. The term of each Option shall be fixed by the
            Committee, but, shall in no event exceed 10 years from the date on
            which such Option is granted.

            3. Time and Method of Exercise. The Committee shall determine the
            time or times at which an Option may be exercised in whole or in
            part and the method or methods by which, and the form or forms
            (including, without limitation, cash, Shares, promissory notes,
            other securities, other Awards or other property, or any combination
            thereof, having a Fair Market Value on the exercise date equal to
            the applicable exercise price) in which, payment of the exercise
            price with respect thereto may be made or deemed to have been made.

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            4. Incentive Stock Options. Notwithstanding anything in the Plan to
            the contrary, the following additional provisions shall apply to the
            grant of stock options which are intended to qualify as Incentive
            Stock Options:

                  (a)   The aggregate Fair Market Value (determined as of the
                        time the option is granted) of the Shares with respect
                        to which Incentive Stock Options are exercisable for the
                        first time by any Participant during any calendar year
                        (under this Plan and all other plans of the Company and
                        its Affiliates) shall not exceed $100,000.

                  (b)   All Incentive Stock Options must be granted within ten
                        years from the earlier of the date on which this Plan
                        was adopted by the Board or the date this Plan was
                        approved by the shareholders of the Company.

                  (c)   Unless sooner exercised, all Incentive Stock Options
                        shall expire and no longer be exercisable no later than
                        10 years after the date of grant; provided, however,
                        that in the case of a grant of an Incentive Stock Option
                        to a Participant who, at the time such Option is
                        granted, owns (within the meaning of Section 422 of the
                        Code) stock possessing more than 10% of the total
                        combined voting power of all classes of stock of the
                        Company or of its Affiliate, such Incentive Stock Option
                        shall expire and no longer be exercisable no later than
                        5 years from the date of grant.

                  (d)   The purchase price per Share for an Incentive Stock
                        Option shall be not less than 100% of the Fair Market
                        Value of a Share on the date of grant of the Incentive
                        Stock Option; provided, however, that, in the case of
                        the grant of an Incentive Stock Option to a Participant
                        who, at the time such Option is granted, owns (within
                        the meaning of Section 422 of the Code) stock possessing
                        more than 10% of the total combined voting power of all
                        classes of stock of the Company or of its Affiliate, the
                        purchase price per Share purchasable under an Incentive
                        Stock Option shall be not less than 110% of the Fair
                        Market Value of a Share on the date of grant of the
                        Inventive Stock Option.

                  (e)   Any Incentive Stock Option authorized under the Plan
                        shall contain such other provisions as the Committee
                        shall deem advisable, but shall in all events be
                        consistent with and contain all provisions required in
                        order to qualify the Option as an Incentive Stock
                        Option.

            5. Reload Options. The Committee may grant Reload Options,
            separately or together with another Option, pursuant to which,
            subject to the terms and conditions established by the Committee,
            the Participant would be granted a new Option when the payment of
            the exercise price of a previously granted option is made by the
            delivery of Shares owned by the Participant pursuant to Section
            6A(3) hereof or the relevant provisions of another plan of the
            Company, and/or when Shares are tendered or withheld as payment of
            the amount to be withheld under applicable income tax laws in
            connection with the exercise of an Option, which new Option would be
            an Option to purchase the number of Shares not exceeding the sum of
            (A) the number of Shares so provided as consideration upon the
            exercise of the previously granted option to which such Reload
            Option relates and (B) the number of Shares, if any, tendered or
            withheld as payment of the amount to be withheld under applicable
            tax laws in connection with the exercise of the option to which such
            Reload Option relates pursuant to the relevant provisions of the
            plan or agreement relating to such option. Reload Options may be
            granted with respect to Options previously granted under the Plan or
            any other stock option plan of the Company or may be granted in
            connection with any Option granted under the Plan or any other stock
            option plan of the Company at the time of such grant. Such Reload
            Options shall have a per share exercise price equal to the Fair
            Market Value of one Share as of the date of grant of the new Option.
            Any Reload Option shall be subject to availability of sufficient
            Shares for grant under the Plan. Shares surrendered as part or all
            of the exercise price

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            of the Option to which it relates that have been owned by the
            optionee less than six months will not be counted for purposes of
            determining the number of Shares that may be purchased pursuant to a
            Reload Option.

      B. Stock Appreciation Rights. The Committee is hereby authorized to grant
      Stock Appreciation Rights to Eligible Persons subject to the terms of the
      Plan and any applicable Award Agreement. A Stock Appreciation Right
      granted under the Plan shall confer on the holder thereof a right to
      receive upon exercise thereof the excess of (i) the Fair Market Value of
      one Share on the date of exercise (or, if the Committee shall so
      determine, at any time during a specified period before or after the date
      of exercise) over (ii) the grant price of the Stock Appreciation Right as
      specified by the Committee, which price shall not be less than 100% of the
      Fair Market Value of one Share on the date of grant of the Stock
      Appreciation Right. Subject to the terms of the Plan and any applicable
      Award Agreement, the grant price, term, methods of exercise, dates of
      exercise, methods of settlement and any other terms and conditions of any
      Stock Appreciation Right shall be as determined by the Committee. The
      Committee may impose such conditions or restrictions on the exercise of
      any Stock Appreciation Right as it may deem appropriate.

      C. Restricted Stock and Restricted Stock Units. The Committee is hereby
      authorized to grant Restricted Stock and Restricted Stock Units to
      Eligible Persons with the following terms and conditions and with such
      additional terms and conditions not inconsistent with the provisions of
      the Plan as the Committee shall determine:

            1.    Restrictions. Shares of Restricted Stock and Restricted Stock
                  Units shall be subject to such restrictions as the Committee
                  may impose (including, without limitation, a waiver by the
                  Participant of the right to vote or to receive any dividend or
                  other right or property with respect thereto), which
                  restrictions may lapse separately or in combination at such
                  time or times, in such installments or otherwise as the
                  Committee may deem appropriate.

            2.    Stock Certificates. Any Restricted Stock granted under the
                  Plan shall be registered in the name of the Participant and
                  shall bear an appropriate legend referring to the terms,
                  conditions and restrictions applicable to such Restricted
                  Stock.

            3.    Forfeiture. Except as otherwise determined by the Committee,
                  upon a Participant's termination of employment (as determined
                  under criteria established by the Committee) during the
                  applicable restriction period, all Shares of Restricted Stock
                  and Restricted Stock Units held by the Participant at such
                  time shall be forfeited and reacquired by the Company;
                  provided, however, that the Committee may, when it finds that
                  a waiver would be in the best interest of the Company, waive
                  in whole or in part any or all remaining restrictions with
                  respect to Shares of Restricted Stock or Restricted Stock
                  Units.

      D. Performance Awards. The Committee is hereby authorized to grant
      Performance Awards to Eligible Persons subject to the terms of the Plan
      and any applicable Award Agreement. A Performance Award granted under the
      Plan (i) may be denominated or payable in cash, Shares (including, without
      limitation, Restricted Stock and Restricted Stock Units), other
      securities, other Awards or other property and (ii) shall confer on the
      holder thereof the right to receive payments, in whole or in part, upon
      the achievement of such performance goals during such performance periods
      as the Committee shall establish. Subject to the terms of the Plan and any
      applicable Award Agreement, the performance goals to be achieved during
      any performance period, the length of any performance period, the amount
      of any Performance Award granted, the amount of any payment or transfer to
      be made pursuant to any Performance Award and any other terms and
      conditions of any Performance Award shall be determined by the Committee.

      E. Dividend Equivalents. The Committee is hereby authorized to grant
      Dividend Equivalents to Eligible Persons under which the Participant shall
      be entitled to receive payments (in cash, Shares, other securities, other
      Awards or other property as determined in the discretion of the Committee)
      equivalent to the amount of cash dividends paid by the Company to holders
      of Shares with respect to a number of Shares determined

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      by the Committee. Subject to the terms of the Plan and any applicable
      Award Agreement, such Dividend Equivalents may have such terms and
      conditions as the Committee shall determine.

      F. Other Stock Grants. The Committee is hereby authorized, subject to the
      terms of the Plan and any applicable Award Agreements, to grant to
      Eligible Persons Shares without restrictions thereon as are deemed by the
      Committee to be consistent with the purpose of the Plan.

      G. Other Stock-Based Awards. The Committee is hereby authorized to grant
      to Eligible Persons, subject to the terms of the Plan and any applicable
      Award Agreements, such other Awards that are denominated or payable in,
      valued in whole or in part by reference to, or otherwise based on or
      related to, Shares (including, without limitation, securities convertible
      into Shares), as are deemed by the Committee to be consistent with the
      purpose of the Plan. Shares or other securities delivered pursuant to a
      purchase right granted under this Section 6(G) shall be purchased for such
      consideration, which may be paid by such method or methods and in such
      form or forms (including, without limitation, cash, Shares, promissory
      notes, other securities, other Awards or other property or any combination
      thereof), as the Committee shall determine, the value of which
      consideration, as established by the Committee, shall not be less than
      100% of the Fair Market Value of such Shares or other securities as of the
      date such purchase right is granted.

      H. General

            1.    Consideration for Awards. Awards shall be granted for no cash
                  consideration or for any cash or other consideration as may be
                  determined by the Committee or required by applicable law.

            2.    Awards May Be Granted Separately or Together. Awards may, in
                  the discretion of the Committee, be granted either alone or in
                  addition to, in tandem with or in substitution, for any other
                  Award or any award granted under any plan of the Company or
                  any Affiliate other than the Plan. Awards granted in addition
                  to or in tandem with other Awards or in addition to or in
                  tandem with awards granted under any such other plan of the
                  Company or any Affiliate may be granted either at the same
                  time as or at a different time from the grant of such other
                  Awards or awards.

            3.    Forms of Payment under Awards. Subject to the terms of the
                  Plan and of any applicable Award Agreement, payments or
                  transfers to be made by the Company or an Affiliate upon the
                  grant, exercise or payment of an Award may be made in such
                  form or forms as the Committee shall determine (including,
                  without limitation, cash, Shares, promissory notes, other
                  securities, other Awards or other property or any combination
                  thereof), and may be made in a single payment or transfer, in
                  installments or on a deferred basis, in each case in
                  accordance with rules and procedures established by the
                  Committee. Such rules and procedures may include, without
                  limitation, provisions for the payment or crediting of
                  reasonable interest on installment or deferred payments or the
                  grant or crediting of Dividend Equivalents with respect to
                  installment or deferred payments.

            4.    Limits on Transfer of Awards. No Award (other than Other Stock
                  Grants) and no right under any such Award shall be
                  transferable by a Participant otherwise than by will or by the
                  laws of descent and distribution and the Company shall not be
                  required to recognize any attempted assignment of such rights
                  by any Participant; provided, however, that, if so determined
                  by the Committee, a Participant may, in the manner established
                  by the Committee, (a) designate a beneficiary or beneficiaries
                  to exercise the rights of the Participant and receive any
                  property distributable with respect to any Award upon the
                  death of the Participant and (b) transfer Awards, except in
                  the case of an Incentive Stock Option, to Family Members
                  pursuant to terms determined by the Committee. Except as
                  otherwise provided in this Plan or in any applicable Award
                  Agreement or amendment thereto (other than an Award Agreement
                  relating to an Incentive Stock Option), pursuant to terms
                  determined by the Committee, each Award or right under any
                  Award shall be exercisable during the Participant's lifetime
                  only by the Participant or, if permissible

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                  under applicable law, by the Participant's guardian or legal
                  representative. Except as otherwise provided in this Plan or
                  in any applicable Award Agreement or amendment thereto (other
                  than an Award Agreement relating to an Incentive Stock
                  Option), no Award or right under any such Award may be
                  pledged, alienated, attached or otherwise encumbered, and any
                  purported pledge, alienation, attachment or encumbrance
                  thereof shall be void and unenforceable against the Company or
                  any Affiliate.

            5.    Term of Awards. The term of each Award shall be for such
                  period as may be determined by the Committee; provided,
                  however, that in the case of an Incentive Stock Option such
                  Option shall not be exercisable after the expiration of 10
                  years from the date such Option is granted.

            6.    Restrictions; Securities Exchange Listing. All Shares or other
                  securities delivered under the Plan pursuant to any Award or
                  the exercise thereof shall be subject to such restrictions as
                  the Committee may deem advisable under the Plan, applicable
                  federal or state securities laws and regulatory requirements,
                  and the Committee may cause appropriate entries to be made or
                  legends to be placed on the certificates for such Shares or
                  other securities to reflect such restrictions. If the Shares
                  or other securities of the Company are traded on a securities
                  exchange, the Company shall not be required to deliver any
                  Shares or other securities covered by an Award unless and
                  until such Shares or other securities have been admitted for
                  trading on such securities exchange.

Section 7. Amendment and Termination; Adjustments

      A. Amendments to the Plan. The Board may amend, alter, suspend,
discontinue or terminate the Plan at any time; provided, however, that,
notwithstanding any other provision of the Plan or any Award Agreement, without
the approval of the shareholders of the Company, no such amendment, alteration,
suspension, discontinuation or termination shall be made that, absent such
approval would violate the rules or regulations of the Nasdaq National Market or
any other securities exchange that is applicable to the Company.

      B. Amendments to Awards. The Committee may waive any conditions of or
rights of the Company under any outstanding Award, prospectively or
retroactively. Except as otherwise provided herein or in an Award Agreement, the
Committee may not amend, alter, suspend, discontinue or terminate any
outstanding Award, prospectively or retroactively, if such action would
adversely affect the rights of the holder of such Award, without the consent of
the Participant or holder or beneficiary thereof.

      C. Correction of Defects, Omissions and Inconsistencies. The Committee may
correct any defect, supply any omission or reconcile any inconsistency in the
Plan or any Award in the manner and to the extent it shall deem desirable to
carry the Plan into effect.

Section 8. Income Tax Withholding

      In order to comply with all applicable national, federal, state or local
income tax laws or regulations, the Company may take such action as it deems
appropriate to ensure that all applicable national, federal, state or local
payroll, withholding, income or other taxes, which are the sole and absolute
responsibility of a Participant, are withheld or collected from such
Participant. In order to assist a Participant in paying all or a portion of the
national, federal, state and local taxes to be withheld or collected upon
exercise or receipt of (or the lapse of restrictions relating to) an Award, the
Committee, in its discretion and subject to such additional terms and conditions
as it may adopt, may permit the Participant to satisfy such tax obligation by
(i) electing to have the Company withhold a portion of the Shares otherwise to
be delivered upon exercise or receipt of (or the lapse of restrictions relating
to) such Award with a Fair Market Value equal to the amount of such taxes or
(ii) delivering to the Company Shares other than Shares issuable upon exercise
or receipt of (or the lapse of restrictions relating to) such Award with a Fair
Market Value equal to the amount of such taxes. The election, if any, must be
made on or before the date that the amount of tax to be withheld is determined.

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Section 9. General Provisions

      A. No Rights to Awards. No Eligible Person, Participant or other Person
shall have any claim to be granted any Award under the Plan, and there is no
obligation for uniformity of treatment of Eligible Persons, Participants or
holders or beneficiaries of Awards under the Plan. The terms and conditions of
Awards need not be the same with respect to any Participant or with respect to
different Participants.

      B. Plan Provisions Control. In the event that any provision of an Award
Agreement conflicts with or is inconsistent in any respect with the terms of the
Plan as set forth herein or subsequently amended, the terms of the Plan shall
control.

      C. No Rights of Shareholders. Except with respect to Shares of Restricted
Stock as to which the Participant has been granted the right to vote, neither a
Participant nor the Participant's legal representative shall be, or have any of
the rights and privileges of, a stockholder of the Company with respect to any
Shares issuable to such Participant upon the exercise or payment of any Award,
in whole or in part, unless and until such Shares have been issued in the name
of such Participant or such Participant's legal representative without
restrictions thereto.

      D. No Limit on Other Compensation Arrangements. Nothing contained in the
Plan shall prevent the Company or any Affiliate from adopting or continuing in
effect other or additional compensation arrangements, and such arrangements may
be either generally applicable or applicable only in specific cases.

