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                                                      Exhibit 10.33
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                                  BIOGEN, INC.
                     SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN
















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                                                                       July 1997
                                                                  Execution Copy

                                   SECTION 1.

                           PURPOSE AND EFFECTIVE DATE


The purpose of this plan is to provide certain key executives of Biogen (or its
subsidiaries) with additional retirement income by supplementing the retirement
benefits provided under the Biogen Retirement Plan. In addition, this plan
provides benefits to participants whose benefit under the Biogen Retirement Plan
is affected by Internal Revenue Code limits on the amount of compensation that
may be taken into account under the Retirement Plan for any plan year (Code
Section 401(a)(17)) or on the amount of benefits that may be provided from the
Retirement Plan (Code Section 415). The effective date of this plan is January
1, 1991. The plan was amended and restated effective April 1, 1996 in
conjunction with the amendment and restatement of the Biogen Retirement Plan.










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                                   SECTION 2.

                                   DEFINITIONS


This section contains definitions of terms used in the plan. Where the context
so requires, the masculine includes the feminine, the singular includes the
plural, and the plural includes the singular.

2.1  APPLICABLE COMPENSATION has the same meaning as in the Retirement Plan,
     without regard, however, to either (a) any dollar limitation on applicable
     compensation that may be imposed under the Retirement Plan, or (b) any
     amount excluded from a participant's applicable compensation under the
     Retirement Plan by reason of the participant's reducing his salary and/or
     bonus under any non-qualified deferred compensation plan or arrangement
     maintained by Biogen (or a subsidiary).

2.2  BIOGEN means Biogen, Inc., a Massachusetts corporation, or any successor to
     all or the major portion of its assets or business which assumes the
     obligations of Biogen, Inc. under this plan.

2.3  BOARD means the Board of Directors of Biogen.

2.4  COMMITTEE means the Retirement Plan committee constituted under the
     Retirement Plan.

2.5  PARTICIPANT means an employee of Biogen (or a subsidiary) who has been
     designated a participant in this plan in accordance with Section 3 hereof.

2.6  PLAN means the Biogen, Inc. Supplemental Executive Retirement Plan, as set
     forth in this plan instrument, and as it may be amended from time to time.





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2.7  PLAN YEAR means the 12-month period beginning each January 1 during the
     continuation of the plan.

2.8  RETIREMENT PLAN means the Biogen Retirement Plan, as amended from time to
     time. Any term defined in the Biogen Retirement Plan will have the same
     meaning when used in this plan unless otherwise defined herein.

2.9  SINGLE SUM CONVERSION BASIS means the assumptions set forth in Section
     2.1(b) of the Retirement Plan.




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                                   SECTION 3.

                                  PARTICIPATION


3.1  PARTICIPATION IN SUPPLEMENTAL PENSION FORMULA. Each person
     listed on Appendix A is a participant in the supplemental
     pension formula under Section 4.1 as of January 1, 1991.

3.2  PARTICIPATION IN EXCESS BENEFIT FORMULAS. Each participant in the
     Retirement Plan (other than a person listed on Appendix A) (a) whose
     applicable compensation for any plan year exceeds the dollar limitation on
     applicable compensation imposed under Code Section 401(a)(17) for such plan
     year, (b) whose benefit under the Retirement Plan is reduced upon his
     retirement or other termination of employment by the limitations imposed
     under Code Section 415, or (c) who has reduced his salary and/or bonus
     under any non-qualified deferred compensation plan or arrangement
     maintained by Biogen (or a subsidiary) will be a participant in the excess
     benefit formula under Section 4.2.

3.3  END OF PARTICIPATION. Except as specifically provided in an appendix, an
     employee's active participation in this plan accruing additional benefits
     under the applicable provisions of the plan will end when his participation
     in the Retirement Plan ends.





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                                   SECTION 4.

