BIOGEN INC
S-8, 1997-12-22
BIOLOGICAL PRODUCTS, (NO DIAGNOSTIC SUBSTANCES)
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<PAGE>   1

    As filed with the Securities and Exchange Commission on December 22, 1997

                                                     REGISTRATION NO. 333 -

================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

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

                                    FORM S-8
                             REGISTRATION STATEMENT
                                    UNDER THE
                             SECURITIES ACT OF 1933

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

                                  BIOGEN, INC.
             (Exact name of Registrant as specified in its charter)

                 MASSACHUSETTS                              04-3002117
         (State or other jurisdiction                    (I.R.S. Employer
       of incorporation or organization)                Identification No.)

                               14 CAMBRIDGE CENTER
                               CAMBRIDGE, MA 02142
                    (Address of Principal Executive Offices)

                               BIOGEN SAVINGS PLAN
                            (Full title of the plan)

                             MICHAEL J. ASTRUE, ESQ.
                         VICE PRESIDENT-GENERAL COUNSEL
                                  BIOGEN, INC.
                               14 CAMBRIDGE CENTER
                               CAMBRIDGE, MA 02142
                  (Name and address code of agent for service)

                                 (617) 679-2000
          (Telephone number, including area code, of agent for service)

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


                         CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
===================================================================================================================
                                                         Proposed              Proposed
          Title of                Amount to be            maximum               maximum               Amount of
 securities to be registered      registered(1)        offering price           aggregate          registration fee
                                                        per share(2)        offering price(2)
- -------------------------------------------------------------------------------------------------------------------

<S>           <C>                   <C>                    <C>                 <C>                    <C>      
Common Stock, $.01 par value        200,000                $35.50              $7,100,000             $2,094.50
===================================================================================================================
</TABLE>

(1) Represents (i) 97,797 shares of common stock, par value $0.01 per share (the
    "Common Stock"), of Biogen, Inc. held by the Biogen Savings Plan, as amended
    (the "Plan"), on December 12, 1997 and (ii) 102,203 additional shares which
    may be required by purchase transactions under the Plan. In addition,
    pursuant to Rule 416(c) under the Securities Act of 1933, as amended (the
    "Securities Act"), this Registration Statement also covers an indeterminate
    amount of interests to be offered or sold pursuant to the Plan.

(2) Estimated solely for the purpose of determining the registration fee
    pursuant to Rule 457(h) under the Securities Act, based on the average of
    the high and low prices for shares of the Common Stock reported on the
    National Market System of the National Association of Securities Dealers
    Automated Quotation System (NASDAQ) on December 17, 1997, a date within five
    business days prior to the date of filing this Registration Statement.

================================================================================

<PAGE>   2



                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

     ITEM 1.  PLAN INFORMATION.

         The document(s) containing the information specified in this Part I
     will be sent or given to employees as specified by Rule 428(b)(1) under the
     Securities Act. Such documents need not be, and are not, filed with the
     Securities and Exchange Commission (the "Commission") either as part of
     this Registration Statement or as prospectuses or prospectus supplements
     pursuant to Rule 424 under the Securities Act. These documents and the
     documents incorporated by reference in this Registration Statement pursuant
     to Item 3 of Part II of this Form S-8, taken together, constitute a
     prospectus that meets the requirements of Section 10(a) of the Securities
     Act.

     ITEM 2.  REGISTRANT INFORMATION AND EMPLOYEE PLAN ANNUAL INFORMATION.

         Upon written or oral request, any of the documents incorporated by
     reference in Item 3 of Part II of this Registration Statement (which
     documents are incorporated by reference into the Section 10(a) prospectus)
     and any other documents required to be delivered to employees pursuant to
     Rule 428(b) under the Securities Act are available without charge. Requests
     for documents should be directed to Kelly Ashton, Manager of Employee
     Benefits, Biogen, Inc., 14 Cambridge Center, Cambridge, MA 02142, telephone
     number: (617) 679-2849.


                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT


     ITEM 3.  INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE.