      E. No Right to Employment. The grant of an Award shall not be construed as
giving a Participant the right to be retained in the employ of the Company or
any Affiliate, nor will it affect in any way the right of the Company or an
Affiliate to terminate such employment at any time, with or without cause. In
addition, the Company or an Affiliate may at any time dismiss a Participant from
employment free from any liability or any claim under the Plan or any Award,
unless otherwise expressly provided in the Plan or in any Award Agreement.
Nothing in this Plan shall confer on any person any legal or equitable right
against the Company or any Affiliate, directly or indirectly, or give rise to
any cause of action at law or in equity against the Company or an Affiliate. The
Awards granted hereunder shall not form any part of the wages or salary of any
Eligible Person for purposes of severance pay or termination indemnities,
irrespective of the reason for termination of employment. Under no circumstances
shall any person ceasing to be an employee of the Company or any Affiliate be
entitled to any compensation for any loss of any right or benefit under the Plan
which such employee might otherwise have enjoyed but for termination of
employment, whether such compensation is claimed by way of damages for wrongful
or unfair dismissal, breach of contract or otherwise. By participating in the
Plan, each Participant shall be deemed to have accepted all the conditions of
the Plan and the terms and conditions of any rules and regulations adopted by
the Committee and shall be fully bound thereby.

      F. Governing Law. The validity, construction and effect of the Plan or any
Award, and any rules and regulations relating to the Plan or any Award, shall be
determined in accordance with the internal laws, and not the law of conflicts,
of the State of Delaware.

      G. Severability. If any provision of the Plan or any Award is or becomes
or is deemed to be invalid, illegal or unenforceable in any jurisdiction or
would disqualify the Plan or any Award under any law deemed applicable by the
Committee, such provision shall be construed or deemed amended to conform to
applicable laws, or if it cannot be so construed or deemed amended without, in
the determination of the Committee, materially altering the purpose or intent of
the Plan or the Award, such provision shall be stricken as to such jurisdiction
or Award, and the remainder of the Plan or any such Award shall remain in full
force and effect.

      H. No Trust or Fund Created. Neither the Plan nor any Award shall create
or be construed to create a trust or separate fund of any kind or a fiduciary
relationship between the Company or any Affiliate and a Participant or any other
Person. To the extent that any Person acquires a right to receive payments from
the Company or any Affiliate pursuant to an Award, such right shall be no
greater than the right of any unsecured general creditor of the Company or any
Affiliate.

                                       9

                                      E-44
<PAGE>

      I. Other Benefits. No compensation or benefit awarded to or realized by
any Participant under the Plan shall be included for the purpose of computing
such Participant's compensation under any compensation-based retirement,
disability, or similar plan of the Company unless required by law or otherwise
provided by such other plan.

      J. No Fractional Shares. No fractional Shares shall be issued or delivered
pursuant to the Plan or any Award, and the Committee shall determine whether
cash shall be paid in lieu of any fractional Shares or whether such fractional
Shares or any rights thereto shall be canceled, terminated or otherwise
eliminated.

      K. Headings. Headings are given to the Sections and subsections of the
Plan solely as a convenience to facilitate reference. Such headings shall not be
deemed in any way material or relevant to the construction or interpretation of
the Plan or any provision thereof.

Section 10. Effective Date of the Plan.

      The Plan shall be effective on December 4, 2001 subject to approval by the
shareholders of the Company on such date.

Section 11. Term of the Plan.

      No Award shall be granted under the Plan ten years after the effective
date or any earlier date of discontinuation or termination established pursuant
to Section 7A of the Plan. However, unless otherwise expressly provided in the
Plan or in an applicable Award Agreement, any Award theretofore granted may
extend beyond such date, and the authority of the Committee provided for
hereunder with respect to the Plan and any Awards, and the authority of the
Board to amend the Plan, shall extend beyond the termination of the Plan.

                                       10

                                      E-45Exhibit 10.15

                                                                   29 OCT 1990:1

             ***

             -----------------------------------------------------

                   DEVELOPMENT, LICENSE AND SUPPLY AGREEMENT

             -----------------------------------------------------

                              SCHERING CORPORATION

                                       AND

                                   ENZON, INC.

             -----------------------------------------------------

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                      E-46
<PAGE>

                                      INDEX

                                                                          Page
                                                                          ----
Article 1   :     Definitions                                               2
Article 2   :     Development Program                                       8
Article 3   :     Manufacture of Commercial Quantities of Agreement
                      Product; Know-How Transfer                           13
Article 4   :     Exclusive License                                        18
Article 5   :     Patent Rights                                            20
Article 6   :     Payments                                                 23
Article 7   :     Royalty Terms                                            24
Article 8   :     Royalty Payment Terms                                    28
Article 9   :     Term and Termination                                     31
Article 10  :     Representations and Warranties; Indemnification          33
Article 11  :     Confidentiality                                          38
Article 12  :     Governing Law                                            41
Article 13  :     Assignment or Delegation                                 42
Article 14  :     Force Majeure                                            43
Article 15  :     Independent Contractor; Publicity                        44
Article 16  :     No Inconsistent Obligations                              45
Article 17  :     Severability; Survival                                   45
Article 18  :     Captions                                                 46
Article 19  :     Waiver                                                   46
Article 20  :     Notice                                                   47
Article 21  :     Entire Agreement                                         47

                                                                      10/29/90:1

                                      E-47
<PAGE>

                                    SCHEDULES

Schedule A-1 - Specifications for Agreement Substance
         A-2 - Specifications for Bulk Concentrate
         A-3 - Specifications for Agreement Product

Schedule B   - Development Program

Schedule C   - Patent Rights

Schedule D   - Criteria for Efficacy in Pilot Study

Schedule E   - Supply Agreement

Schedule F   - Press Release

                                                                      10/29/90:1

                                      E-48
<PAGE>

                    DEVELOPMENT, LICENSE AND SUPPLY AGREEMENT

This agreement is made and effective as of November 14, 1990 by and between

                  ENZON, Inc.
                  40 Cragwood Road
                  South Plainfield, NJ 07080-2406, a Delaware corporation,

hereinafter referred to as "ENZON",
and

                  SCHERING Corporation
                  2000 Galloping Hill Road
                  Kenilworth, New Jersey 07033, a New Jersey corporation,

hereinafter referred to as SCHERING:

WHEREAS, SCHERING markets a proprietary pharmaceutical alpha interferon product
(hereinafter "Intron A") in various finished pharmaceutical dosage forms; and

WHEREAS, ENZON has developed a technology for coupling polyethylene glycol
("PEG") with polypeptides to create longer-acting therapeutic products; and

WHEREAS, SCHERING and ENZON have performed preliminary in vitro testing of a
compound made from Intron A and PEG and now desire, subject to the terms of this
Agreement, to further develop, test, conduct clinical trials of, and market the
resulting product; and

                                                                      10/29/90:1

                                      E-49
<PAGE>

WHEREAS the parties desire to enter into an agreement whereby ENZON shall
develop, under the terms of this Agreement, a longer-acting product in which
PEG is coupled with Intron A, develop scale-up and manufacturing procedures, and
manufacture such product for SCHERING, consistent with all applicable Good
Manufacturing Practices ("GMP's") and SCHERING shall, subject to the terms and
conditions of this Agreement, test, clinically develop and market worldwide, the
resulting Agreement Product as hereinafter defined;

NOW, THEREFORE, in consideration of the mutual premises and covenants set forth
below, the parties agree as follows:

Article 1 - Definitions

1.1 "Affiliate" shall mean any corporation or business entity controlled by,
controlling, or under common control with the respective party. For this purpose
"control" shall mean the direct or indirect beneficial ownership of at least
fifty percent (50%) of the voting stock of, or at least fifty percent (50%)
interest in the income of, such corporation or other business entity.

1.2 "Agreement Product" shall mean any pharmaceutical product in which Agreement
Substance as an active ingredient is coupled with PEG. The specifications for
the Agreement Product to be developed by SCHERING under this Agreement, in its
diluted form, will be set forth in Schedule A-3 hereto, which may be amended
from time-

                                    -2-                               10/29/90:1

                                      E-50
<PAGE>

to-time, as mutually agreed by the parties hereto, or as needed to conform with
the IND (hereinafter defined) for Agreement Product. Agreement Product shall be
supplied to SCHERING by ENZON in the form of Bulk Concentrate, as hereinafter
defined.

1.3 "Agreement Substance" shall mean alpha interferon, including, without
limitation, the alpha interferon the specifications for which are set forth in
Schedule A-1 attached hereto (Specifications for Agreement Substance).

1.4 "Bulk Concentrate" shall mean Agreement Product in bulk concentrated form
meeting the criteria set forth in Schedule A-2 hereof. (Specifications for Bulk
Concentrate).

1.5 "Development Program" shall mean all development work undertaken by the
parties hereto with respect to Agreement Substance or Agreement Product,
relating to the development program for Agreement Product (Schedule B) hereto,
including the applicable objectives, protocols and timetables, by phases and
cumulatively, as well as any modifications to the development program set forth
[in Schedule B hereto made by Schering] in Schedule B hereto requested by either
party and accepted by the other party in writing, which acceptance shall not be
unreasonably withheld.

1.6 "FDA" shall mean the United States Food and Drug Administration or any
successor agency thereof, as well as any

                                      -3-                             10/29/90:1

                                      E-51
<PAGE>

foreign equivalent thereof, including, where applicable, price-approval
authorities.

1.7 "FDA Approval" shall mean written notification of the approval (without
qualification which would materially adversely affect SCHERING's ability to
market Agreement Product) from the FDA of a PLA for Agreement Product for which
SCHERING seeks such approval, which allows SCHERING to commercially sell
Agreement Product.

1.8   (a)   "Know-How" shall mean all confidential scientific, medical, and
            technical data, instructions, processes, formulae, unpatented
            inventions, expert opinion and information, whether written or
            otherwise, necessary (as set forth in the PLA for Agreement Product)
            for the development, manufacture, use or sale of Agreement
            Substance, Bulk Concentrate or Agreement Product. Know-How shall
            include, without limitation, all biological, chemical,
            pharmacological, toxicological, pharmaceutical, physical and
            analytical, research, testing, safety, quality control,
            manufacturing, marketing, test and clinical data and information
            necessary (as set forth in the PLA for Agreement Product) for the
            manufacture, use or sale of Agreement Substance, Bulk Concentrate or
            Agreement Product. For purposes

                                       -4-                            10/29/90:1

                                      E-52
<PAGE>

            of this Agreement, Know-How shall not include any Know-How (as
            defined in this Article 1.8(a)) which SCHERING can clearly
            demonstrate was developed by SCHERING or its Affiliates without the
            aid, application or use of any ENZON Existing or Development
            Know-How (as hereinafter defined) or the participation of any
            SCHERING employee or consultant who has knowledge of the contents of
            such ENZON Existing or Development Know-How ("SCHERING Independent
            Know-How"). Know-How also shall not include any information, data,
            test result, study result, trial result, expert opinion or analysis
            related to the pharmacology, toxicology, pharmacokinetics, clinical
            trials or manufacturing steps to be taken by SCHERING, covered under
            SCHERING'S PLA (hereinafter referred to as "SCHERING Information").
            Know-How as defined in this subparagraph (a) is divided into the
            three categories described below, and the two subcategories
            described in Articles 4.1 and 4.2 hereof.

      (b)   "ENZON Existing Know-How" shall mean all Know-How owned or
            controlled by ENZON or its Affiliates on or before April 3, 1990.

                                      -5-                            10/29/90:1

                                      E-53
<PAGE>

      (c)   "SCHERING Existing Know-How" shall mean all Know-How owned or
            controlled by SCHERING or its Affiliates on or before April 3, 1990.

      (d)   "Development Know-How" shall mean all Know-How developed by or on
            behalf of either or both parties and/or their Affiliates under this
            Agreement.

1.9 "IND" shall mean an Investigational New Drug Application or its foreign
equivalent, as required to be filed with the FDA.

1.10 "Net Sales" shall mean the proceeds actually received from sales of
Agreement Product in any country in the Territory by SCHERING or any of its
Affiliates or sublicensees to independent third party customers in bona fide,
arm's-length transactions, less:

      (a)   actual allowances for returns, damages or otherwise, and discounts,
            rebates and allowances to customers, including cash, credit or free
            goods allowances; and

      (b)   freight or other transportation charges, including insurance,
            actually allowed or paid on account of the delivery of Agreement
            Product to purchasers thereof; and

                                       -6-                            10/29/90:1

                                      E-54
<PAGE>

      (c)   taxes (except income taxes) or duties paid, absorbed or otherwise
            imposed on the sale, including, without limitation, value added
            taxes.

1.11 "Patent Rights" shall mean: (a) all claims of any patent applications and
issued patents arising under this Agreement; and (b) any and all reissues,
extensions (or other governmental actions which provide exclusive rights to the
patent holder in the patented subject matter beyond the original patent
expiration date), substitutions, confirmations, registrations, revalidations,
additions, continuations, continuations-in-part or divisions of or to any of
the foregoing which are hereafter granted in the Territory. For purposes of this
Agreement, Patent Rights shall not include: (1) any patentable invention related
to the SCHERING Information, or (2) any patentable invention which SCHERING can
clearly demonstrate was invented or developed by SCHERING or its Affiliates,
without the aid, application or use of any ENZON Know-How (as hereinafter
defined) or the participation of any SCHERING employee or consultant who has
knowledge of the contents of the ENZON Know-How ("SCHERING Independent Patent
Rights"). The parties shall list Patent Rights as they arise in Schedule C
hereto, which shall be updated by the parties periodically to reflect changes in
the status of the Patent Rights.

                                       -7-                            10/29/90:1

                                      E-55
<PAGE>

1.12 "PLA" shall mean a Product License Application, or its foreign equivalent,
as required to be filed with the FDA to obtain approval to sell the Agreement
Product.

1.13 "Territory" shall mean all countries of the world.

1.14 "Valid Claim" shall mean any claim contained in any pending patent
application or issued patent included within the Patent Rights, which has not
been abandoned or declared invalid in a non-appealable order, as the case may
be, and which would be infringed by the manufacture, use or sale of Agreement
Product in the absence of the licenses granted hereunder.

Article 2 - Development Program

2.1 The parties have agreed upon, and set forth in Schedules A-2 and A-3 hereof,
specifications and criteria for Bulk Concentrate and Agreement Product.

2.2 The parties have executed a Letter of Agreement dated April 3, 1990, wherein
ENZON agreed, inter alia, to provide SCHERING with a *** sample batch and
a *** production batch of Agreement Product in exchange for certain
payments.

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                       -8-                           10/29/90:1

                                      E-56
<PAGE>

2.3 The parties have agreed upon, and set forth in Schedule B hereto, a
Development Program for Agreement Product. ENZON and SCHERING shall use diligent
efforts to carry out their designated responsibilities under the Development
Program in a timely fashion, except in those cases where a party has obtained
the prior written approval of the other to modify a portion of the Development
Program. The Development Program shall be reviewed from time-to-time as shall
be agreed by the parties hereto, and the terms thereof may only be changed as
mutually agreed in writing by the parties hereto or as suggested by the FDA
(with SCHERING's concurrence), or as ordered by the FDA. ENZON's obligations
under the Development Program are subject to its receipt of Agreement Substance
from SCHERING in a timely manner.

2.4 All trials for Agreement Product specified in the Development Program,
including any toxicological, pre-clinical or clinical trials that may be
required for drug registrations and approvals in the Territory, shall be the
responsibility of SCHERING. SCHERING shall use diligent efforts to obtain such
approvals and shall bear all costs of such trials; provided, however, that the
manufacture and supply of Agreement Product used in the trials referenced in the
Development Program shall be the responsibility of ENZON, but at SCHERING's sole
cost and expense, as set forth in Article 3.1 hereof, and subject to the
provisions of Article 2.8 and the other provisions of Article 3 hereof.

                                       -9-                            10/29/90:1

                                      E-57
<PAGE>

2.5 Except as expressly provided otherwise herein, ENZON shall be solely
responsible for all direct and indirect costs and expenses of its employees,
agents, consultants, or subcontractors and for materials, plant and equipment
which ENZON employs or utilizes in carrying out its designated responsibilities
under the Development Program or in manufacturing Agreement Product for any
purpose hereunder.

2.6 The parties hereto shall keep each other informed of their progress with
respect to their respective responsibilities under the Development Program by
means of a quarterly written summary report in a form to be agreed upon by the
parties hereto. In addition, SCHERING will have access to ENZON's facilities to
observe progress of the Development Program, to the extent reasonably necessary
to successfully complete the Development Program, upon reasonable advance
notice. Each party hereto shall provide the other party with copies of all
Know-How in such other party's possession reasonably required in order to
enable SCHERING to obtain FDA Approval, including, without limitation, the
obligations of Article 2 hereof. Whenever reasonably feasible, such copies may
be given directly to the FDA rather than to the other party when in connection
with obtaining the FDA approval. ENZON shall, at SCHERING's reasonable request,
take all reasonable steps necessary to allow SCHERING to reference ENZON's Drug
Master File for Agreement Product; provided, however, that SCHERING shall not
have the right to have access to, view or have copies of

                                      -10-                            10/29/90:1

                                      E-58
<PAGE>

ENZON's Drug Master File for Agreement Product. The right of reference and to
receive Know-How provided in this Article 2.6 shall not entitle SCHERING to use
or receive ENZON's Know-How for manufacturing the Agreement Product, except as
expressly permitted hereunder.