                                BENEFIT FORMULAS


4.1  SUPPLEMENTAL PENSION FORMULA.

     (a) For each plan year during the existence of this plan, a participant in
         the supplemental pension formula will be credited with a supplemental
         pension amount equal to 1 1/2% of his applicable compensation during
         such year up to the Social Security taxable wage base for such year
         (prorated for any partial year of employment as an employee of Biogen
         (or a U.S. subsidiary)), plus 2 1/2% of his applicable compensation
         during such year above the Social Security taxable wage base for such
         year (as so prorated). In addition, a participant will be credited with
         a supplemental pension amount in accordance with the preceding sentence
         for 1989 and 1990 provided such individual was an employee of Biogen
         (or subsidiary) during such year.

     (b) A participant's accrued supplemental pension benefit hereunder as of
         any date of reference will be the sum of the supplemental pension
         amounts he is credited with under the preceding paragraph.

4.2  EXCESS BENEFIT FORMULAS. A participant's accrued excess benefit under this
     section will be the benefit he would have earned under the accrued benefit
     formula in Section 6.1(a) of the Retirement Plan if (a) his applicable
     compensation taken into account had never been limited under Code Section
     401(a)(17), (b) the limitations on benefits imposed under Code Section 415
     (as specified in Sections 12.1 to 12.3 of the Retirement Plan) were
     disregarded, and (c) he had never reduced his salary and/or bonus under a
     non-qualified deferred compensation plan or arrangement maintained by
     Biogen (or a subsidiary).




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4.3  VESTED INTEREST. A participant who terminates employment before his normal
     retirement date will have the same vested percentage in his supplemental
     pension benefit or excess benefit under this plan that he has in his
     accrued benefit under the Retirement Plan.

4.4  ADDITIONAL SPECIAL BENEFITS. In addition to any supplemental pension
     benefits, excess benefits or other benefits provided under the terms of the
     plan, a participant who is specified in an appendix to this plan will be
     entitled to benefits calculated in accordance with the provisions of such
     appendix.







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                                   SECTION 5.

                        PROVISIONS ON PAYMENT OF BENEFITS


5.1  SUPPLEMENTAL PENSION BENEFIT AND EXCESS BENEFITS. Supplemental pension
     benefit amounts or excess benefit amounts as calculated under the formulas
     in Sections 4.1 and 4.2 above are payable as an annual pension beginning on
     the participant's normal retirement date in the form of an annuity for the
     life of the participant only. If a participant's supplemental pension
     benefits or excess benefits hereunder are actually paid beginning on an
     earlier date or in a different form in accordance with Section 5.2, the
     amount will be adjusted as provided in Section 5.2.

5.2  FORM AND TIME OF BENEFIT PAYMENTS. A participant's vested supplemental
     pension benefit or excess benefit under Section 4.1 or Section 4.2 will be
     paid in the same form and beginning on the same date that his benefit under
     the Retirement Plan is payable. Notwithstanding the preceding sentence, if
     a participant elects a lump sum form of payment, such election shall be
     made prior to the date the participant actually retires, and will be
     subject to the approval of the committee.

     (a) ANNUITY OPTIONS. If the participant's supplemental pension benefit or
         excess benefit is payable in any form other than a lump sum or a life
         annuity, the actuarial factors used to convert his supplemental pension
         benefit or excess benefit amount from a life annuity to such other form
         of payment will be the same as the factors used for such purpose in the
         Retirement Plan.





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     (b) LUMP SUM OPTION.

         (i)   Supplemental pension: If a participant's supplemental pension
               under Section 4.1(b) is payable as a lump sum, the amount shall
               be the supplemental pension amount under Section 4.1 converted to
               a single sum benefit using the assumptions set forth in Section
               2.9.

         (ii)  Excess pension: If a participant's excess benefit under Section
               4.2 is payable as a lump sum, the amount will be the lump sum
               benefit he would have received under the Retirement Plan without
               regard to Internal Revenue Code Section 401(a)(17) and
               regulations thereunder, Internal Revenue Code Section 415 and
               regulations thereunder and any reductions of his salary and/or
               his bonus under a non-qualified deferred compensation plan or
               arrangement maintained by Biogen (or a subsidiary).

     (c) EARLY RETIREMENT.

         (i)   Supplemental pension: If the participant's supplemental pension
               benefit is payable as an annuity starting before his normal
               retirement date, his benefit will equal the supplemental pension
               amount he is credited with under Section 4.1, reduced by 5/9ths
               of 1% per month for each of the first 60 months, and by 5/18ths
               of 1% per month for each additional month, that the start of
               payments precedes his normal retirement date.