         The following documents, which have been filed by the Registrant or the
     Plan with the Commission, are incorporated in this Registration Statement
     by reference:

         (a)      The Registrant's Annual Report on Form 10-K for the year ended
                  December 31,1996.

         (b)      The Registrant's Quarterly Reports on Form 10-Q for the
                  quarters ended March 31, 1997, June 30, 1997 and September 30,
                  1997.

         (c)      The description of the Common Stock contained in the
                  Registrant's Registration Statement on Form 8-B (File No.
                  0-12042) filed under the Securities Exchange Act of 1934 as
                  amended (the "Exchange Act"), including any amendment or
                  report filed for the purpose of updating such description.

          All reports and other documents filed by the Registrant or the Plan
     after the date hereof pursuant to Sections 13(a), 13(c), 14 and 15(d) of
     the Exchange Act prior to the filing of a post-effective amendment which
     indicates that all securities offered hereby have been sold or which
     deregisters all securities then remaining unsold, shall be deemed to be
     incorporated by reference in this Registration Statement and to be part
     hereof from the date of filing of such reports and documents.

           Any statement contained in a document incorporated or deemed to be
     incorporated by reference in this Registration Statement shall be deemed to
     be modified or superseded for purposes of this Registration Statement to
     the extent that a statement contained herein or in any other subsequently
     filed document which also is 



<PAGE>   3

     incorporated or deemed to be incorporated by reference in this Registration
     Statement modifies or supersedes such statement. Any statement so modified
     or superseded shall not be deemed, except as so modified or superseded, to
     constitute a part of this Registration Statement.

     ITEM 4.  DESCRIPTION OF SECURITIES.

           Not applicable.

     ITEM 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL.

         The validity of the issuance of the shares of Common Stock registered
     under this Registration Statement has been passed upon for the Registrant
     by Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. of Boston,
     Massachusetts. Members of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo,
     P.C. and certain members of their families and trusts for their benefit own
     an aggregate of approximately 3,520 shares of the Common Stock.


     ITEM 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.

         Incorporated herein by reference from Registration Statement on 
     Form S-3, No. 33-43721.

     ITEM 7.  EXEMPTION FROM REGISTRATION CLAIMED.

           Not applicable.

     ITEM 8.  EXHIBITS.

         (4.1)    Form of Common Stock Certificate (Filed as Exhibit 4.1 to
                  Registration Statement on Form S-3, No. 33-51639, and
                  incorporated herein by reference).

         (4.2)    Articles of Organization (Filed as Exhibit 3.1 to Annual
                  Report on Form 10-K for the year ended December 31, 1996, File
                  No. 0-12042, and incorporated herein by reference).

         (4.3)    By-Laws, as amended (Filed as Exhibit 3.2 to Annual Report on
                  Form 10-K for the year ended December 31, 1993, File No.
                  0-12042, and incorporated herein by reference).

         (4.4)    Rights Agreement, dated as of May 8, 1989, between the
                  Registrant and First National Bank of Boston, as the Rights
                  Agent, including Certificate of Designation of Series A Junior
                  Participating Preferred Stock (Filed as Exhibit 1 to
                  Registration Statement on Form 8-A, File No. 0-12042, filed
                  May 26, 1989, and incorporated herein by reference).

         (4.5)    Biogen Savings Plan, as amended.

         (4.6)    Trust Agreement, dated, April 1, 1994, between Registrant and
                  Fidelity Management Trust Company for the Biogen Savings Plan,
                  as amended.

         (5)      Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, PC
                  as to the legality of shares being registered.

         (23.1)   Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, PC
                  (included in opinion of counsel filed as Exhibit 5).


<PAGE>   4

         (23.2)   Consent of Price Waterhouse LLP.

         (24)     Power of Attorney to file future amendments (set forth on the
                  signature page of this Registration Statement).

     The Registrant hereby undertakes to submit or has submitted the Plan and
     any amendments thereto to the Internal Revenue Service ("IRS") in a timely
     manner, and undertakes to make or has made all changes required by the IRS
     in order to qualify the Plan.

     ITEM 9.  UNDERTAKINGS.