2.7 ENZON and SCHERING shall each designate one (1) senior development employee
as technical liaison to handle all technical matters and communications relating
to the Development Program.

2.8 SCHERING has provided ENZON with certain Know-How relating to the Agreement
Substance and shall provide ENZON with any additional Know-How in SCHERING's
possession as SCHERING and ENZON shall reasonably agree is required in
connection with ENZON's obligations under the Development Program.

2.9 SCHERING shall supply to ENZON in a timely fashion, at no cost, quantities
of Agreement Substance necessary to make the Agreement Product called for in the
Development Program and to manufacture Agreement Product as required under
Article 2.4 hereof. ENZON agrees to advise SCHERING, at least sixty (60) days in
advance of the requested delivery thereof, of its requirements for the Agreement
Substance.

2.10 SCHERING shall use its best efforts (comparable to efforts it uses with
other products of similar status) to file the

                                     -11-                             10/29/90:1

                                      E-59
<PAGE>

foreign equivalent of a Product License Application for Agreement Product in at
least three (3) of the following countries: France, West Germany, Italy, Spain,
United Kingdom, Japan and Canada, within twelve (12) months after the submission
of the initial PLA in the United States of America for Agreement Product, and
similarly file in the remaining countries set forth in this Article 2.10 within
twenty-four (24) months after such submission.

2.11 ENZON shall deliver to SCHERING in a timely fashion sufficient Agreement
Product for SCHERING to carry out the activities contemplated by Article 2.4
hereof.

2.12 ***

2.13 During the term of this Agreement, anything to the contrary contained
herein notwithstanding, ENZON shall not develop any product containing PEG and
alpha interferon for itself or with or for any third party, nor shall it enter
into discussions or negotiations with any such third party concerning any
product containing PEG and alpha interferon.

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                      -12-                            10/29/90:1

                                      E-60
<PAGE>

Article 3 - Manufacture of Commercial Quantities of Agreement Product; Know-How
Transfer

3.1 SCHERING shall notify ENZON, no later than the date of the first PLA filing,
that it intends to introduce Agreement Product into commerce. Upon such
notification, ENZON shall commence preparing to supply commercial quantities of
Agreement Product to SCHERING under a supply agreement (the "Supply Agreement")
executed by the parties concurrently with this Agreement and attached hereto as
Schedule E. SCHERING shall pay ENZON for Agreement Product to be supplied to
SCHERING for the Development Program and for commercial sales at the price
established pursuant to the terms of the Supply Agreement.

3.2 The provisions of Article 14 (Force Majeure) of this Agreement and Article
XIV (Force Majeure) of the Supply Agreement notwithstanding:

      (a)   The parties recognize the importance of an uninterrupted supply of
            commercial quantities of Agreement Product to SCHERING under the
            terms and conditions of the Supply Agreement. As soon as it shall
            become apparent to ENZON that circumstances resulting in any failure
            or delay in delivery hereunder, beyond the time period for timely
            delivery set forth in Article III.D. of the Supply

                                      -13-                            10/29/90:1

                                      E-61
<PAGE>

            Agreement, will continue for more than sixty (60) days, ENZON shall
            promptly notify SCHERING, in writing, of that fact and shall consult
            with SCHERING as to the best practical method for assuring SCHERING
            a source of supply of Agreement Product during the continuance of
            such circumstances. Provided such failure or delay is not caused by
            SCHERING's failure to deliver Agreement Substance in a timely manner
            to ENZON or otherwise by SCHERING's breach of this Agreement, ENZON
            shall provide SCHERING with all reasonable technical and other
            assistance in this regard ("Manufacturing Assistance") including
            disclosure of only such Know-How as may be reasonably necessary (as
            set forth in the PLA for Agreement Product) for manufacturing or
            quality control of Agreement Product. Manufacturing Assistance shall
            include, without limitation: (1) designation by ENZON of a
            third-party manufacturer acceptable to SCHERING, or (2) assisting
            SCHERING or any of its Affiliates to manufacture its own
            requirements of Agreement Product; such assistance by ENZON shall
            continue only for so long as such circumstances continue. The
            decision as to whether SCHERING or any of its Affiliates, or a third
            party source, shall be designated to supply Agreement Product during
            the

                                      -14-                            10/29/90:1

                                      E-62
<PAGE>

            time ENZON cannot do so directly, shall be made jointly by the
            parties, taking into account the best interest of both the parties,
            including factors such as (without limitation) cost, time required
            to initiate production of commercial quantities, protection of
            intellectual property rights, avoiding benefitting competitors,
            regulatory requirements, quality control, etc.

      (b)   (1) ENZON shall be required to allow any manufacturing program for
            Agreement Product initiated under this Article 3.2 to continue after
            ENZON regains its ability to supply Agreement Product to SCHERING
            hereunder only for so long as may be reasonably necessary for
            SCHERING and/or any of its Affiliates to fully amortize any
            incremental capital expenses it may have reasonably incurred under
            such manufacturing program, using generally accepted accounting
            principles normally used by SCHERING in keeping its own books and
            records. Third-party sources designated by ENZON shall be acceptable
            in view of the then-current health regulations.

            (2) If the failure or delay first mentioned in Article 3.2(a) above
            is the fault of ENZON and

                                      -15-                            10/29/90:1

                                      E-63
<PAGE>

            within its control, any additional cost of Agreement Product under
            such manufacturing program, whether performed by SCHERING or any of
            its Affiliates or any such third-party source, over the cost agreed
            to by SCHERING in the Supply Agreement, will be borne by ENZON;
            provided ENZON was afforded the opportunity to mitigate or reduce
            such cost prior to it being incurred by SCHERING and/or any of its
            Affiliates.

      (c)   ENZON shall have the option, in its sole discretion, to delete
            Article 3.2 from this Agreement. If ENZON exercises this option and
            so advises SCHERING in writing (the "Option Notice") as far in
            advance as possible, taking into account the current inventory of
            Agreement Product and the time required for SCHERING to arrange for
            another source of Agreement Product, but in no case less than nine
            (9) months (or such shorter period as may be mutually agreed upon by
            the parties hereto) in advance of the effective date of the Option
            Notice, then Article 3.2 shall be deleted and have no further
            effect, as of the effective date of the Option Notice; provided,
            however, that ENZON's obligation to provide Manufacturing Assistance
            to SCHERING and the provisions of Article 3.2(c)(1) through (4)

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                                      E-64
<PAGE>

            inclusive shall survive such deletion and continue until SCHERING
            shall reasonably be capable of manufacturing Agreement Product (in
            the manner set forth in the PLA for Agreement Product). In such
            case, as of the effective date of such notice: (1) ENZON shall have
            no right or obligation to thereafter supply Agreement Product to
            SCHERING under this Agreement or the Supply Agreement, (2) ENZON
            shall grant to SCHERING a paid-up license in the Territory, at no
            additional cost to SCHERING, for the term of this Agreement, of all
            Know-How necessary (as set forth in the PLA for Agreement Product)
            and Patent Rights for manufacturing Agreement Product, (3) SCHERING
            will have no further obligations to purchase Agreement Product from
            ENZON, and (4) SCHERING shall waive and give up any right or claim
            to damages or recoupment of expenses or costs which are based on
            ENZON's failure or delay in supplying Agreement Product and which
            arose under this Article 3.2; provided, however, ENZON shall
            continue to be responsible, as set forth in 3.2(b)(1), for any
            additional cost of Agreement Product manufactured during the nine
            (9) month period between the date the Option Notice is given and the
            effective date of the Option Notice. In any event, SCHERING shall be
            required to pay ENZON

                                      -17-                            10/29/90:1

                                      E-65
<PAGE>

            royalties on Net Sales under Article 7 hereof in connection with all
            Agreement Product manufactured under this Agreement.

Article 4 - Ownership of Intellectual Property; Exclusive Licenses

4.1 ENZON shall own the entire right, title and interest in and to the
Development Know-How related to Agreement Product and/or PEG, whether created
by SCHERING, by ENZON or jointly by the parties ("ENZON Development Know-How").

4.2 SCHERING shall own the entire right, title and interest in and to the
Development Know-How covering and related to Agreement Substance, whether
created by SCHERING, by ENZON or jointly by the parties ("SCHERING Development
Know-How").

4.3 ENZON shall own the entire right, title and interest in and to all Patent
Rights related to Agreement Product and/or PEG, whether invented by SCHERING, by
ENZON, or jointly by the parties hereto ("ENZON Patent Rights").

4.4 SCHERING shall own the entire right, title and interest in and to all Patent
Rights related to Agreement Substance, whether invented by ENZON, by SCHERING,
or jointly by the parties hereto (the "SCHERING Patent Rights").

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                                      E-66
<PAGE>

4.5 For all other Development Know-How and Patent Rights, the party that
invented or created same shall own the entire right, title and interest therein,
if the other party is not a joint inventor or creator thereof. If the parties
jointly invented or created same, they shall be joint owners thereof.

4.6 SCHERING shall own all right, title and interest in and to all SCHERING
Information.

4.7 Subject to the other terms of this Agreement, ENZON hereby grants to
SCHERING and SCHERING hereby accepts, exclusive licenses, exclusive even as to
ENZON, under the ENZON Patent Rights the ENZON Existing Know-How and the ENZON
Development Know-How, during the term of this Agreement: (1) to use and sell
Agreement Product in the Territory, with the right to sub1icense SCHERING's
Affiliates or third parties and (2) to make and have made Agreement Product,
with the right to sublicense SCHERING's affiliates, under the conditions stated
in Article 3.2 hereof.

4.8 Subject to the other terms of this Agreement and for the sole purpose of
ENZON's development and manufacture of Agreement Product for SCHERING, SCHERING
hereby grants ENZON the right, under the SCHERING Information, the SCHERING
Patent Rights, the SCHERING Existing Know-How, the SCHERING Development
Know-How, and the RC License (as hereinafter defined) during the term of this
Agreement, to use Agreement Substance in the development and manufacture of

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Agreement Product and to manufacture Agreement Product, except under the
condition referred to in Article 3.2(a)(2) hereof.

4.9 Upon expiration of this Agreement as provided in Article 9.1 hereof, either
in its entirety or with respect to any country in the Territory, ENZON shall
disclose to SCHERING and SCHERING and its Affiliates may use, either in the
Territory or in any such country, without any further payment, for any purpose
whatsoever related to Agreement Product (but not for any other purpose or other
product), all ENZON Patent Rights, ENZON Existing Know-How, and ENZON
Development Know-How.

4.10 No license or right of any sort shall arise under this Agreement, whether
by implication or estoppel, with respect to U.S. Patent 4,179,337 and its
foreign counterparts, nor to any patent or patent right with respect to
Agreement Substance, except as specifically set forth herein.

Article 5 - Patent Rights

5.1 With respect to Patent Rights, it is agreed:

            5.1.1 ENZON shall diligently prosecute, maintain and defend all
            patent applications and issued patents included within the ENZON
            Patent Rights. All costs for patent filing, prosecution, and
            maintenance of such

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            Patent Rights shall be borne by ENZON. If ENZON chooses not to
            defend such Patent Rights in any country against any third party,
            then SCHERING may defend such Patent Rights in such country with
            respect to such third party, at SCHERING's sole cost, and SCHERING
            will retain for its own benefit any judgment therefrom. If such
            defense is successful, SCHERING may thereafter deduct its reasonable
            expenses (including attorney's fees) related to such defense (net of
            any judgment monies received by SCHERING) from future royalties due
            in such country under Article 7 hereof.

            5.1.2 ENZON shall notify SCHERING immediately (i) whenever it files
            a patent application related to the ENZON Patent Rights in any
            country; and (2) whenever it is considering discontinuance of the
            prosecution or maintenance of such a patent application. If ENZON
            shall fail to file a patent application related to the ENZON Patent
            Rights in any country it shall so notify SCHERING immediately in
            writing, and if ENZON does not initiate the filing process within
            ninety (90) days after written request to do so from SCHERING, or if
            ENZON does not continue prosecution or maintenance of such a patent
            application related to the ENZON Patent Rights which it has
            initiated pursuant to Article 5.1.2 above within ninety (90) days
            after written notice from SCHERING,

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            SCHERING shall have the option to file or continue prosecution in
            its own name and at SCHERING's cost. In either such case ENZON shall
            do all acts necessary to assign and vest title to and transfer
            control of such patent application in that country in a timely
            fashion to SCHERING for no additional consideration.

            5.1.3 Each party hereto shall notify the other party hereto of its
            knowledge of any infringement of the Patent Rights within sixty (60)
            days after becoming aware of such infringement.

            5.1.4 With respect to SCHERING Independent Patent Rights, it is
            agreed that SCHERING shall, no later than the date of filing of the
            first patent application for any patentable invention included in
            the SCHERING Independent Patent Rights, provide to ENZON on a
            non-confidential basis a statement of the subject matter of such
            patent application, using good-faith efforts to make such statement
            as complete as possible without divulging any confidential or
            proprietary information, it being understood that SCHERING shall not
            be required to divulge any confidential or proprietary information
            in such statement. Thereafter, ENZON may request, and SCHERING shall
            provide, within twenty (20) days after ENZON's request, (under
            appropriate conditions of confidentiality

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            imposing restrictions on ENZON similar to those SCHERING requires
            for its other confidential information) a copy of the contents of
            such patent application.

Article 6 - Payments

6.1 Subject to the terms of this Agreement, SCHERING agrees to make the
following payments to ENZON in U.S. Dollars as indicated:

      (a)   One hundred and fifty thousand dollars ($150,000) within fifteen
            (15) days after the date of execution by the last of the parties to
            sign this Agreement.

      (b)   Four hundred fifty thousand dollars ($450,000) within fifteen (15)
            days after SCHERING's filing of the first IND in the United States
            of America for Agreement Product.

      (c)   Two million five hundred thousand dollars ($2,500,000) within
            fifteen (15) days after confirmation of SCHERING's successful
            completion ("Successful Completion") of the initial clinical trial
            demonstrating that Agreement Product meets the criteria set forth in
            Schedule D attached hereto (Criteria for Efficacy in Pilot Study).

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      (d)   One million dollars ($1,000,000) within fifteen (15) days after
            SCHERING's submission of the first PLA for Agreement Product in the
            United States of America.

      (e)   Two million dollars ($2,000,000) within fifteen (15) days after
            receipt of the first FDA Approval to occur in the United States of
            America.

      (f)   None of the foregoing payments shall be payable to ENZON more than
            once under any circumstance.

6.2 If, upon completion of the clinical trial described in Article 6.1(c)
hereof, Agreement Product does not meet the applicable criteria set forth in
Schedule D, and SCHERING decides at that time to terminate this Agreement as
permitted by Article 9.2(b) hereof, ENZON shall promptly refund *** to SCHERING.

Article 7  - Royalty Terms

7.1 Subject to Article 7.2 below, SCHERING shall pay or cause to be paid
royalties to ENZON on Net Sales as follows:

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

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      (a)   For as long as SCHERING pays a royalty, under the license agreement
            dated December 3, 1987, between SCHERING and Research Corporation,
            in connection with SCHERING's sale of an Agreement Product in any
            country in the Territory, SCHERING shall pay ENZON a royalty in such
            country of *** SCHERING shall provide to ENZON reasonable evidence
            of such payments to Research Corporation at ENZON's request.