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         (ii)  Excess pension: If the participant's excess benefit is payable as
               an annuity starting before his normal retirement date, his
               benefit shall equal the early retirement annuity amount he would
               have received under the Retirement Plan without regard to
               Internal Revenue Code Sections 401(a)(17) and regulations
               thereunder, Internal Revenue Code Section 415 and regulations
               thereunder, and any reductions in his salary and/or bonus under
               any non-qualified deferred compensation plan or arrangement
               maintained by Biogen (or a subsidiary).

5.3  REDUCTION FOR RETIREMENT PLAN BENEFITS. Notwithstanding any other provision
     of this plan, the amount payable to a participant under this plan (as
     calculated under the applicable provisions of the plan) will be reduced by
     the amount actually paid to such participant as a benefit from the
     Retirement Plan. If Retirement Plan benefits to a participant are paid in a
     different form or commence at a different time compared to benefits under
     this plan, for purposes of calculating the reduction for Retirement Plan
     benefits under the preceding sentence, the Retirement Plan benefit amount
     will be converted to an amount payable in the same form and commencing at
     the same time as the participant's benefit under this plan, with such
     conversion performed in accordance with the actuarial assumptions specified
     in the Retirement Plan or, if applicable, in Section 2.9.







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                                   SECTION 6.

                                 DEATH BENEFITS


6.1  APPLICATION OF THIS SECTION. This section specifies the benefits payable
     upon the death of a participant, either before or after the date his
     benefit payments hereunder begin. Except as specified in this section, no
     benefits are payable upon the death of a participant.

6.2  PRERETIREMENT DEATH BENEFIT.

     (a) ELIGIBILITY. If a participant in this plan dies after the date when he
         has a vested interest hereunder but before the date when his benefit
         payments begin, his beneficiary will receive the preretirement death
         benefit described under this section.

     (b) PAYMENT. Preretirement death benefit payments will be made or will
         begin on the same date that payments of the Retirement Plan's
         preretirement death benefit under Section 9.2 of the Retirement Plan is
         made or begins.

     (c) AMOUNT. The amount of each monthly payment to the beneficiary will be
         the amount the participant would have received on the payment starting
         date (had he survived to such date), based upon his vested accrued
         supplemental pension benefit or excess benefit hereunder as of the
         earlier of his date of death or his date of retirement or other
         termination of employment, with actuarial reduction if applicable, for
         payments starting before the participant's normal retirement date.
         Notwithstanding the preceding sentence, for purposes of converting a
         participant's Retirement Plan cash balance account to a life annuity,
         the applicable factor shall be based on the attained age of the
         beneficiary as of the date payments begin.






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     (d) If the preretirement death benefit under the Retirement Plan is payable
         as a commuted single sum payment, the beneficiary's benefit under this
         section will also be paid as a commuted single sum payment. Such single
         sum payment shall equal the lump sum benefit the participant would have
         received under Section 5.2(b) (had he survived to such date and elected
         such form of payment).

     (e) OFFSET FOR RETIREMENT PLAN BENEFITS. Notwithstanding any other
         provision of this Section 6.2, the amount payable to a beneficiary
         under this section (as calculated under the applicable provisions of
         the plan) will be reduced by the amount payable to such beneficiary as
         a preretirement death benefit from the Retirement Plan.

6.3  DEATH AFTER BENEFIT PAYMENTS BEGIN. If a participant dies while receiving
     benefit payments hereunder, his surviving spouse, contingent annuitant or
     beneficiary will receive the benefit, if any, payable under the form of
     payment in effect for such participant.






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                                   SECTION 7.

                                   DISABILITY


7.1  A participant who suffers a total and permanent disability for purposes of
     the Retirement Plan will continue to participate in this plan during his
     period of covered disability (as defined in the Retirement Plan). Such a
     participant will continue to accrue supplemental pension benefits or excess
     benefits under this plan in accordance with the applicable provisions of
     Section 4 as if such participant had continued to be an active employee of
     Biogen (or a subsidiary) during his period of covered disability and as if
     he had continued receiving applicable compensation during his period of
     covered disability at the same rate as his rate of applicable compensation
     in effect immediately before the onset of his disability and the Social
     Security taxable wage base had remained at the same level as in effect for
     the year when his disability began. The accrual of additional supplemental
     pension benefits or excess benefits under the preceding sentence will end
     when his period of covered disability ends in accordance with the terms of
     the Retirement Plan.