          (a)     The undersigned Registrant hereby undertakes:

                  (1)      To file, during any period in which offers or sales
                           are being made, a post-effective amendment to this
                           Registration Statement:

                           (i)      To include any prospectus required by
                                    Section 10(a)(3) of the Securities Act of
                                    1933;

                           (ii)     To reflect in the prospectus any facts or
                                    events arising after the effective date of
                                    this Registration Statement (or the most
                                    recent post-effective amendment thereof)
                                    which, individually or in the aggregate,
                                    represents a fundamental change in the
                                    information set forth in this Registration;

                           (iii)    To include any material information with
                                    respect to the plan of distribution not
                                    previously disclosed in this Registration
                                    Statement or any material change to such
                                    information in this Registration Statement;

                           Provided, however, that paragraphs (a)(1)(i) and
                           (a)(1)(ii) do not apply if this Registration
                           Statement is on Form S-3 or Form S-8, and the
                           information required to be included in a
                           post-effective amendment by those paragraphs is
                           contained in periodic reports filed by the Registrant
                           pursuant to Section 13 or Section 15(d) of the
                           Securities Exchange Act of 1934 that are incorporated
                           by reference in this Registration Statement.

                  (2)      That, for the purpose of determining any liability
                           under the Securities Act of 1933, each post-effective
                           amendment shall be deemed to be a registration
                           statement relating to the securities offered therein,
                           and the offering of such securities at that time
                           shall be deemed to be the initial bona fide offering
                           thereof.

                  (3)      To remove from registration by means of a
                           post-effective amendment any of the securities being
                           registered which remain unsold at the termination of
                           the offering.

          (b)     The undersigned Registrant hereby undertakes that, for
                  purposes of determining any liability under the Securities Act
                  of 1933, each filing of the Registrant's annual report
                  pursuant to Section 13(a) or Section 15(d) of the Securities
                  Exchange Act of 1934 (and, where applicable, each filing of an
                  employee benefit plan's annual report pursuant to Section
                  15(d) of the Securities Exchange Act of 1934) that is
                  incorporated by reference in this Registration Statement shall
                  be deemed to be a new registration statement relating to the
                  securities offered therein, and the offering of such
                  securities at that time shall be deemed to be the initial bona
                  fide offering thereof.


<PAGE>   5

         (c)      Insofar as indemnification for liabilities arising under the
                  Securities Act of 1933 may be permitted to directors, officers
                  and controlling persons of the Registrant pursuant to the
                  foregoing provisions, or Commission such indemnification is
                  against public policy as expressed in the Act and is,
                  therefore, unenforceable. In the event that a claim for
                  indemnification against such liabilities (other than the
                  payment by the Registrant of expenses incurred or paid by a
                  director, officer or controlling person of the Registrant in
                  the successful defense of any action, suit or proceeding) is
                  asserted by such director, officer or controlling person in
                  connection with the securities being registered, the
                  Registrant will, unless in the opinion of its counsel the
                  matter has been settled by controlling precedent, submit to a
                  court of appropriate jurisdiction the question whether such
                  indemnification by it is against public policy as expressed in
                  the Act and will be governed by the final adjudication of such
                  issue.



                                   SIGNATURES

          The Registrant. Pursuant to the requirements of the Securities Act of
     1933, the Registrant certifies that it has reasonable grounds to believe
     that it meets all of the requirements for filing on Form S-8 and has duly
     caused this Registration Statement to be signed on its behalf by the
     undersigned, thereunto duly authorized, in the City of Cambridge,
     Commonwealth of Massachusetts, on December 17, 1997.


                                      BIOGEN, INC.