      (b)   In each calendar year during the term of this Agreement, in
            connection with SCHERING's sales of Agreement Product which do not
            require payment of a royalty under Article 7.1(a) above, if
            Competition, as defined in Article 7.1(d) below, exists in any such
            country, SCHERING shall pay ENZON a royalty *** for so long as the
            sum of Net Sales in all countries in the Territory during such
            calendar year *** and *** after the sum of Net Sales in all
            countries in the Territory during such calendar year ***

      (c)   In each calendar year during the term of this Agreement, in
            connection with SCHERING's sales of

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

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            Agreement Product which do not require payment of a royalty under
            Article 7.1(a) above, if Competition does not exist in any such
            country, SCHERING shall pay ENZON a royalty of *** for so long as
            the sum of Net Sales in all countries in the Territory during such
            calendar year does not ***, and *** after the sum of Net Sales in
            all countries in the Territory during such calendar year ***

      (d)   For purposes of this Article 7, "Competition" shall mean sales by a
            party or parties, other than SCHERING, its Affiliates, licensees,
            distributors or agents, in any country of the Territory, of a
            competitive product which contains alpha interferon (as a
            significant active ingredient) coupled with PEG to produce a
            longer-lasting product, ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

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            or countries, there exists no effective patent or know-how
            protection) and: (1) the sales of such competitive product made by
            any one party equal *** or, (2) the sales of such competitive
            product by two or more parties equal, with respect to each such
            party, *** but, considering the sales of such competitive product
            made by all such parties in the aggregate, equal ***

7.2 If SCHERING shall hereafter be required, in respect of its sale of Agreement
Product, to pay royalties (hereinafter "Third-Party Royalties") to any third
party whose patents may be infringed by such sales, SCHERING, in order to
reimburse itself for any such Third-Party Royalties, may deduct from royalties
due ENZON hereunder the net amount of any such Third-Party Royalties paid, but
in no event shall royalty due ENZON for any calendar quarter with respect to
Agreement Product be thereby reduced by reason of the provisions of this Article
7.2 to less than one-half of the royalty which would otherwise be due under the
other provisions of this Article 7. This Article 7.2 shall only apply if such
infringement is based on PEG coupled with Agreement Substance, and shall not
apply or be effective in connection with an infringement

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

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related to Agreement Substance alone. ***

Article 8 - Royalty Payment Terms

8.1 Royalties payable hereunder with respect to Net Sales shall be paid
quarterly within sixty (60) days after the close of the calendar quarter during
which proceeds from such Net Sales were received. All royalty payments shall be
accompanied by SCHERING's statement of Net Sales, together with SCHERING's
royalty calculations.

8.2 Royalties shall be payable in and remitted in United States dollars, by
SCHERING or its appropriate Affiliate. If Net Sales are invoiced in other than
U.S. dollars, such Net Sales shall be converted to U.S. dollars in accordance
with generally accepted accounting principles (as employed by SCHERING in
translating its published financial results for SEC reporting purposes) at the
closing rates of exchange in effect on the last SCHERING business day of the
month prior to the month in which the invoice was issued. If, due to
restrictions or prohibitions imposed by national or international authority,
SCHERING is unable to make the conversions as aforesaid or SCHERING or
SCHERING's affiliates are

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

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unable to make any such royalty payment, the parties shall consult with a view
to finding a prompt and acceptable solution and SCHERING will, from time to
time, deal with such monies as ENZON may lawfully direct. The parties shall use
diligent efforts to cause such royalties to be paid, without reduction, and in
any event SCHERING's ultimate obligation for such payments shall not be
eliminated.

8.3 If the royalties set forth herein are higher than the maximum royalties
permitted by the law or regulations in any country in the Territory, the royalty
payable for sales in such country shall be equal to the maximum permitted
royalty under such law or regulations. In such case, the parties shall take
whatever actions are legally permissible to pay the entire amount of such
royalty above the permitted amount to ENZON, such as by an increase in the
period of time during which royalties shall be paid by SCHERING hereunder.

8.4 If any taxes, withholding or otherwise, are levied by any taxing authority
in connection with accrual or payment of any royalties payable under this
Agreement, SCHERING or its Affiliate shall have the right to pay such taxes to
the local tax authorities on behalf of ENZON and the payment to ENZON of the net
amount due after reduction by the amount of such taxes, together with evidence
of payment of such taxes, shall fully satisfy SCHERING's royalty

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obligations under this Agreement. SCHERING shall give ENZON reasonable notice of
any such payment.

8.5 Royalties shall be due hereunder and calculated only on the final sale of
Agreement Product by SCHERING or any SCHERING Affiliate or SCHERING sublicensee
to independent third parties in arm's-length transactions. No multiple royalties
shall be payable upon account of the sale of Agreement Product, regardless of
whether more than one patent exists within the Patent Rights in any country or
countries in the Territory.

8.6 SCHERING shall keep complete and adequate records with respect to the sale
of Agreement Product and royalties payable hereunder. ENZON shall have the right
to have such records of SCHERING inspected and examined, at ENZON's expense, for
the purpose of determining correctness of royalty payments made hereunder. Such
inspection and examination shall be made by an independent certified public
accountant to whom SCHERING shall have no reasonable objection. Such accountant
shall not disclose to ENZON any information other than that necessary to verify
the accuracy of reports and payments made pursuant to this Agreement. It is
understood that such examination with respect to any quarterly accounting period
hereunder shall take place not later than two (2) years following the expiration
of the said period. No more than one examination per year shall take place.

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8.7 If, in order to facilitate direct royalty payments by a SCHERING Affiliate,
it is desirable that a separate license agreement be entered into between ENZON
and such Affiliate, ENZON agrees to grant such license directly to such
Affiliate by means of an agreement which shall be consistent with the provisions
of this Agreement.

Article 9 - Term and Termination

9.1 This Agreement shall be made effective on the date first written above and
shall continue in effect, unless terminated earlier as expressly provided
herein, for a term which shall expire, on a country-by-country basis, upon the
longer of (a) the life in each such country of the last to expire of the ENZON
Patent Rights for which a Valid Claim exists, including any patent extension (or
any other governmental action which has the effect of extending the period of
market exclusivity of the patent owner) which is permitted by law and obtained,
or (b) fifteen (15) years from the date on which Agreement Product is first
approved for commercial marketing in each such country ("Term").

9.2 SCHERING shall have the right to terminate this Agreement at any time,
either in its entirety or on a country-by-country basis:

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      (a)   after sixty (60) days' written notice to ENZON, if ENZON has not
            cured the cause of such right to terminate if based upon this
            Article 9.2(a), if the Agreement Product resulting from the
            Development Program shall fail to meet the specifications and
            criteria set forth in Schedule A-3;

      (b)   after the completion of the clinical trial set forth in Article
            6.1(c) hereof; provided, however, that if Successful Completion, as
            defined in Article 6.1(c) hereof shall have taken place, SCHERING's
            right to terminate under this Article 9.2(b) shall be contingent
            upon SCHERING's having made the payment set forth in Article 6.1(C)
            hereof.

In the event of any termination by SCHERING hereunder, SCHERING shall be
required to make only those payments hereunder which have accrued and are due
and owing prior to the effective date of such termination.

9.3 Upon the failure of either party hereto to comply with any of its respective
material obligations contained herein, the other party shall be entitled,
without prejudice to any other rights conferred on it herein, to terminate this
Agreement upon not less than sixty (60) days notice of such default given to the

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defaulting party, provided that the party in defau1t has failed to cure such
default within such sixty (60) day period.

9.4 If either party hereto shall become insolvent or shall make an assignment
for the benefit of creditors, or proceedings in voluntary or involuntary
bankruptcy shall be instituted in behalf of or against such party, or a receiver
or trustee of such party's property shall be appointed, the other party hereto
may, without other action or notice, forthwith terminate this Agreement.

9.5 Except as otherwise provided herein, in the event of termination under
Articles 9.2, 9.3 or 9.4, each party shall return to the other party, and no
longer use for any reason or purpose, all the Know-How belonging solely to the
other party.

Article 10 - Representations and Warranties; Indemnification

10.1 ENZON hereby represents and warrants that it has sole, full and complete
right, title and interest in all Patent Rights and ENZON Existing Know-How.
ENZON further represents and warrants that, to the best of its knowledge as of
the effective date of this Agreement, the manufacture, use or sale of Agreement
Product by ENZON will not infringe any third party's patent rights related to
Agreement Product or PEG. ENZON shall fully indemnify, defend and hold SCHERING
harmless for all direct damages and expenses, including reasonable attorney's
fees, (but not for consequential

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damages such as lost or reduced profits, royalties, or similar items) which may
be incurred by SCHERING for any breach of the warranties contained in this
Article 10.1, but not if such infringement is related to the Agreement
Substance.

10.2 Each party hereto represents and warrants that it has the right to enter
into this Agreement and has no obligation, or knows of no obstacle, which would
prevent either party from carrying out its obligations and responsibilities
thereunder, including manufacture, commercialization and/or sale of Agreement
Product on a world-wide basis.

10.3 SCHERING represents and warrants that the use by ENZON of Agreement
Substance supplied by Schering to ENZON only in connection with ENZON's
activities under this Agreement and the Supply Agreement will not infringe the
patent rights and/or know-how rights of any third party with respect to
Agreement Substance. SCHERING further represents and warrants that, to the best
of its knowledge as of the effective date of this Agreement, it has: (a) the
right to make, have made, use and sell: (1) alpha interferon, and (2) under a
license agreement with Research Corporation dated December 3, 1987, in
connection with U.S. Patent 4,179,337 and its foreign counterparts (the "RC
License"), products combining PEG and alpha interferon; and (b) the right to
make the grants to ENZON contained in Article 4.8 hereof. SCHERING shall fully
indemnify, defend and hold ENZON harmless for all direct damages and expenses,

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including reasonable attorney's fees, (but not for consequential damages such as
lost or reduced profits, royalties, or similar items) which may be incurred by
ENZON for any breach of the warranties contained in this Article 10.3.

10.4 SCHERING shall indemnify, defend and hold ENZON harmless from and against
any and all claims, including reasonable attorney's fees, for loss, damages, or
injury to persons or property by whomsoever raised, arising out of SCHERING's
negligence in manufacturing, marketing, distribution and sale of Agreement
Substance or Agreement Product.

10.5 In addition to the warranty set forth in Article 10.1 above, ENZON shall
indemnify, defend and hold SCHERING harmless from and against any and all
claims, including reasonable attorney's fees, for loss, damages, or injury to
persons or property by whomsoever raised, arising out of ENZON's negligence in
the development, testing or manufacture of Agreement Substance or Agreement
Product.

10.6 The coverage of Articles 10.4 and 10.5 above shall not be deemed to extend
to the infringement of any patent right.

10.7  (a)   ENZON shall provide to SCHERING, no later than thirty (30) days
            after the date first written above, a certificate of insurance
            evidencing liability

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            insurance, including products liability and contractual liability
            insurance, covering ENZON's obligations under this Agreement, and
            satisfactory to SCHERING, in the amount of one million dollars
            ($1,000,000). Such insurance shall be kept valid and in full force
            by ENZON until the initiation of Phase III clinical trials of
            Agreement Product by SCHERING hereunder, and ENZON shall provide
            SCHERING with certificates of renewal or re-issuance as they become
            available.

      (b)   ENZON shall provide to SCHERING, no later than thirty (30) days
            after the initiation of Phase III clinical trials of Agreement
            Product by SCHERING hereunder, a certificate of insurance evidencing
            liability insurance, including products liability and contractual
            liability insurance, covering ENZON's obligations under this
            Agreement, and satisfactory to SCHERING, in the amount of five
            million dollars ($5,000,000), hereinafter the "Face Amount". Such
            insurance shall be kept valid and in full force by ENZON during the
            term of this Agreement, and ENZON shall provide SCHERING with
            certificates of renewal or re-issuance as they become available;
            provided, however, that during any period in which the cost to ENZON
            of the insurance

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            required under this Article 10.7(b) reasonably shall be greater than
            one hundred fifty percent (150%) of the adjusted cost ("Adjusted
            Cost") to ENZON of such insurance during the first full year such
            insurance was in effect, the Face Amount shall be reduced to an
            amount which ENZON reasonably can purchase for a sum equal to one
            hundred fifty percent (150%) of the Adjusted Cost. The Adjusted Cost
            in each year shall be determined by applying to the prior year's
            Adjusted Cost (or for the second year such insurance shall be in
            effect, the prior year's actual cost) the percentage increase or
            decrease in the U.S. gross national product shown in the final
            annual Implicit Price Deflator for Gross National Product table
            produced for the prior calendar year by the Bureau of Economic
            Analysis of the U.S. Department of Commerce (or any successor agency
            thereof).

      (c)   Failure to demonstrate such proof of valid, in-force, insurance
            coverage, or failure to maintain such coverage, shall be grounds for
            immediate termination of this Agreement by SCHERING.

10.8 The aforesaid obligations of the indemnifying party shall be subject to the
indemnified party fulfilling the following obligations:

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      (a)   The indemnified party shall bring any claim, action, damage,
            expense, etc. to the attention of the indemnifying party as soon as
            reasonably possible;

      (b)   The indemnified party shall fully cooperate with the indemnifying
            party in the defense of any claims, actions, etc., which defense
            shall be controlled by the indemnifying party (provided, however,
            that the indemnified party shall be entitled to counsel at its own
            expense) ;

      (c)   The indemnified party shall not, except at its own cost, voluntarily
            make any payment or incur any expense with respect to any claim or
            suit without the prior written consent of the indemnifying party,
            which such party shall not be required to give.

Article 11 - Confidentiality

11.1 Both ENZON and SCHERING recognize that Know-How disclosed by each party to
each other, including ENZON Existing Know-How, ENZON Development Know-How,
SCHERING Existing Know-How, SCHERING Development Know-How, and any
jointly-owned Development Know-How, is of proprietary value to and is to be
considered highly confidential and shall be treated by the parties during the
term of this

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Agreement (or any extension thereof) and for a period of five (5) years after
the expiration or any termination thereof in the following manner:

      (a)   SCHERING agrees to keep confidential and not to disclose to others,
            without the express written permission of ENZON, ENZON Existing
            Know-How, ENZON Development Know-How, and any jointly-owned
            Development Know-How.

      (b)   ENZON agrees to keep confidential and not to disclose to others,
            without the express written permission of SCHERING, SCHERING
            Existing Know-How, SCHERING Development Know-How, and any
            jointly-owned Development Know-How.

11.2 Neither party shall be prevented from disclosing any Know-How which:

      (a)   can be demonstrated by written records to have been known to the
            recipient at the time of receipt, or

      (b)   was subsequently otherwise legally acquired by such party from a
            third party having an independent right to disclose the Confidential
            Information, or

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      (c)   which was at the time of disclosure, is now or later becomes
            publicly known without breach of this Agreement by either party, or

      (d)   which is required to be disclosed by law, regulation, or act of any
            governmental agency.

11.3 Anything to the contrary in Article 11.1. above notwithstanding, SCHERING
and its Affiliates shall be permitted to disclose Know-How to:

      (a)   regulatory agencies in support of applications to market Agreement
            Product;

      (b)   clinicians or others in connection with research related to such
            applications, or to the preparation or filing of such applications,
            provided, however, that SCHERING shall use diligent efforts to
            secure signed confidentiality agreements prior to any disclosure
            under this Article 11.3(b); and

      (c)   distributors as reasonably necessary for purposes of obtaining
            approval to market Agreement Product or to market or promote
            Agreement Product, provided, however, that SCHERING shall use
            diligent efforts to

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            secure signed confidentiality agreements prior to any disclosure
            under this Article 11.3(c).

11.4 Upon the expiration or any termination of (but not during) this Agreement
except termination for cause by SCHERING under the terms of Articles 9.3 and 9.4
hereof), ENZON shall have the right to disclose to potential licensees, under
appropriate confidentiality restrictions similar to those ENZON normally uses
for the disclosure of its own valuable proprietary information, summaries of the
SCHERING Information (but not the underlying data or the tests or analyses
themselves). These disclosures, however, may not contain any proprietary
information pertaining to Agreement Substance.

11.5 During the term of this Agreement, SCHERING and its Affiliates may use
ENZON Patent Rights, ENZON Existing Know-How, and ENZON Development Know-How for
any purpose whatsoever related to Agreement Product (but not for any other
purpose or other product) only pursuant to Article 4.7 hereof. This clause 11.5
shall not be deemed to impose any requirement for ENZON to disclose Know-How
except as otherwise expressly provided in this Agreement.

Article 12 - Governing Law

This Agreement shall be governed by and interpreted in accordance with the
substantive law of the State of New Jersey, without regard to its choice of law
principles.

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Article 13 - Assignment or Delegation

13.1 Except as provided in Article 13.2 below, neither party may without written
approval of the other:

      (a)   assign this Agreement or transfer its interest or any part thereof
            under this Agreement to any third party, or

      (b)   designate and cause any third party to perform all or part of its
            activities hereunder, or to have the benefit of all or part of
            its rights hereunder, except that ENZON may continue to have that
            portion of its manufacturing operation which is being performed by
            other parties as of the date of this Agreement continue to be so
            performed.