7.2  If a participant dies while disabled, his beneficiary will receive the
     pre-retirement death benefit determined under Section 6.2.

7.3  A participant may not receive benefits under this plan at any time when he
     is receiving disability income benefits under any long-term disability
     income plan maintained by Biogen.







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                                   SECTION 8.

                          BENEFITS NOT CURRENTLY FUNDED


8.1  Nothing in this plan will be construed to create a trust or to obligate
     Biogen (or any subsidiary) to segregate a fund, purchase an insurance
     contract, or in any other way currently to fund the future payment of any
     benefits hereunder, nor will anything herein be construed to give any
     participant or any other person rights to any specific assets of Biogen or
     of any other employer or entity.

8.2  Notwithstanding Section 8.1, Biogen in its sole discretion may establish a
     trust of which it is treated as the owner under Section 671 of the Internal
     Revenue Code (a "grantor trust") to provide for the payment of benefits
     hereunder, subject to such terms and conditions as Biogen may deem
     necessary or advisable to ensure that benefits are not includable, by
     reason of the trust, in income of trust beneficiaries before actual
     distribution and that the existence of the trust does not cause the plan or
     any other arrangement to be considered funded for purposes of Title I of
     the Employee Retirement Income Security Act of 1974, as amended.







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                                   SECTION 9.

                                 ADMINISTRATION


The plan will be administered by the committee, which will have full power and
authority to construe, interpret and administer the plan. Decisions of the
committee will be final and binding on all persons. The committee in its
discretion may adopt, amend, and rescind rules and regulations relating to the
administration of the plan.








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                                   SECTION 10.

                              RIGHTS NON-ASSIGNABLE


No participant, surviving spouse, or any other person will have any right to
assign or otherwise to alienate the right to receive payments under the plan, in
whole or in part.







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                                   SECTION 11.

                            AMENDMENT OR TERMINATION


Biogen reserves the right at any time by action of the board to terminate this
plan or to amend its provisions in any way. In addition, if the Retirement Plan
is terminated, this plan will automatically terminate also as of the same
effective date. Notwithstanding the foregoing, no termination or amendment of
the plan may reduce the benefits payable under the plan to any person with
respect to a participant whose employment with Biogen (or a subsidiary) was
terminated before such termination or amendment, and no termination or amendment
may reduce the benefits to be paid with respect to a participant on the date of
such termination or amendment below the amount which such participant would have
received if his employment had terminated on the date before such termination or
amendment.


In witness whereof, Biogen, Inc. has caused this instrument to be restated and 
effective April 1, 1996 and to be executed as of the ______ day of ______, 1997.


                                                BIOGEN, INC.






                             By:
                                    --------------------------------------------
                                    Frank A. Burke, Jr. V.P., Human Resources





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                                   APPENDIX A

            LIST OF PARTICIPANTS IN THE SUPPLEMENTAL PENSION FORMULA


The following employees are participants in the supplemental pension formula
under Section 4.1, effective as of January 1, 1991:


                               Frank A. Burke, Jr.
                                John W. Catterall
                             Adrian W. Dawson, M.D.
                             Frederic A. Eustis, III
                                  James Mullen
                              Vicki L. Sato, Ph.D.
                                Michael R. Slater
                                  Alan W. Tuck
                                James L. Vincent




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                                   APPENDIX B

         SPECIAL ADDITIONAL BENEFITS APPLICABLE TO CERTAIN PARTICIPANTS


B.1   Special Additional Benefits for James L. Vincent

      (a)  APPLICABILITY. This Section B.1 provides for special additional
           benefits for James L. Vincent, Chairman and Chief Executive Officer
           of Biogen, Inc. ("Vincent"), in order to carry out the provisions of
           Paragraph 6 of the letter agreement dated November 21, 1996 from
           Biogen, Inc. to Vincent (the "Letter Agreement"). Capitalized or
           other terms used in this Section B.1 not otherwise defined by or in
           accordance with this plan have the meanings given to them in such
           Letter Agreement or in the Employment Agreement referred to in the
           first paragraph of the Letter Agreement.