                                      By: /s/ James R. Tobin
                                          -------------------------------------
                                          James R. Tobin
                                          President and Chief Executive Officer


<PAGE>   6


         Each person whose signature appears below constitutes and appoints
James R. Tobin, Michael J. Astrue and Timothy M. Kish, and each of them, his
true and lawful attorneys-in-fact and agents, with full power of substitution
and resubstitution in each of them, for him and in his name, place and stead,
and in any and all capacities, to sign any and all amendments (including
post-effective amendments) to this Registration Statement, and to file the same,
with all exhibits thereto and other documents in connection therewith, with the
Securities and Exchange Commission, granting unto said attorneys-in-fact and
agents, and each of them, full power and authority to do and perform each and
every act and thing requisite or necessary to be done in or about the premises,
as fully to all intents and purposes as he might or could do in person, hereby
ratifying and confirming all that said attorneys-in-fact and agents or any of
them or their or his substitute or substitutes may lawfully do or cause to be
done by virtue hereof.

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the date indicated.

<TABLE>
<CAPTION>
Signature                           Title                               Date
- ---------                           -----                               ----

                                    
<S>                                 <C>                                 <C> 
/s/ James R. Tobin                  President, Chief  Executive         December 17, 1997
- -------------------------           Officer and Director 
James R. Tobin                      (principal executive 
                                    officer)             


/s/ James L. Vincent                Chairman and Director               December 17, 1997
- -------------------------
James L. Vincent


/s/ Timothy M. Kish                 Vice President-Finance and          December 17, 1997
- -------------------------           Chief Financial Officer 
Timothy M. Kish                     (principal financial and
                                    accounting officer)     


/s/ Alexander G. Bearn              Director                            December 12, 1997
- -------------------------
Alexander G. Bearn


/s/ Alan Belzer                     Director                            December 12, 1997
- -------------------------
Alan Belzer



/s/ Harold W. Buirkle               Director                            December 12, 1997
- -------------------------
Harold W. Buirkle
</TABLE>



<PAGE>   7

<TABLE>
<S>                                 <C>                                 <C> 
/s/ Mary L. Good                    Director                            December 12, 1997
- -------------------------
Mary L. Good


/s/ Thomas F. Keller                Director                            December 12, 1997
- -------------------------
Thomas F. Keller


/s/ Roger H. Morley                 Director                            December 12, 1997
- -------------------------
Roger H. Morley


/s/ Kenneth Murray                  Director                            December 17, 1997
- -------------------------
Kenneth Murray


/s/ Phillip A. Sharp                Director                            December 17, 1997
- -------------------------
Phillip A. Sharp


/s/ James W. Stevens                Director                            December 12, 1997
- -------------------------
James W. Stevens
</TABLE>


     The Plan. Pursuant to the requirements of the Securities Act of 1933, the
plan administrator has duly caused this Registration Statement to be signed on
its behalf by the undersigned, thereunto duly authorized, in the City of
Cambridge, Commonwealth of Massachusetts on December 12, 1997.

                                             BIOGEN SAVINGS PLAN
                                             ADMINISTRATOR


                                             By: /s/ Frank A. Burke, Jr.
                                                 ------------------------------
                                                 Frank A. Burke, Jr.
                                                 Vice President-Human Resources


<PAGE>   8



                                  BIOGEN, INC.

                          INDEX TO EXHIBITS FILED WITH
                         FORM S-8 REGISTRATION STATEMENT

Exhibit
Number            Description
- -------           -----------

4.5               Biogen Savings Plan, as amended.

4.6               Trust Agreement, dated April 1, 1994, between Registrant and
                  Fidelity Management Trust Company for the Biogen Savings Plan,
                  as amended.

5                 Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, PC
                  as to the legality of shares being registered.

23.1              Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, PC
                  (included in opinion of counsel filed as Exhibit 5).

23.2              Consent of Price Waterhouse LLP.

24                Power of Attorney to file future amendments (set forth on the
                  signature page of this Registration Statement).