13.2 Either party hereto may assign this Agreement, without the other party's
consent, to a third party in a transaction in which all the assets of the
assigning party relating to Agreement Product are sold or assigned; provided
such third party was not selling a product competitive to the Agreement prior to
such assignment.

In the event of any such permitted assignment, transfer or designation, the
assignee, transferee or designee shall assume and be bound by the provisions of
this Agreement.

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Article 14 - Force Majeure

14.1 Except as otherwise provided in Article 3.2 hereof, neither party hereto
shall be liable in damages for, nor shall this Agreement be terminable or
cancellable by reason of, any delay or default in such party's performance
hereunder if such default or delay is caused by events beyond such party's
reasonable control, including, but not limited to, acts of God, regulation or
law or other action or failure to act of any government or agency thereof
(including, without limitation, any action or inaction of the FDA), war or
insurrection, civil commotion, destruction of production facilities or materials
by earthquake, fire, flood or storm, labor disturbances, epidemic, or failure of
suppliers (including each of SCHERING and ENZON with respect to their obligation
to supply the other), public utilities or common carriers; provided however,
that the party seeking relief hereunder shall immediately notify the other party
of such cause(s) beyond such party's reasonable control.

14.2 Each party shall endeavor to resume its performance hereunder as quickly as
possible if such performance is delayed or interrupted as set forth in Article
14.1 above.

Article 15 - Independent Contractor; Publicity

15.1 It is understood that both parties hereto are independent contractors and
engage in the operation of their own respective

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<PAGE>

businesses and neither party hereto is to be considered the agent of the other
party for any purpose whatsoever. Each party shall be fully responsible for its
own employees and consultants, and the employees of one party shall not be
deemed to be employees of the other party for any purpose whatsoever.

15.2 Both parties agree not to use or refer to, without the other party's
written permission, which shall not be unreasonably withheld, the other's name
or agreements with the other party, or the terms of this Agreement, in any
public statement, whether oral or written, including, but not limited to,
communications with the media, or prospectuses; provided, however, that either
party may, without the consent of the other party, make whatsoever disclosure is
required by law, and particularly by the Securities and Exchange Act of 1934, as
amended, the regulations promulgated thereunder, and any applicable NASDAQ
rule. The parties agree that the press release attached hereto as Schedule F may
be made public.

15.3 Except as allowed under Article 15.2 above, ENZON shall not, without the
written permission of SCHERING's Vice-President, Business Development, publish
or report any information related to Agreement Substance or Agreement Product,
such approval shall not unreasonably be withheld, and shall be deemed to have
been given unless notice in writing to the contrary has been given to ENZON
within sixty (60) days from receipt by SCHERING of a request to approve such
publication or report.

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                                      E-92
<PAGE>

Article 16 - No Inconsistent Obligations

Each party hereto represents and warrants that it has no obligations or
commitments inconsistent with this Agreement and that it knows of no reason the
Development Program may not proceed as set forth in Schedule B hereto.

Article 17 - Severability; Survival

      (a)   Should any part of this Agreement be held unenforceable or in
            conflict with the applicable laws or regulations of any
            jurisdiction, the invalid or unenforceable part or provision shall
            be replaced with a provision which accomplishes, to the extent
            possible, the original business purpose of such invalid or
            unenforceable part or provision in a valid and enforceable manner,
            and the remainder of this Agreement shall remain binding upon the
            parties hereto.

      (b)   The terms of this Agreement which by their intent or meaning have
            validity beyond the term of this Agreement shall survive the
            termination or expiration of this Agreement.

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                                      E-93
<PAGE>

Article 18 - Captions

18.1 The captions of this Agreement are solely for the convenience of reference
and shall not affect its interpretation.

18.2 Schedules A-1, A-2, A-3, B, C, D, E and F are attached to and made a part
of this Agreement.

Article 19 - Waiver

The failure of either party at any time to times or enforce or require
performance of any provision of this Agreement shall in no way operate as a
waiver or affect the right of such party at a later time to enforce the same. No
waiver by either party of any condition or the breach of any term or agreement
contained in this Agreement, whether by conduct or otherwise, in any one or more
instances, shall be deemed to be or shall be construed as a further or
continuing waiver of any such condition or breach, or a waiver of any other
condition or of any other breach of any other term contained in this Agreement.

Article 20 - Notice

Any notice, payment, report or other correspondence (hereinafter collectively
referred to as "Correspondence") required or permitted to be given hereunder
shall be sent by certified mail, by telefax

                                      -46-                            10/29/90:1

                                      E-94
<PAGE>

or delivered by hand to the party to whom such Correspondence is required or
permitted to be given hereunder. Any such Correspondence shall be deemed to be
received upon receipt as evidenced by the written and dated receipt signed by
the receiving party or its agent or employee, or as evidenced by telefax
records.

All Correspondence to ENZON shall be addressed as follows:

                  Enzon, Inc.
                  40 Cragwood Road
                  South Plainfield, NJ 07080-2406
                  Attention: Vice President, Corporate Development
                  with a copy to: General Counsel, Law Department

All Correspondence to SCHERING shall be addressed as follows:

                  Schering Corporation
                  2000 Galloping Hill Road
                  Kenilworth, New Jersey 07033
                  Attention: Vice President, Business Development
                  with a copy to: Legal Director, Research and Licensing

Any entity may change the address to which correspondence to it is to be
addressed by notification as provided herein.

Article 21 - Entire Agreement

This Agreement, including Schedules A through F attached hereto, and the Letter
Agreement between the parties dated April 3, 1990, (the "Letter Agreement")
represent the entire understanding between the parties with respect to the
subject matter hereof, and supersede all other prior agreements, negotiations,
understandings, representations, statements and writings between the parties
relating thereto. No modification, alteration, waiver or change in

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                                      E-95
<PAGE>

any of the terms of this Agreement shall be valid or binding upon the parties
hereto unless made in writing and duly executed by each of the parties hereto.
In the event of any conflict between this Agreement and the Letter Agreement,
this Agreement shall control.

IN WITNESS WHEREOF, this Agreement has been executed by the parties to be
effective as of the day and year first written above.

                                                        --------------
                                                        APPROVED AS TO
                                                          LEGAL FORM
                                                          [INITIALS]
                                                        --------------

ENZON, INC.                             SCHERING CORPORATION

By: /s/ Abraham                         By: /s/ Donald R.
   ---------------------------             ---------------------------------

Title: President and CEO                Title: President, Pharmaceuticals
      ------------------------               -------------------------------

                                      -48-                            10/29/90:1

                                      E-96
<PAGE>

                                   APPENDIX I

                 Initial Specifications for Agreement Substance

            TEST                        SPECIFICATION
            ----                        -------------

            ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

10/25/90

                                      E-97
<PAGE>

                                   APPENDIX II

                Additional Specifications for Agreement Substance

            TEST                                   SPECIFICATIONS
            ----                                   --------------

            ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

10/25/90

                                      E-98
<PAGE>

                                  SCHEDULE A-1

                     SPECIFICATIONS FOR AGREEMENT SUBSTANCE
                     (SCM 30500 Drug Substance in Solution)

                 TEST                                 SPECIFICATION
                 ----                                 -------------

                 ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

10/19/90

                                      E-99
<PAGE>

                                  SCHEDULE A-2

                       SPECIFICATIONS FOR BULK CONCENTRATE
                          (PEG-SCH 30500 in Solution)

               TEST                                SPECIFICATION
               ----                                -------------

               ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

10/19/90

                                     E-100
<PAGE>

                                  SCHEDULE A-3

                      SPECIFICATIONS FOR AGREEMENT PRODUCT
                 (Modified PEG-Intron A Solution, 50x10^6 IU/ml.)

             TEST                                SPECIFICATION
             ----                                -------------

             ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

10/19/90

                                     E-101
<PAGE>

                                   SCHEDULE B

                            SCH 30500--PEG INJECTION
                                DEVELOPMENT PLAN

                                              START/
                                            COMPLETION
              ACTIVITY                        TARGET             RESPONSIBILITY
-------------------------------------       -----------          --------------

            ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

10/10/90

                                     E-102
<PAGE>

                                      -2-

                                              START/
                                            COMPLETION
              ACTIVITY                        TARGET             RESPONSIBILITY
-------------------------------------       -----------          --------------

               ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-103
<PAGE>

                                       -3-

                                              START/
                                            COMPLETION
              ACTIVITY                        TARGET             RESPONSIBILITY
-------------------------------------       -----------          --------------

            ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-104
<PAGE>

                             PROJECTED NDA TIMELINE

                                       ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-105
<PAGE>

                                   SCHEDULE C

                                  PATENT RIGHTS

         (To be updated by the parties as required under Article 1.11.]

                                                                      10/25/90:1

                                     E-106
<PAGE>

                                   SCHEDULE D

                       CRITERIA FOR DEMONSTRATING EFFICACY
                            IN A PILOT CLINICAL STUDY

                                       ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

10/10/90

                                     E-107
<PAGE>

                                   SCHEDULE E

                                SUPPLY AGREEMENT

                         [omitted no longer in effect]

                                     E-108
<PAGE>

                                              Amendment No. 1  December 10, 1991

                                                                       FILE COPY

                              SCHERING CORPORATION           J.M.K. SEP 23 1991

                GALLOPING HILL ROAD [LOGO] KENILWORTH, N.J 07033

                                              CABLES: SCHERING KENILWORTH
                                              TELEX: 138315
                                                     138280
                                              TELEPHONE: (201) 298-4OOO

                           September 16, 1991

Enzon, Inc.
40 Cragwood Road
South Plainfield, NJ 07080-2406

ATTN: Glenn Kazo

Dear Mr. Kazo:

      Pursuant to Article 1.5 of our Development, License and Supply Agreement
(the "Agreement"), dated November 14, 1990, this will confirm our agreement to
amend the Development Plan, as defined in the Agreement, in accord with the
attached memo from M. Zupon, dated April 29, 1991, and with the amended Schedule
B to the Agreement, both of which are attached hereto.

      Please indicate your acceptance of the foregoing by signing one copy of
this letter as indicated below and returning it to us for our files.

                                Very Truly yours,

                                SCHERING CORPORATION            ---------------
                                                                APPROVED AS TO
                                                                  LEGAL FORM
                                By: /s/ William J. Breiner
                                   --------------------------      [INITIALS]
                                    William J. Breiner              LAW DEPT
                                    Vice President              ----------------
                                    Business Development

Accepted and Agreed:

ENZON, INC.

By /s/ Glenn M. Kazo   Glenn M. Kazo
  ----------------------------------------

Title Vice President Corporate Development

Date 12/10/91

C/ADAG2691

cc: John Caruso, Esq. (Enzon)
    Karin Cast
    Ann Martin

                                     E-109
<PAGE>

[LOGO] Schering-Plough                                                     Memo

Meeting Attendees                                       Date: April 29, 1991

***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-110
<PAGE>

***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                            /s/ Michael A. Zupon
                                            -----------------------------

MAZ/nf Mtg(2)
Attachment

                                     E-111
<PAGE>

                                 ATTACHMENT VII

                                   SCHEDULE B

***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-112
<PAGE>

                                      -2-

***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-113
<PAGE>

                                       -3-

***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-114
<PAGE>

                                              Amendment No. 2  October 30, 1992

[Amendment No. 2 is no longer in effect and therefore is not included in this
filing]

                                     E-115
<PAGE>

                                                     Amendment No. 3  March 1993

[Amendment No. 3 is no longer in effect and therefore is not included in this
filing]

                                     E-116
<PAGE>

                                                   Amendment No. 4 June 30, 1995

[LOGO] ENZON, Inc.
       =========================================================================

June 30, 1995

Schering Corporation
2000 Galloping Hill Road
Kenilworth, New Jersey 07033

Gentlemen:

      This letter sets forth the terms and conditions under which Schering
Corporation (hereinafter called "SCHERING") and Enzon, Inc. (hereinafter called
"ENZON") agree to further amend their Development, License and Supply Agreement
dated November 14, 1990 (hereinafter the "Agreement"). Except as stated
otherwise in this letter amendment, capitalized words used herein shall have the
same meaning as those defined in the Agreement. A "Stock Purchase Agreement"
between the parties is effective concurrently with this letter amendment. This
letter amendment is effective June 30, 1995.

      The Agreement shall be amended as follows:

Amendment 1

Under the March 2, 1993 letter amendment to the Agreement, paragraph 6, SCHERING
decided to continue development of an "Optimal Formulation" and ENZON provided
SCHERING with several batches of such Optimal Formulation. Additionally,
SCHERING elected to similarly pursue a second "Optimal Formulation" through the
filing of an IND. No additional payment is due ENZON pursuant to Article 6.1(b)
of the Agreement in connection with the second Optimal Formulation and all
payments required under the March 2, 1993 letter amendment to the Agreement have
been made by SCHERING.

================================================================================
          40 Kingsbridge Road Piscataway, NJ 08854-3998 (908) 980-4500
                              FAX: (908) 980-5911

                                     E-117
<PAGE>

Amendment 2

Schedules A-1, A-2, B, C, and D, which were attached to the Agreement and part
thereof on November 14, 1990, are hereby deleted and replaced by the new
schedules of that same designation, which are attached to this letter amendment.
Schedule A-3 is deleted and when SCHERING develops new and final specifications
they will be put into the Agreement as a new Schedule A-3. Further, new
Schedules G and H are attached to and made a part of this letter amendment. All
references below to Schedules, and future interpretation of this letter
amendment and the Agreement, shall be made with respect to these new Schedules.

Amendment 3

In Article 1.2, delete -- ENZON -- and -- are -- in the fourth line and replace
them, respectively, with -- SCHERING -- and -- will be --. Add at the end of
Article 1.4, -- SCHERING may amend the specifications in Schedule A-2 after the
effective date of this letter amendment if in its reasonable judgment it is
necessary to do so. ENZON shall not be responsible for such specifications as
amended unless it chooses to supply Bulk Concentrate pursuant to Article
3.1, --.

Amendment 4

In Article 1.5, strike the last three lines and insert -- in Schedule B hereto
made by SCHERING -- .

Amendment 5

In Article 2.3, delete -- ENZON and -- in the second line and delete the last
sentence in this Article. Further, insert therein

                                        2

                                     E-118
<PAGE>

-- Neither party will unreasonably withhold consent to changes in the
Development Program --.

Amendment 6

In Article 2.4, strike the last six lines and insert -- approvals and bear all
costs of such trials, including the manufacture and supply of Agreement Product
used in the trials referenced in the Development Program --.

Amendment 7

Delete Article 2.6 and insert the following:

      "The parties hereto shall keep each other informed of their activities
      hereunder and the status of the Development Program by means of a
      quarterly written summary report, a form to be agreed upon by such
      parties. ENZON shall, at SCHERING'S reasonable request, take all
      reasonable steps necessary to allow SCHERING to reference ENZON'S drug
      master file for Agreement Product, which exists as of the effective date
      of this letter amendment, but have no obligation to hereafter add to or
      modify it, except any obligation which may arise from the exercise by
      ENZON of it's option rights to supply SCHERING'S United States
      requirements for Bulk Concentrate under Article 3.1."

Amendment 8

In Article 2.8 strike the last three lines and insert -- any additional Know-How
in SCHERING'S possession as SCHERING shall determine is required under the
Development Program --.

                                        3

                                     E-119
<PAGE>

Amendment 9

Delete the present Article 2.9. Insert therein -- SCHERING shall supply to ENZON
in a timely fashion, at no cost, quantities of Agreement Substance necessary to
make the Bulk Concentrate called for under Article 3.1.--

Amendment 10

Delete Article 2.11.

Amendment 11

***

Amendment 12

Delete Articles 3.1 and 3.2 and insert the following:

      "3.1 SCHERING shall notify ENZON when it receives FDA approval for
      Agreement Product no later than ten (10) days after such receipt. During
      the two year period after the date of such FDA approval, ENZON shall have
      the exclusive option, even as to SCHERING, to elect to manufacture for
      SCHERING and its Affiliates and their sublicensees all of their
      requirements for Bulk Concentrate to be sold in the U.S. during the Term
      of this Agreement. If ENZON elects to supply such Bulk Concentrate, ENZON
      shall supply, and SCHERING shall purchase, such Bulk Concentrate under the
      terms and conditions of the "Supply Agreement" attached as Schedule

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                       4

                                     E-120
<PAGE>

      E to the Agreement. ENZON shall not have to supply, and SCHERING shall not
      have to accept, Bulk Concentrate before seven months have expired since
      the giving of such notice."