      (b)  SPECIAL ADDITIONAL BENEFITS. In addition to the benefits payable to
           Vincent under the other provisions of this plan, Vincent will be
           entitled to benefits hereunder calculated in accordance with the
           following provisions:

           (i)   during the period following the Change Date while Vincent
                 continues to serve as Chairman (including during any period
                 while Vincent has a condition or other disability for which
                 Vincent is entitled to payment of disability benefits for a
                 "covered disability", as defined in the Biogen Retirement
                 Plan), Vincent will continue to accrue additional retirement
                 benefits calculated under the supplemental pension formula in
                 Section 4.1, treating Vincent's service as Chairman as if it
                 were full-time service as an eligible employee and treating the
                 salary and bonus paid to Vincent for service as Chairman (which
                 during any period of "covered disability" shall be deemed to be
                 the salary and bonus for the year immediately preceding such
                 period) as Vincent's "applicable compensation" for purposes of
                 the supplemental pension formula;

           (ii)  in the event of a termination of Vincent's employment, before
                 or after the Change Date, (I) by Biogen, other than for cause
                 or permanent total disability, (II) by Vincent, under
                 circumstances constituting Constructive Termination, or (III)
                 by Vincent after Vincent has reached age 61, in each case
                 before Vincent has reached age 65, Vincent will receive
                 additional retirement benefits calculated under the
                 supplemental pension formula as if Vincent has continued to
                 serve as Chairman until age 65 and had continued to receive the
                 same compensation as the annual rate of compensation in effect
                 for Vincent immediately prior to such termination of
                 employment; and

           (iii) in the event of a termination of Vincent's employment as
                 Chairman before age 65, under circumstances entitling Vincent
                 to benefit payments under subparagraph (i) or (ii), Vincent may
                 thereafter elect to commence receiving such benefit payments
                 under either clause (I) or (II) below:

                 (I)  in the event Vincent's employment is terminated under
                      circumstances entitling Vincent to a payment under
                      paragraph 11(c)(v) of the Letter Agreement, starting on
                      the earlier of:



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                      (A) Vincent's normal retirement date, or

                      (B) the date which follows the date of termination of
                          Vincent's employment by a number of months equal to
                          the Severance Multiple (as defined in paragraph
                          11(c)(v) of the Letter Agreement) multiplied by 12,

                 (II) in the event Vincent's employment is terminated under
                      circumstances not entitling Vincent to a payment under
                      paragraph 11(c)(v) of the Letter Agreement, starting on
                      the date of such termination.

                 In any event, benefit payments under either clause (I) or (II)
                 above will not be reduced for payments received before
                 Vincent's normal retirement date.

                 For example, if the Change Date occurs on Vincent's 57th
                 birthday, and Vincent's employment is terminated by Biogen on
                 Vincent's 58th birthday under circumstances entitling Vincent
                 to a payment under paragraph 11(c)(v) of the Letter Agreement,
                 then Vincent's Severance Multiple would be 5.5 which,
                 multiplied by 12, results in a period of 66 months. Therefore,
                 in this example, pursuant to subparagraph (iii)(I)(B), Vincent
                 could elect to commence receiving benefit payments at the time
                 Vincent reaches age 63 1/2 (which is 66 months after the
                 assumed date of termination of Vincent's employment at age 58)
                 without reduction for payments received before Vincent's normal
                 retirement date.

           The rules governing the presumed form of payment for calculations of
           benefits amounts under the foregoing provisions of this paragraph,
           and the rules governing the conversion of actual benefits into any
           other form of payment elected by Vincent, will be the same rules used
           for such purposes under the Biogen Retirement Plan and under the
           provisions of this plan.

           Vincent's gross retirement benefit amount will be the amount
           calculated under this Section B.1 and the actual amount payable to
           Vincent under this Section B.1 will be such amount reduced by the
           amounts Vincent actually receives under the other provisions of this
           plan and the Biogen Retirement Plan.





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