<PAGE>   1


                                                                     Exhibit 4.5






                               BIOGEN SAVINGS PLAN












                         As amended and restated by the
                                Eighth Amendment






<PAGE>   2




                               BIOGEN SAVINGS PLAN


                                Table of Contents

                                                                          Page
                                                                          ----


ARTICLE 1
                                 INTRODUCTION..............................  1
      1.1   Establishment of Plan..........................................  1
      1.2   Compliance with Code and ERISA.................................  1
      1.3   Exclusive Benefit of Participants..............................  1
      1.4   Limitation on Rights Created By Plan...........................  1
      1.5   Application of Plan's Terms....................................  1
      1.6   Benefits Not Guaranteed........................................  1

ARTICLE 2
                                  DEFINITIONS..............................  1
      2.1   Applicable compensation........................................  1
      2.2   Beneficiary....................................................  2
      2.3   Biogen.........................................................  2
      2.4   Code...........................................................  2
      2.5   Employee.......................................................  2
      2.6   Employer.......................................................  2
      2.7   ERISA..........................................................  2
      2.8   Participant....................................................  2
      2.9   Plan...........................................................  2
      2.10  Plan administrator.............................................  2
      2.11  Plan year......................................................  2
      2.12  Trust agreement................................................  2
      2.13  Trust fund.....................................................  2
      2.14  Trustee........................................................  2

ARTICLE 3
                                 PLAN SERVICE..............................  2
      3.1   Plan Service...................................................  2
      3.2   Employment Defined.............................................  3
      3.3   Service with Affiliated Companies..............................  3
      3.4   Affiliated Company Defined.....................................  3

ARTICLE 4
                                 PARTICIPATION.............................  3
      4.1   Eligible Class.................................................  3
      4.2   Participation..................................................  4
      4.3   End of Participation...........................................  4
      4.4   Reentry of Former Active Participant...........................  4

ARTICLE 5
                       SAVINGS DEPOSITS BY PARTICIPANTS....................  4


                                       i


<PAGE>   3

      5.1   Savings Deposits...............................................  4
      5.2   401(k) Limits..................................................  5
      5.3   Changes in Savings Deposits....................................  7
      5.4   Collection of Savings Deposits. ...............................  7
      5.5   Rollover Contributions.........................................  7

ARTICLE 6
                            EMPLOYER CONTRIBUTIONS.........................  8
      6.1   Matching Employer Contributions................................  8
      6.2   401(m) Limits..................................................  8
      6.3   Form and Time of Contribution..................................  9
      6.4   Biogen Stock Defined...........................................  9
      6.5   Return of Contribution Made in Error or Not Deductible.........  9

ARTICLE 7
                             ACCOUNTS AND CREDITS.......................... 10
      7.1   Establishment of Accounts...................................... 10
      7.2   Crediting Participants' Savings Deposits....................... 10
      7.3   Crediting Matching Contributions and Forfeitures............... 10
      7.4   Crediting Rollovers............................................ 10
      7.5   Charges to Accounts............................................ 10
      7.6   Maximum Additions.............................................. 10

ARTICLE 8
             INVESTMENT FUNDS AND CREDITING INVESTMENT EXPERIENCE.......... 11
      8.1   Investment Funds............................................... 11
      8.2   Investment Directions and Transfers Among Funds................ 12
      8.3   Valuation of Assets and Crediting Investment Experience........ 13

ARTICLE 9
                             LOANS TO PARTICIPANTS......................... 13
      9.1   Loans to Participants.......................................... 13
      9.2   Accounting for Loans........................................... 15

ARTICLE 10
                         WITHDRAWALS AND DISTRIBUTIONS..................... 15
      10.1  In-Service Withdrawals from Savings Deposits................... 15
      10.2  In-Service Withdrawals from Rollover Account................... 16
      10.3  Distribution Upon Retirement or Disability..................... 16
      10.4  Distribution Upon Termination of Employment.................... 17
      10.5  Right to Defer................................................. 17
      10.6  Distribution Upon Death of Participant......................... 17
      10.7  Manner of Payment.............................................. 18
      10.8  Direct Rollovers............................................... 18
      10.9  Rehire Before Distribution..................................... 19

ARTICLE 11
                   AMENDMENT, MERGER AND TERMINATION OF PLAN............... 19
      11.1  Amendment of Plan.............................................. 19
      11.2  Merger of Plans................................................ 19


                                       ii


<PAGE>   4

      11.3  Termination.................................................... 20
      11.4  Effect of Termination.......................................... 20