      "3.2(a) In consideration of the payments made by SCHERING to ENZON under
      the June 30, 1995 letter amendment, ENZON shall promptly transfer after
      June 30, 1995 previously undisclosed Know-How, owned and controlled by
      ENZON as of June 30, 1995, which is necessary to acquire polyethylene
      glycol("PEG") from third party vendors for conversion to Bulk Concentrate
      and to manufacture Bulk Concentrate, and to obtain regulatory approvals to
      manufacture Bulk Concentrate and Agreement Product and to market Agreement
      Product. This transfer is being made by ENZON for the sole purpose of
      SCHERING developing, and obtaining FDA approval to manufacture, market and
      sell the Bulk Concentrate and Agreement Product. SCHERING shall use such
      Know-How only as necessary for such purposes. It is the intention of the
      parties that such Know-How not in any way be used for or relied on by
      SCHERING to independently develop its own technology for combining PEG
      with polypeptide. Further to the obligations specified in Article 11,
      SCHERING will not disclose such ENZON Know-How to any party other then a
      SCHERING Affiliate, a government authority required to approve the
      registration or sale of Agreement Product, or a vendor producing PEG for
      SCHERING provided such vendor has an obligation of confidentiality to
      SCHERING in connection

                                        5

                                     E-121
<PAGE>

      therewith. Further, SCHERING shall only disclose or allow access of such
      ENZON Know-How to only those persons described above in this Article
      3.2(a) who need such knowledge or access, as strictly interpreted, in
      connection with the above stated purposes. As stated in Article 1.8(a) in
      connection with SCHERING Independent Know-How, in the event SCHERING shall
      develop any idea, invention or technology for combining PEG or any other
      polyalkyleneoxide with any polypeptide, chemical or other compound the
      presumption shall be that such development occurred, was based on and
      originated out of such ENZON Know-How, unless SCHERING can reasonably
      clearly demonstrate that it was developed by SCHERING or its Affiliates
      without the aid, application, or use of such ENZON Know-How or the
      participation of any SCHERING employee or consultant who had knowledge of
      such Know-How. Further the second last sentence in Article 1.8(a)
      relating to "SCHERING Information", shall be modified and interpreted to
      exclude such ENZ0N Know-How or steps taken by SCHERING based on such
      ENZON Know-How."

      "3.2(b) ENZON shall transfer to SCHERING the Know-How referred to in
      Article 3.2(a) promptly after June 30, 1995, generally in accordance with
      the time table set forth in new Schedule G attached to and made a part of
      the June 30, 1995 letter amendment. It is the intention of the parties
      that ENZON provide to SCHERING, for example, but not by way of limitation
      on SCHERING, the "recipe" for making and testing the Agreement Product,

                                        6

                                     E-122
<PAGE>

      but not ENZON's thought processes, theories, or reasons that led to such
      recipe. Further, as an example only, ENZON shall provide SCHERING with its
      standard operating procedures, batch records and raw material in-process,
      final release assay protocols and available assay validations used in
      connection with Agreement Product. However, ENZON shall not transfer to
      SCHERING any of the information or documentation that led to ENZON's
      selection or arrangement of the steps or information in these items.
      Notwithstanding the remainder of this Article 3.2(b), such Know-How
      shall include all elements of ENZON'S thought processes, theories or
      reasons or previously referred to steps or information which ENZON relied
      on in the manufacture of Bulk Concentrate. The foregoing limitations on
      ENZON'S disclosure of such Know-How to SCHERING shall not apply to any of
      such Know-How which is necessary to be disclosed to a government agency
      for approval by that agency to manufacture Bulk Concentrate and / or sell
      Agreement Product, provided such disclosure is done under confidentiality.

      "3.2(c) Schedule G, entitled "Information Needed and Activities To Be
      Completed For Transfer Of Know-How", lists particular information and
      documents which ENZON shall transfer to SCHERING relating to the Know-How
      described in Article 3.2 (a), but is not all inclusive of such Know-How.
      In connection with the transfer of such Know-How to SCHERING under Article
      3.2(b) ENZON will also provide to SCHERING and its Affiliates, and their

                                       7

                                     E-123
<PAGE>

      designees with regard to PEG and manufacture of Bulk Concentrate meeting
      specifications included in Schedule A-2 as of June 30, 1995, reasonable
      technical assistance in utilizing such Know-How. SCHERING shall pay
      ENZON's reasonably documented out of pocket expenses for such assistance.
      Further, upon completion or transfer of such information and documents and
      of the activities specified in Schedule G, SCHERING shall make the One
      Million Dollar($1,000,000) payment to ENZON described in "Amendment 15"
      of the June 30, 1995 letter amendment."

Amendment 13

In Article 4.7, in the third line, delete -- licenses -- and insert -- license
--. Further, delete the last five lines and insert -- during the Term of this
Agreement, to make, have made, use and sell Bulk Concentrate and Agreement
Product in the Territory. This license shall include the right to sublicense
SCHERING'S Affiliates, with respect to making and having made the Bulk
Concentrate and Agreement Product, third party vendors with respect to the
manufacture of conforming PEG and its Affiliates or third parties, with respect
to using and selling Agreement Product. Notwithstanding the exclusive license
granted under the previous sentence, ENZON shall retain the right to manufacture
and supply Agreement Product to SCHERING under the terms and conditions covered
under Article 3.1, if ENZON exercises its option under that Article --.

Amendment 14

In Article 4.8, delete the first two lines on page 20 and insert --

                                        8

                                     E-124
<PAGE>

Bulk Concentrate and to manufacture Bulk Concentrate, under the terms and
conditions specified in Article 3.1 -- . In Article 4.9, in the sixth line, add
-- Bulk Concentrate or -- before the words -- Agreement Product --.

Amendment 15

In addition to the batches of Bulk Concentrate covered under the March 2, 1993
amendment to the Agreement sold to SCHERING, ENZON has delivered eleven (11)
batches of such Bulk Concentrate to SCHERING for total of eight hundred
eighty-five thousand dollars ($885,000), which SCHERING has paid to ENZON. In
addition to the payments called for under Article 6.1(a) - (f), SCHERING shall
also pay ENZON two million dollars ($2,000,000) within twenty(20) days after the
effective date of this letter amendment and a further additional one million
dollars ($1,000,000) on the completion of the transfer of Know-How from ENZON to
SCHERING covered under Article 3.2(c) of this letter amendment.

Amendment 16

Delete the Article 6.1 (C) and add the following

      "(c) One million dollars ($1,000,000) within ten days after dosing the
first patient enrolled in the Phase 2 Clinical Trial, as that phase is described
in the Development Program (as per Schedule D(a)) and one million five hundred
thousand dollars ($1,500,000) within ten days after SCHERING's initiation of
large scale clinical trials as described in Schedule D(b)."

Amendment 17

In Article 9.2(a), in the third line, after --, -- delete the

                                        9

                                     E-125
<PAGE>

ba1ance of Article 9.2(a) and insert the following:

      "if any of the requirements specified in new Schedule H attached hereto
      and made a part of the June 30, 1995 letter amendment are not met or."

Delete Article 9.2 (b) and insert the following:

      " after completion of the initial clinical trial demonstrating that
      Agreement Product meets the criteria set forth in Schedule D attached
      hereto; provided, however, that if such successful completion shall have
      taken place, SCHERING'S right to terminate under this Article 9.2 (b)
      shall be contingent upon SCHERING'S having made to ENZON the total
      payments of *** set forth in Article 6.1(c)."

Amendment 18

In Article 10 add the following new representations and warranties:

      "10.9 As of June 30, 1995, ENZON and it's subsidiaries are each a
      corporation duly organized and existing in good standing under the laws of
      the jurisdiction of their incorporation, except, in the case of such
      subsidiaries, as would not have a Material Adverse Effect(as defined
      below), and has the requisite corporate power to own its properties and
      carry on its business as now being conducted. ENZON and it's subsidiaries
      are each duly qualified to do business and in good standing in every
      jurisdiction in which the nature of the business conducted or property
      owned by them makes such

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                       10

                                     E-126
<PAGE>

      qualification necessary and where the fai1ure so to qualify would have a
      Material Adverse Effect. "Material Adverse Effect" means any material
      adverse effect on the operation, properties or financial condition of
      ENZON and it's subsidiaries taken as a whole."

      "10.10 As of June 30, 1995 ENZON has the requisite corporate power and
      authority to enter into and perform its obligations under the 1995 letter
      amendment, the execution and delivery of the June 30, 1995 letter
      amendment by ENZON and the consummation by it of the transaction
      contemplated herein have been duly authorized by ENZON's Board of
      Directors and no further consent or authorization of ENZON's Board of
      Directors or stockholders is required. The June 30, 1995 letter amendment
      has been duly executed and delivered by ENZ0N, and constitutes a valid and
      binding obligation of ENZON enforceable against ENZON in accordance with
      it's terms, except as such enforceability may be limited by applicable
      bankruptcy, insolvency, or reorganization, moratorium, liquidation, or
      similar laws relating to, or effecting generally the enforcement of
      creditor's rights and remedies or by other equitable principles of general
      application."

      "10.11 ENZON represents and warrants that the representations and
      warranties provided in Article 10.1 and 10.2 of the Agreement shall also
      apply and be binding on ENZON with respect to such Know-How transfer to
      SCHERING under Article 3.2(b) above and to SCHERING's or

                                       11

                                     E-127
<PAGE>

      it's Affiliates as manufacture of Bulk Concentrate or Agreement Product
      based on the use of such Know-How."

      "10.12 As of June 30, 1995, ENZON represents and warrants that there is no
      judicial or administrative action or other proceeding pending or, to the
      best of it's knowledge, threatened, nor to the best of it's knowledge is
      there any governmental investigation pending or threatened that questions
      the validity of any of the transactions contemplated under the June 30,
      1995 letter amendment."

      "10.13 Enzon represents and warrants that with respect to the manufacture
      of "M-PEG", "SC-PEG", and Bulk Concentrate ENZON will provide to SCHERING
      all of such Know-How referred to in Article 3.2(a), and the reasonable
      technical assistance in manufacturing Bulk Concentrate based on such
      Know-How referred to in Article 3.2(b), and that such transfer would
      enable a party skilled in the art of manufacture of substances similar to
      Bulk Concentrate to be able to reproduce Bulk Concentrate meeting the
      specifications of Schedule A-2 as they exist on the effective date of the
      June 30, 1995 letter amendment in connection with an eight gram batch
      thereof."

Amendment 19

In Article 11.5., the fourth and fifth lines, delete -- (but not for any other
purpose or other product) -- and insert -- (but not in contravention of
SCHERING'S obligations or the prohibitions

                                       12

                                     E-128
<PAGE>

covered under Article 3.2) --.

[Amendment 20, 21 and 22 relate to the Supply Agreement between Enzon, Inc. and
Schering Corporation which is no longer in effect, therefore, these Amendments
are not included in this filing]

Amendment 23

                                       13

                                     E-129
<PAGE>

In Article VI of the Supply Agreement, delete the last sentence and insert -- in
that case SCHERING shall be free to cancel such purchase order, in addition to
exercising any other legal remedies which SCHERING may have under the Agreement.
Further, in connection with ENZON's breach of it's obligations under this Supply
Agreement, Article 9.3 and Article 14 of the Agreement as they relate to
SCHERING'S right to terminate the Supply Agreement shall be incorporated by
reference into this Supply Agreement --.

Amendment 24

      All rights and licenses granted under or pursuant to the Agreement by
ENZON to SCHERING are, for all purposes of Section 365(n) of Title 11 of the
U.S. Code ("Title 11"), licenses of rights to intellectual property as defined
in Title 11. ENZON agreed during the Term to create and maintain current copies
or, if not amenable to copying, detailed descriptions or other appropriate
embodiments, to the extent feasible, of all such intellectual property. If a
case is commenced by or against ENZON under Title 11, then, unless and until
this Agreement is rejected as provided in Title 11, ENZON (in any capacity,
including debtor-in-possession) and its successors and assigns (including,
without limitation, a Title 11 Trustee) shall, as SCHERING may elect in a
written request, immediately upon such request (i) perform all of the
obligations provided in the Agreement to be performed by ENZON including, where
applicable and without limitation, providing to SCHERING portions of such
intellectual property (including embodiments thereof) held by ENZON and such
successors and assigns or otherwise available to them or (ii) provide to
SCHERING all such intellectual property (including all, embodiments thereof)
held by ENZON and such successors and assigns or otherwise

                                      14

                                     E-130
<PAGE>

available to them. If a Title 11 case is commenced by or against ENZON, this
Agreement is rejected as provided in Title 11 and SCHERING elects to retain its
rights hereunder as provided in Title 11, then ENZON (in any capacity, including
debtor-in-possession) and its successors and assigns (including, without
limitation, a Title 11 Trustee) shall provide to SCHERING all such intellectual
property (including all embodiments thereof) held by ENZON and such successors
and assigns or otherwise available to them immediately upon SCHERING's written
request therefor. Whenever ENZON or any of its successors or assigns provides
to SCHERING any of the intellectual property licensed hereunder (or any
embodiment thereof) pursuant to this paragraph, SCHERING shall have the right to
perform the obligation of ENZON hereunder with respect to such intellectual
property, but neither such provision nor such performance by SCHERING shall
release ENZON from any such obligation or liability for failing to perform it.
All rights, powers and remedies of SCHERING provided herein are in addition to
and not in substitution for any and all other rights, powers and remedies now or
hereafter existing at law or in equity (including, without limitation, Title 11)
in the event of the commencement of a Title 11 case by or against ENZON.
SCHERING, in addition to the rights, powers and remedies expressly provided
herein, shall be entitled to exercise all other such rights and powers and
resort to all other such remedies as may now or hereafter exist at law or in
equity (including, without limitation, Title 11) in such event. The parties
agree that they intend the foregoing SCHERING's rights to extend to the maximum
extent permitted by law, including without limitation for purposes of Title 11,
(i) the right of access to any intellectual property (including all embodiments
thereof) of ENZON, or any third party with whom ENZON contracts to perform an

                                       15

                                     E-131
<PAGE>

obligation of ENZON under the Agreement, and in the case of the third party,
which is necessary for the development, registration and manufacture of Bulk
Concentrate and Agreement Product and (ii) the right to contract directly with
any third party as described in (i) in the sentence to complete the contracted
work.

      Except as expressly set forth above, all other non-conflicting terms and
conditions of the Agreement shall remain in full force and effect.

Please indicate your acceptance of the foregoing by signing the enclosed copy of
this letter amendment and returning it to us.

Very truly yours,

ENZON, INC.

By: /s/
   ---------------------------

Title: President and Chief Executive Officer

ACCEPTED:

SCHERING CORPORAITON            ------------
                                LEGAL REVIEW
By: /s/ David Poorvin
  ---------------------------    [INITIALS]
                                ------------
Title: Vice President
      ------------------------------------

JAC/cam

                                       16

                                     E-132
<PAGE>

                                  SCHEDULE A-1

                   TEST                      SPECIFICATION
                   ----                      -------------

                   ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-133
<PAGE>

                                  SCHEDULE A-2

                   TEST(1)                   SPECIFICATION
                   -------                   -------------

                   ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-134
<PAGE>

                                  SCHEDULE B

Activity                Start/Completion Target               Responsibility
--------                -----------------------               --------------

           ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-135
<PAGE>

                                  SCHEDULE C

                                 PATENT RIGHTS

                                    DATE                                 DATE
TITLE       INVENTOR    COUNTRY    FILED    SERIAL NO.    PATENT NO.    ISSUED
-----       --------    -------    -----    ----------    ----------    ------

       ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-136
<PAGE>

                                  SCHEDULE D

                                      ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-137
<PAGE>

                                  SCHEDULE G

                                      ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-138
<PAGE>

                                  SCHEDULE H

                                      ***

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-139
<PAGE>

                                                   Amendment No. 5 June 24, 1999

                               AMENDMENT AGREEMENT

      This "Amendment Agreement" effective as of the last date on the signature
page hereof, by and between Enzon, Inc. ("ENZON") and Schering Corporation
("SCHERING") amends and supplements that certain Development, License and
Supply Agreement between ENZON and SCHERING dated November 14, 1990, as amended
by: the Letter Amendment effective June 30, 1995, and the amendment letters of
March 2, 1993, October 30, 1992 and December 10, 1991 (collectively, the
"Agreement").

      WHEREAS, ENZON has developed technology relating to branched chain PEG and
holds patent rights relating thereto, the U-PEG Patent Rights (as defined
below); and

      WHEREAS, SCHERING desires to obtain a license under the U-PEG Patent
Rights; and

      WHEREAS, the parties further desire to modify certain provisions of the
Agreement relating to the manufacture of Agreement Product, payment of royalties
on sales of Agreement Product, and rights under Patent Rights and to use
Know-How;

      NOW THEREFOR, The parties hereby agree to further amend the Agreement as
follows:

      Except as expressly defined herein, all capitalized terms shall have the
meanings set forth in the Agreement, as amended.