ARTICLE 12
                               NAMED FIDUCIARIES........................... 20
      12.1  Identity of Named Fiduciaries.................................. 20
      12.2  Responsibilities and Authority of Plan Administrator........... 20
      12.3  Responsibilities and Authority of Trustee...................... 20
      12.4  Responsibilities of Biogen..................................... 21
      12.5  Responsibilities Not Shared.................................... 21
      12.6  Dual Fiduciary Capacity Permitted.............................. 21
      12.7  Actions by Biogen.............................................. 21
      12.8  Procedure for Allocation and Delegation of Responsibilities.... 21
      12.9  Advice......................................................... 21
      12.10 Indemnification................................................ 21

ARTICLE 13
                            THE PLAN ADMINISTRATOR......................... 22
      13.1  Appointment.................................................... 22
      13.2  Notice to Trustee.............................................. 22
      13.3  Administration of Plan......................................... 22
      13.4  Reporting and Disclosure....................................... 22
      13.5  Records........................................................ 23
      13.6  Compensation and Expenses...................................... 23
      13.7  Decisions, Rules and Regulations............................... 23
      13.8  Secretary...................................................... 23
      13.9  Claims Review Procedure........................................ 23

ARTICLE 14
                                 MISCELLANEOUS............................. 24
      14.1  Qualified Domestic Relations Orders............................ 24
      14.2  Nonalienation of Benefits...................................... 24
      14.3  Payment to Minors and Incompetents............................. 24
      14.4  Current Address of Payee; Unclaimed Payments................... 24
      14.5  Disputes over Entitlement to Benefits.......................... 25
      14.6  Payment of Benefits............................................ 25
      14.7  Top-Heavy Plan Provisions...................................... 25
      14.8  Rules of Construction.......................................... 26
      14.9  Text Controls.................................................. 27
      14.10 Applicable State Law........................................... 27
      14.11 Paperless Administration....................................... 27
      14.12 Correction of Mistakes in Plan Operation....................... 27
      14.13 Veterans' Rights............................................... 27


                                      iii
<PAGE>   5


                                 INDEX OF TERMS


The items listed below are defined or explained in the plan sections or articles
                                   indicated.

Accounts..........................0      Matching contribution percentage
Affiliated company................0      ERROR REFERENCE SOURCE NOT FOUND.
Annual additions..................0      Matching employer contributions...0
Applicable compensation...........0      Maximum additions.................0
                                                                            
Basic savings deposits............0      Named fiduciaries.......Article 0,0
Beneficiary.......................0                                         
Biogen............................0      Participant.......................0
Biogen stock......................0      Participation.....................0
                                         Plan..............................0
Claim.............................0      Plan service......................0
Claimant..........................0      Plan year.........................0
Code..............................0                                         
CommitteeArticle 0, ERROR! REFERENCE     Qualified domestic relations       
SOURCE NOT FOUND.                         order............................0
                                                                            
Deferral percentage...............0      Retirement........................0
Defined benefit plan fraction.....0      Rollover contributions..........5.5
Defined contribution plan                                                   
 fraction.........................0      Savings deposits..................0
Direct rollovers..................0      Supplemental savings deposits.....0
Disability........................0                                         
                                         Top-heavy.........................0
Eligible class....................0      Total compensation................0
Employee..........................0      Trust agreement...................0
Employer..........................0      Trust fund........................0
Employment........................0      Trustee...........................0
Entry Date........................0                                         
ERISA.............................0      Valuation date....................0
                                         Vested interest...................0
Financial hardship................0                                         
                                         Withdrawals.....................0,0
Higher paid group.................0      

Limitation year...................0
Loans.............................0
Lower paid group..................0

                                    ARTICLE 1
                                  INTRODUCTION

  1.1    ESTABLISHMENT OF PLAN.  Biogen established this plan effective as of 
January 1, 1987. The plan has subsequently been amended as specified in 
Appendix A.