1.    Add the following new provisions to Article 1:

      "1.15 "U-PEG Patent Rights" shall mean United States patent number ***
      together with its foreign counterparts and any reissues, re-examinations,
      extensions, substitutions, confirmations, registrations, revalidations,
      additions, continuations, continuations-in-part and divisions of any of
      the foregoing. U-PEG Patent Rights shall not be included in Patent Rights
      under the Agreement.

      1.16 "Roche Patent Rights" shall mean United States patent which may be
      infringed by Agreement Product, together with its foreign counterparts and
      any reissues, re-examinations, extensions, substitutions, confirmations,
      registrations, revalidations, additions, continuations,
      continuations-in-part and divisions of any of the foregoing, which are
      owned or controlled by F. Hoffmann La Roche Ltd. or its Affiliates
      (collectively "Roche").

      1.17 "Licensed ENZON Patent Rights" shall mean any ENZON Patent Rights
      invented by SCHERING, or jointly by SCHERING and ENZON, and owned and/or
      assigned to ENZON pursuant to Section 4.3 of the Agreement."

2.    Delete Articles 2.9, 3.1 and the Supply Agreement set forth in Schedule E
      in their entirety.

3.    Delete Article 1.4 and insert in its place the following:

      "1.4 "Bulk Concentrate" shall mean Agreement Product in bulk concentrated
      form meeting the criteria set forth in Schedule A-2 hereof.
      (Specifications for Bulk Concentrate). SCHERING may amend the
      specifications in Schedule A-2 if in its reasonable judgment it is
      necessary to do so. ENZON shall not be responsible for such specifications
      as amended."

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-140
<PAGE>

4.    Delete Article 2.6 and insert in its place the following:

      "2.6 The parties hereto shall keep each other informed of their activities
      hereunder and the status of the Development Program by means of a
      quarterly written summary report in a form to be agreed upon by such
      parties. ENZON shall, at SCHERING's reasonable request, take all
      reasonable steps necessary to allow SCHERING to reference ENZON's drug
      master file for Agreement Product, which exists as of June 30, 1995, but
      have no obligation to hereafter add to or modify it."

5.    Delete Article 4.7 and insert in its place the following:

      "4.7 Subject to the other terms of this Agreement, ENZON hereby grants to
      SCHERING and SCHERING hereby accepts, an exclusive license, exclusive even
      as to ENZON, under the ENZON Patent Rights, the ENZON Existing Know-How
      and the ENZON Development Know-How, during the term of this Agreement, to
      make, have made, use, import, export, offer for sale and sell Bulk
      Concentrate and Agreement Product in the Territory. This license shall
      include the right to sublicense: (i) SCHERING's Affiliates with respect to
      making and having made the Bulk Concentrate and Agreement Product, (ii)
      third party vendors with respect to the manufacture of conforming PEG, and
      (iii) its Affiliates or third parties with respect to using importing,
      exporting, offering for sale and selling Agreement Product."

6.    Delete Article 4.8 and insert in its place the following:

      "4.8 Subject to the other terms of this Agreement and for the sole purpose
      of ENZON's development and manufacture of Agreement Product for SCHERING,
      SCHERING hereby grants ENZON the right, under the SCHERING Information,
      the SCHERING Patent Rights, the SCHERING Existing Know-How, the SCHERING
      Development Know-How, and the RC License (as hereinafter defined) during
      the term of this Agreement, to use Agreement Substance in the development
      of Bulk Concentrate and Agreement Product for SCHERING."

7.    Add the following new provision to Article 4:

      "4.11 ENZON hereby grants to SCHERING and SCHERING hereby accepts a
      non-exclusive license in the Territory under the U-PEG Patent Rights
      during the term of this Agreement to make, have made, use, import, export,
      offer for sale and sell pharmaceutical products in which alpha interferon
      as an active ingredient is coupled with PEG. The licenses granted to
      SCHERING under this Section 4.11 shall include the right to grant
      sublicenses to SCHERING's Affiliates and/or to any third party.

      4.12 ENZON hereby grants to SCHERING and its Affiliates, and SCHERING and
      its Affiliates hereby accept, a non-exclusive license in the Territory
      under the Licensed ENZON Patent Rights for all purposes during the term of
      this Agreement. The license granted under this Section 4.12 includes,
      without limitation, the right to make, have made, use, import, export,
      offer for sale and sell pharmaceutical products comprising one or more
      active ingredients coupled with PEG, and the limited right to grant
      sublicenses to (i) third party contractors in connection with the
      discovery, development and commercialization of such products by or on
      behalf of SCHERING or its Affiliates, and/or (ii) SCHERING's, or its
      Affiliates', third party licensees and/or distributors of such products."

                                     E-141
<PAGE>

8. It is the intent of the parties that ENZON shall be primarily responsible for
the prosecution, maintenance and defense of the U-PEG Patent Rights, at ENZON's
expense, and that the provisions of Article 5 of the Agreement which govern the
performance of such activities with regard to ENZON Patent Rights will also
control with respect to the U-PEG Patent Rights.

9.    Delete Article 7.1(b) and insert in its place the following:

      "(b)  In each calendar year during the term of this Agreement, in
            connection with SCHERING's sales of Agreement Product which do not
            require payment of a royalty under Article 7-1(a) above, if
            Competition (as defined in Article 7.1(d) below) exists in any such
            country, SCHERING shall pay ENZON a royalty of ***."

10.   Delete Article 7.1(c) and insert in its place the following:

      "(c)  In each calendar year during the term of this Agreement, in
            connection with SCHERING's sales of Agreement Product which do not
            require payment of a royalty under Article 7.1(a) above, if
            Competition does not exist in any such country, SCHERING shall pay
            ENZON a royalty of ***."

11.   Delete Article 7.1(d) and insert in its place the following:

      "(d)  For purposes of this Article 7, "Competition" shall mean sales by a
            party or parties (other than Roche, or SCHERING and its Affiliates,
            or the licensees, distributors or agents of any of the foregoing) in
            any country of the Territory, of a competitive product which
            contains alpha interferon (as a significant active ingredient)
            coupled with PEG to produce a longer-lasting product, with respect
            to which product claims are made which are substantially similar to
            those of Agreement Product, and the promotion, marketing or sale of
            such competitive product does not infringe any of the Patent Rights
            or Know-How exclusively licensed to SCHERING hereunder (or for
            which, because of weak or ineffective intellectual property law in a
            country or countries, there exists no effective patent of know-how
            protection) and: (1) the sales of such competitive product made by
            any one party equal *** or, (2) the sales of such competitive
            product by two or more parties equal, with respect to such party,
            *** but, considering the sales of such competitive product made by
            all such parties in the aggregate, equal ***."

12.   Add new Articles 7.3 and 7.4 as follows:

      "7.3 Notwithstanding the terms of Article 7.2, SCHERING will not be
      entitled to deduct any amount paid as Third-Party Royalties to F. Hoffmann
      La Roche Ltd. or its Affiliates ("Roche") as a result of any Roche Patent
      Rights which may be infringed by SCHERING's sales of Agreement Product.
      SCHERING shall have the right, in its sole discretion, to negotiate the
      settlement of any claim or action related to the infringement of

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-142
<PAGE>

      any Roche Patent Rights. SCHERING will allow ENZON to participate in such
      negotiations.

      7.4 ***

13.   Add the following new provision to the end of Article 8.1:

      "SCHERING's obligation to pay royalties on sales of Agreement Product
      shall expire, unless terminated earlier as expressly provided in Article
      9, on a country-by-country basis upon the longer of(a) the life in each
      country of the last to expire of the ENZON Patent Rights for which a Valid
      Claim exists, including any patent extension (or other governmental action
      which has the effect of extending the period of market exclusivity of the
      patent owner) which is permitted by law and obtained, or (b) fifteen (15)
      years from the date on which Agreement Product is first approved for
      commercial marketing in each such country."

l4.   Delete Article 9.1 and insert in its place the following:

      "9.1 This Agreement shall be made effective on the date first written
      above and shall continue in effect, unless terminated earlier as expressly
      provided herein, for a term which shall expire, on a country-by-country
      basis, upon the longer of (a) the duration of SCHERING's obligation to pay
      royalties in each country, as set forth Article 8.1, or (b) the life in
      each country of the last to expire of the U-PEG Patent Rights and/or
      Licensed ENZON Patent Rights, including any patent extension (or other
      governmental action which has the effect of extending the period of market
      exclusivity of the patent owner) which is permitted by law and obtained
      ("Term")."

15.   In Article 10.3, in lines three and four, delete the term -- and the
      Supply Agreement --.

16.   Delete Article 10.7 and insert in its place the following:

      "10.7 During the term of this Agreement Enzon shall maintain an adequate
      insurance program for liability insurance, including products liability
      and contractual liability insurance, to cover its obligations under this
      Agreement."

17. It is the intent of the parties that during and after the term of the
Agreement SCHERING shall have the right to utilize any Development Know-How
developed by SCHERING, either alone or jointly with ENZON, as well as any
Licensed ENZON Patent Rights, SCHERING Independent Know-How and SCHERING
Existing Know-How, to independently develop and commercialize products in which
one or more active ingredients, whether proteins (other than Agreement

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-143
<PAGE>

Substance) or other compounds, are coupled with PEG ("Other Products"). The
parties acknowledge and agree (i) that any and all Other Products are outside
the scope of the Agreement, (ii) that nothing contained in the Agreement or this
Amendment Agreement shall be construed, either expressly, by estoppel or
otherwise, as granting or otherwise providing to ENZON any license or other
rights in or to such Other Products, and (iii) that ENZON will not be entitled
to any milestone payments or royalties under the Agreement or this Amendment
Agreement with respect thereto. In order to give effect to the intent of the
parties the Agreement is hereby amended as follows:

      (a)   Delete Article 2.12 and insert in its place the following:

            "2.12 Nothing herein shall be construed to prevent SCHERING from
            independently developing its own technology for combining PEG with
            polypeptides and other compounds for any use whatsoever."

      (b)   Delete Article 3.2(a) and insert in its place the following:

            "3.2(a) In consideration of the payments made by SCHERING to ENZON
            under the June 30, 1995 letter agreement, ENZON has transferred to
            SCHERING Know-How, owned and controlled by ENZON as of June 30,
            1995, which is necessary to acquire PEG from third party vendors for
            conversion to Bulk Concentrate and to manufacture Bulk Concentrate,
            and to obtain regulatory approvals to manufacture Bulk Concentrate
            and Agreement Product, and to market Agreement Product. Further to
            its obligations specified in Article 11, SCHERING will not disclose
            such ENZON Know-How to any party other than a SCHERING Affiliate, a
            government authority required to approve the registration or sale of
            Agreement Product or other products developed by SCHERING, or a
            vendor producing PEG for SCHERING provided such vendor has an
            obligation of confidentiality to SCHERING in connection therewith.
            Further, SCHERING shall only disclose or allow access of such ENZON
            Know-How to only those persons described in this Article 3.2(a) who
            need such knowledge or access, as strictly interpreted, in
            connection with the above stated purposes.

18.   Add the following new representations and warranties to Article 10:

      "10.14 ENZON represents and warrants (a) that it has the sole, full and
      complete right, title and interest in the U-PEG Patent Rights, (b) that it
      has the full right, power and authority to grant the licenses and other
      rights granted to SCHERING under this Amendment Agreement, and (c) that as
      of the effective date of this Amendment Agreement there are no claims,
      judgments or settlements against or owed by ENZON relating to the U-PEG
      Patent Rights, and to the best of ENZON's knowledge there are no pending
      or threatened claims or litigation, relating to the U-PEG Patent Rights
      which have not been disclosed by ENZON to SCHERING, except Civil Action
      98-5597 commenced by ENZON against Shearwater Polymers, Inc. in the State
      of Alabama.

      10.15 Each of SCHERING and ENZON represents and warrants that it has the
      right to enter into this Amendment Agreement and has no obligation, or
      knows of no obstacle, which would prevent it from carrying out its
      obligations and responsibilities hereunder.

      10.16 ENZON represents and warrants that the representations and
      warranties set forth in Article 10.9 remain true and in full force and
      effect as of the effective date of this Amendment Agreement.

                                     E-144
<PAGE>

      10.17 ENZON represents and warrants that as of the effective date of this
      Amendment Agreement it has the requisite corporate power and authority to
      enter into and perform its obligations hereunder, that the execution and
      delivery of this Amendment Agreement by ENZON and the consummation of the
      transactions contemplated hereunder have been duly authorized by ENZON's
      Board of Directors and no further consent or authorization of ENZON's
      Board of Directors or stockholders is required. This Amendment Agreement
      has been duly executed and delivered by ENZON and constitutes a valid and
      binding obligation of ENZON, enforceable against ENZON in accordance with
      its terms, except as such enforceability may be limited by applicable
      bankruptcy, insolvency, or reorganization, moratorium, liquidation, or
      similar laws relating to, or affecting generally the enforcement of
      creditor's rights and remedies or by other equitable principles of general
      application.

      10.18 ENZON represents and warrants (a) that to the best of ENZON's
      knowledge based on diligent inquiry, as of the effective date of this
      Amendment Agreement the U-PEG Patent Rights are free and clear of any
      liens, charges or encumbrances, and no other person, corporate or private
      entity, or governmental entity or subdivision thereof, has or shall have
      any claim of ownership with respect to such U-PEG Patent Rights, (b) that
      during the Term of the Agreement ENZON will use reasonable diligent
      efforts not to diminish the rights under the U-PEG Patent Rights granted
      to SCHERING hereunder, including without limitation, by not committing or
      permitting any actions or omissions which would cause the breach of any
      third party agreements relating to the U-PEG Patent Rights, that it will
      provide SCHERING promptly with notice of any such alleged breach, and that
      as of the effective date of this Amendment Agreement, ENZON is in
      compliance in all material respects with all such third party agreements,
      if any."

19. The parties acknowledge and agree that the licenses under the U-PEG Patent
Rights granted to SCHERING under this Amendment Agreement are licenses of
rights to intellectual property and are subject to the provisions set forth in
Amendment 24 of the Letter of Agreement dated June 30, 1995.

      Except as expressly amended and supplemented hereby, all other terms of
the Agreement shall remain in full force and effect.

      IN WITNESS WHEREOF, the parties hereto have caused this Amendment
Agreement to be executed in duplicate by their duly authorized representatives.

                                                               ------------
                                                               LEGAL REVIEW

                                                                [INITIALS]
                                                               ------------

ENZON, INC.                              SCHERING CORPORATION

BY: /s/ John P. Caruso                   BY: /s/ David Poorvin, Ph.D.
   ------------------------------           -----------------------------------

NAME: John P. Caruso                     NAME:  David Poorvin, Ph.D.
     ----------------------------             ---------------------------------

TITLE: VP - Admin & Gen. Counsel         TITLE: Vice President
      ---------------------------              --------------------------------

DATE: 6/24/99                            DATE:  23 June 1999
     ----------------------------             ---------------------------------

                                     E-145
<PAGE>

                                              Amendment No. 6 September 13, 1999

                                   AMENDMENT

      This "Amendment" effective as of the last date on the signature page
hereof, by and between Enzon, Inc. ("ENZON") and Schering Corporation
("SCHERING") amends and supplements that certain Development, License and Supply
Agreement between ENZON and SCHERING dated November 14, 1990, as amended by:
the Amendment Agreement effective June 24, 1999, the Letter Amendment effective
June 30, l995, and the amendment letters of March 2, 1993, October 30, 1992 and
December 10, 1991 (collectively, the "Agreement").

      WHEREAS, the parties desire to clarify the scope of certain provisions of
the Agreement relating to (i) non-competition restrictions imposed on ENZON,
(ii) ENZON's rights under ENZON Patent Rights to which SCHERING may acquire
title under the terms of the Agreement, and (iii) assignment or change of
control;

      NOW THEREFOR, The parties hereby agree to further amend the Agreement as
follows:

      Except as expressly defined herein, all capitalized terms shall have the
meanings set forth in the Agreement, as amended.

1.    Delete Article 2.13 and insert in its place the following:

      "2.13 During the term of this Agreement, anything to the contrary
      contained herein notwithstanding, ENZON and its Affiliates shall not
      develop any product containing PEG and alpha interferon for itself or with
      or for any third party, nor shall it or its Affiliates enter into
      discussions or negotiations with any third party concerning any product
      containing PEG and alpha interferon."

2.    Delete Article 5.1.2 and insert in its place the following:

      "5.1.2 ENZON shall notify SCHERING immediately (1) whenever it files a
      patent application related to the ENZON Patent Rights in any country; and
      (2) whenever it is considering discontinuance of the prosecution or
      maintenance of such a patent application. If ENZON shall fail to file a
      patent application related to the ENZON Patent Rights in any country it
      shall notify SCHERING immediately in writing, and (a) if ENZON does not
      initiate the filing process within ninety (90) days after written request
      to do so from SCHERING, or (b) if ENZON does not continue prosecution or
      maintenance of such a patent application related to the ENZON Patent
      Rights which it has initiated pursuant to the Article 5.1.1 above within
      ninety (90) days after written notice from SCHERING, SCHERING shall have
      the option to file or continue prosecution in its own name and at
      SCHERING's cost. In either such case (a) or (b), ENZON shall do all acts
      necessary to assign and vest title to and transfer control of such patent
      application in that country in a timely fashion to SCHERING for no
      additional consideration. SCHERING hereby covenants that during the term
      of this Agreement SCHERING and its Affiliates shall not file or initiate
      any suit or other legal action or proceedings against ENZON with respect
      to any products, with the exception of products containing both PEG and
      alpha interferon, for infringement of any patents acquired, or obtained by
      SCHERING from patent applications filed and/or prosecuted by SCHERING,
      under this Section 5 1.2 in the

                                     E-146
<PAGE>

      Territory, The covenant not to sue set forth in this Section 5.1.2 is
      personal to ENZON and shall not be transferred or assigned to any third
      party."

3.    Delete Article 13.2 and insert in its place the following:

      "13.2 Either party hereto may assign this Agreement, without the other
      party's consent, to a third party in a transaction in which all the assets
      of the assigning party relating to Agreement Product are sold or assigned;
      provided such third party was not selling (directly or through an
      Affiliate) a product competitive to the Agreement Product prior to such
      assignment.

      In the event of any such permitted assignment, transfer or designation,
      the assignee, transferee or designee shall assume and be bound by the
      provisions of this Agreement."

4.    Add new Article 13.3 as follows:

      "13.3 In the event that any assignment pursuant to Section 13.1 or 13.2
      causes ENZON's rights and obligations hereunder to pass to any third party
      which itself or through an Affiliate is developing, manufacturing,
      marketing, distributing or selling a product competitive to the Agreement
      Product, then SCHERING's obligation under Section 2.6 to keep ENZON
      informed and provide quarterly reports with respect to the Development
      Program shall immediately terminate."

      Except as expressly amended and supplemented hereby, all other terms of
the Agreement shall remain in full force and effect.

      IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
executed in duplicate by their duly authorized representatives.

                                                               ------------
                                                               LEGAL REVIEW

                                                                [INITIALS]
                                                               ------------

ENZON, INC.                              SCHERING CORPORATION

BY: /s/ John Caruso                      BY: /s/ David Poorvin, Ph.D.
   ------------------------------           -----------------------------------

NAME: John Caruso                        NAME:  David Poorvin, Ph.D.
     ----------------------------             ---------------------------------

TITLE: VP - Admin & GC                   TITLE: Vice President
      ---------------------------              --------------------------------

DATE: 9/13/99                            DATE:   8 September 1999
     ----------------------------             ---------------------------------

                                     E-147
<PAGE>

                                                                 Amendment No. 7

                                    AMENDMENT

            WHEREAS, Schering Corporation ("Schering") and Enzon, Inc. ("Enzon")
are parties to that certain Development, License and Supply Agreement having an
effective date of November 14, 1999, as amended (the "Agreement"),

            WHEREAS, Schering's parent company is a party to Hoffman-LaRoche
Inc. v. Schering-Plough Corp., Civil Action No. 00-79 (JAG) pending in the U.S.
District Court for the District of New Jersey (the "Roche v. Schering case"),

            WHEREAS, Enzon commenced on September 5, 2000 a suit for patent
infringement against Hoffman-La Roche Inc. and Roche Laboratories Inc. in
Federal court in New Jersey (the "Enzon v. Roche" case), and

            WHEREAS, Schering and Enzon wish to clarify the effect of the
Agreement, including paragraph 2.13 thereof, on (i) the grant of a license or
sublicense under the U-PEG Patent Rights (as defined in the Agreement) to
Hoffman-LaRoche Inc., Roche Laboratories Inc., or their parents, subsidiaries,
affiliates, successors, assigns or third parties acting on their behalf
(collectively, "Roche"); and (ii) the settlement of Roche v. Schering.

            THEREFORE, the Agreement is hereby amended as follows:

      1.    Schering shall have sole authority to negotiate and settle (i) Roche
            v. Schering; and (ii) the Enzon v. Roche and/or related cases ("the
            Cases") (including the right to assume control of the Cases in the
            event Enzon decides to discontinue such cases for any reason) or
            license Roche under the U-PEG Patent Rights in the field of
            pegylated alpha interferon; and Enzon shall not settle the Cases
            with or license Roche or directly or

                                  CONFIDENTIAL

                                     E-148
<PAGE>

            impliedly a third party to enable Roche to make, use, sell, offer to
            sell or import pegylated alpha interferon products under the U-PEG
            Patent Rights without Schering's written consent (except as provided
            below).

      2.    If a settlement between Schering and Roche results in withdrawal of
            PEG-INTRON from the United States market, then Enzon shall have the
            right to negotiate and settle the Cases with Roche under terms as
            Enzon sees fit, including the grant of a license to Roche under the
            U-PEG Patent Rights.

      3.    Schering shall not sublicense Roche under the U-PEG Patent Rights
            except as part of a settlement that (i) licenses or releases
            Schering under Roche's U.S. patent(s) covering PEG-INTRON; and (ii)
            releases Enzon from any counterclaims or demands asserted against
            Enzon by Roche (on behalf of Roche and not on behalf of a third
            party) relating to the U-PEG patents or in the field of pegylated
            alpha interferon products.

      4.    If Schering grants a sublicense under the U-PEG Patent Rights to
            Roche to settle the Roche v. Schering or Enzon v. Roche or related
            cases that results in royalties or royalty equivalents from Roche to
            Schering in connection with a license to Roche under the U-PEG
            patent rights, such royalties and royalty equivalents that exceed
            the value of any royalties or royalty equivalents paid by Schering
            to Roche under a license to Schering under the Roche patents
            relating to urethane linked pegylated interferon shall be divided
            equally between Schering and Enzon. A license under the Roche
            patents for Schering to market PEG-INTRON shall not be considered
            royalty or royalty equivalents to Schering and is not subject to
            division with Enzon hereunder. In the event of a dispute between
            Schering and Enzon regarding whether Schering has received royalties
            or royalty equivalents from Roche that exceed the value of any

                                     E-149
<PAGE>

            royalties or royalty equivalents paid by Schering to Roche, such
            dispute shall be resolved pursuant to the arbitration provision in
            the Agreement.

      5.    This Amendment contains the entire agreement of the parties
            concerning the subject matter set forth herein. Except as expressly
            amended by this Amendment, all terms of the Agreement and all
            previous amendments shall remain in full force and effect.

            IN WITNESS WHEREOF, the parties have caused this Amendment to be
executed in duplicate by their duly authorized representatives.

ENZON, INC.                              SCHERING CORPORATION

By:                                      By: /s/
   ------------------------------           -----------------------------------
   Authorized Representative                  Authorized Representative

Date:                                    Date:
     ----------------------------             ---------------------------------

                                     E-150
<PAGE>

                                                 Amendment No. 8 August 10, 2001

                                    AMENDMENT

      This "Amendment" effective as of the last date on the signature page
hereof, by and between Enzon, Inc. ("ENZON") and Schering Corporation
("SCHERING") amends and supplements that certain Development, License and Supply
Agreement between ENZON and SCHERING dated November 14, 1990, as amended (the
"Agreement").

      WHEREAS, the parties desire to clarify the scope of certain provisions of
the Agreement relating to SCHERING's rights under the ENZON Patent Rights and
the Licensed ENZON Patent Rights;

      NOW THEREFORE, the parties hereby agree to further amend the Agreement as
follows:

      Except as expressly defined herein, all capitalized terms shall have the
meanings set forth in the Agreement, as amended.

1.    Amend Article 1.2 of the Agreement by adding the following new provision
      to the end of the first sentence of that Article:

      ", whether or not developed by SCHERING under this Agreement."

2.    Add the following new provision to the end of Article 2.13:

      "The parties acknowledge and agree that nothing in this Article 2.13 shall
      be construed as preventing Enzon from acquiring, through merger,
      acquisition or other similar transaction, all or substantially all of the
      business and assets of Shearwater Polymers, Inc. ("Shearwater") or from
      entering into discussions or negotiations with Shearwater with regard to
      such an acquisition."

3.    Amend Article 2 of the Agreement by adding the following new provisions:

      "2.14 During the term of this Agreement and subject to receipt of the
      necessary regulatory approvals, SCHERING shall use commercially
      reasonable efforts to market, promote, distribute and sell an Agreement
      Product developed by SCHERING or its Affiliates under this Agreement in
      the United States, United Kingdom, France, Germany, Italy and Spain and in
      any other countries where in SCHERING's opinion it is commercially viable
      to do so. Such commercially reasonable efforts shall be consistent with
      the usual practice followed by SCHERING in pursuing the commercialization
      and marketing of its other pharmaceutical products of similar potential,
      value and status. The parties acknowledge and agree that all business
      decisions relating to the commercialization of the Agreement Product,
      including, without limitation, decisions relating to registration, design,
      manufacture, sale, commercialization, pricing, distribution, marketing and
      promotion of

                                     E-151
<PAGE>

                                     No. 8

      Agreement Products, shall be within the sole discretion of SCHERING
      exercised in good faith.

      2.15 SCHERING's obligations with respect to commercialization of Agreement
      Product under Article 2.14 are expressly conditioned upon the continuing
      absence of any adverse condition or event which warrants a delay in
      commercialization of the Agreement Product, including, but not limited to,
      an adverse condition or event relating to the safety or efficacy of the
      Agreement Product, or unfavorable labeling, pricing or pricing
      reimbursement approvals, or lack of regulatory approval, and such
      obligations shall be delayed or suspended so long as in SCHERING's
      reasonable opinion any such condition or event exists."

4.    Amend Article 4 of the Agreement by adding the following new provision:

      "4.13 The rights granted to SCHERING under the ENZON Patent Rights in
      Article 4.7 and the Licensed ENZON Patent Rights in Article 4.12 shall, to
      the extent not previously granted, additionally include the right to grant
      a royalty bearing, non-transferable, non-exclusive sublicense, with no
      right to grant sublicenses to others, to Hoffmann-LaRoche Inc., F.
      Hoffmann-LaRoche Ltd. and their respective Affiliates (collectively,
      "Roche") in the Territory (as such term is defined in the Settlement and
      License Agreement by and between SCHERING and Roche, dated as of August
      10, 2001 (the "Settlement Agreement") to Commercialize Licensed Roche
      Product under the Enzon Patent Rights (as such terms are defined in the
      Settlement Agreement)."

5.    Delete Article 7.1(b) in its entirety and insert in its place the
      following:

            (b)   In each calendar year during the term of this Agreement, in
                  connection with Net Sales of Agreement Product which do not
                  require payment of a royalty under Article 7.1(a) above, if
                  Competition (as defined in Article 7.1(d) below) exists in any
                  such country, SCHERING shall pay ENZON a royalty of ***.

6.    Delete Article 7.1(c) in its entirety and insert in its place the
      following:

            (c)   In each calendar year during the term of this Agreement, in
                  connection with Net Sales of Agreement Product which do not
                  require payment of a royalty under Article 7.1(a) above, if
                  Competition does not exist in any such country, SCHERING shall
                  pay ENZON a royalty of ***.

7.    Amend Article 7.1 of the Agreement by adding the following new provision:

*** Indicates the omission of confidential material pursuant to a request for
confidential treatment made in accordance with Rule 24b-2 under the Securities
Exchange Act of 1934, as amended. The Confidential material is being filed
separately with the Secretary to the Securities and Exchange Commission.

                                     E-152
<PAGE>

            (c)   The parties understand and agree that, except as expressly set
                  forth in section 4 of the Amendment to the Agreement dated
                  September 22, 2000, nothing in this Agreement shall give rise
                  to any right by ENZON to receive or claim entitlement to any
                  royalty payments from SCHERING and its Affiliates, nor shall
                  SCHERING or its Affiliates have any obligation to make any
                  royalty payments to ENZON or its Affiliates, under this
                  Agreement in connection with any sublicense or other rights
                  granted by SCHERING in the Territory to Roche under the
                  Settlement Agreement and/or the commercialization of Agreement
                  Products by Roche pursuant to such sublicense under the
                  Settlement Agreement (including, without limitation, the
                  manufacture of Agreement Products on behalf of Roche by
                  Shearwater and its Affiliates), provided that royalties on
                  Agreement Product sold by SCHERING or its Affiliates will
                  become payable at such time, if any, as Roche becomes an
                  Affiliate of SCHERING. It is understood and agreed that the
                  provisions of the immediately preceding sentence shall apply
                  on a country-by-country basis only for so long as such
                  sublicense granted by SCHERING to Roche under the Settlement
                  Agreement remains in effect."

8.    Delete Article 9.2(b) in its entirety.

9.    Amend Article 13 of the Agreement by adding the following new provision:

      "l3.4 Nothing in this Agreement shall preclude Roche from becoming an
      Affiliate of SCHERING if Roche should otherwise come within the definition
      of Affiliate set forth in Article 1.1 of the Agreement.

      Except as expressly amended and supplemented hereby, all other terms of
the Agreement shall remain in full force and effect.

      IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
executed in duplicate by their duly authorized representatives.

ENZON, INC.                              SCHERING CORPORATION

BY: /s/ Arthur Higgins                   BY:
   ------------------------------           -----------------------------------

NAME: Arthur Higgins                     NAME:  David Poorvin, Ph.D.
     ----------------------------             ---------------------------------

TITLE: President and CEO                 TITLE: Vice President
      ---------------------------              --------------------------------

DATE: August 10, 2001                    DATE:
     ----------------------------             ---------------------------------

                                     E-153
<PAGE>

            (e)   The parties understand and agree that, except as expressly set
                  forth in section 4 of the Amendment to the Agreement dated
                  September 22, 2000, nothing in this Agreement shall give rise
                  to any right by ENZON to receive or claim entitlement to any
                  royalty payments from SCHERING and its Affiliates, nor shall
                  SCHERING or its Affiliates have any obligation to make any
                  royalty payments to ENZON or its Affiliates, under this
                  Agreement in connection with any sublicense or other rights
                  granted by SCHERING in the Territory to Roche under the
                  Settlement Agreement and/or the commercialization of Agreement
                  Products by Roche pursuant to such sublicense under the
                  Settlement Agreement (including, without limitation, the
                  manufacture of Agreement Products on behalf of Roche by
                  Shearwater and its Affiliates), provided that royalties on
                  Agreement Product sold by SCHERING or its Affiliates will
                  become payable at such time, if any, as Roche becomes an
                  Affiliate of SCHERING. It is understood and agreed that the
                  provisions of the immediately preceding sentence shall apply
                  on a country-by-country basis only for so long as such
                  sublicense granted by SCHERING to Roche under the Settlement
                  Agreement remains in effect."

8.    Delete Article 9.2(b) in its entirety.

9.    Amend Article 13 of the Agreement by adding the following new provision:

      "13.4 Nothing in this Agreement shall preclude Roche from becoming an
      Affiliate of SCHERING if Roche should otherwise come within the definition
      of Affiliate set forth in Article 1.1 of the Agreement.

      Except as expressly amended arid supplemented hereby, all other terms of
the Agreement shall remain in full force and effect.

      IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
executed in duplicate by their duly authorized representatives.

                                                               ------------
                                                               LEGAL REVIEW

                                                                [INITIALS]
                                                               ------------

ENZON, INC.                              SCHERING CORPORATION

BY:                                      BY: /s/ David Poorvin, Ph.D.
   ------------------------------           -----------------------------------

NAME:                                    NAME:  David Poorvin, Ph.D.
     ----------------------------             ---------------------------------

TITLE:                                   TITLE: Vice President

      ---------------------------              --------------------------------

DATE:                                    DATE:  10 August 2001
     ----------------------------             ---------------------------------

                                     E-154

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