  1.2    COMPLIANCE WITH CODE AND ERISA. This plan is intended to qualify as a
profit-sharing plan under Section 401(a) of the Internal Revenue Code of 1986,
as amended from time to time, and a qualified cash or deferred 


                                       iv


<PAGE>   6

arrangement described in Section 401(k) of said Code, and to comply with the
applicable provisions of the Employee Retirement Income Security Act of 1974, as
amended from time to time. This plan is also intended to comply in all respects
with Rule 16b-3 or its successors promulgated under the Securities Exchange Act
of 1934 ("1934 Act") with respect to participants who are subject to Section 16
of the 1934 Act. The plan will be interpreted in a manner that comports with
these intentions.

  1.3    EXCLUSIVE BENEFIT OF PARTICIPANTS. The plan is for the exclusive 
benefit of participants and their beneficiaries. Employer and employee
contributions are made to the trust fund for the purpose of accumulating a fund
for distribution to participants and their beneficiaries in accordance with the
plan. Except as provided in Section 0, no part of the trust fund or any
distribution therefrom will be used for or diverted to purposes other than for
the exclusive benefit of participants and their beneficiaries and defraying
those reasonable expenses of administering the plan and trust fund not paid by
the employers.

  1.4    LIMITATION ON RIGHTS CREATED BY PLAN. Nothing appearing in the plan 
will be construed (a) to give any person any benefit, right or interest except
as expressly provided herein, or (b) to create a contract of employment or to
give any employee the right to continue as an employee or to affect or modify
his terms of employment in any way.

  1.5    APPLICATION OF PLAN'S TERMS. The benefits and rights of a participant 
and his beneficiaries under the plan will be determined in accordance with the
terms of the plan that are in effect on the date that contributions on a
participant's behalf are made or credited to his accounts, or on the date of the
participant's retirement, death or other termination of employment, whichever
may be applicable.

  1.6    BENEFITS NOT GUARANTEED. The employers, the trustee and the plan 
administrator do not guarantee the payment of benefits hereunder. Benefits will
be paid only from the assets of the trust fund and are limited to the amount of
assets therein.

                                    ARTICLE 2
                                   DEFINITIONS

         This article contains a number of definitions of terms used in the
plan. Other terms are defined, explained or clarified in other articles. This is
done for convenience of plan administration. There is no other significance to
the location of a definition.

  2.1    APPLICABLE COMPENSATION of an employee for any calendar year or other
period of reference means his total compensation for services paid by his
employer during such period including wages, salary, overtime, cash bonuses and
shift differential. Except for purposes of Section 0, applicable compensation
also includes contributions made by his employer to this plan or any other
employee benefit program on behalf of the employee in accordance with a salary
reduction agreement under Code Section 125 or 401(k) with the employee. However,
applicable compensation does not include reimbursed expenses, life insurance
premiums included in his compensation for income tax purposes, stock options or
stock bonuses, or any other items not constituting direct compensation for
services. Compensation also does not include payments to or benefits received
under this or any other public or private employee benefit plan (other than
salary reduction contributions under this plan or other employee benefit
program).
The amount of applicable compensation taken into account under the plan for any
participant in a particular plan year will not exceed $150,000 (as adjusted in
accordance with Code Section 401(a)(17)).

  2.2    BENEFICIARY means a person, class of persons or trust designated by a 
participant or, if there is no such designation, by the plan to receive a death
benefit hereunder.

  2.3    BIOGEN means Biogen, Inc. (a Massachusetts corporation) or its 
successor.



                                       2


<PAGE>   7

  2.4    CODE means the Internal Revenue Code of 1986, as amended from time to 
time, or any successor statute enacted in its place.

  2.5    EMPLOYEE means an individual classified as an employee under the 
regular personnel classifications and practices of his employer. An individual
is not an employee for purposes of this plan if the individual is classified as
a consultant or contractor under his employer's regular personnel
classifications and practices, or he is a party to an agreement to provide
services to his employer without participating in this plan, notwithstanding
that such individual may be treated as a common law employee for payroll tax or
other legal purposes.

  2.6    EMPLOYER means Biogen, Inc. or any successor organization to it, or any
other entity that adopts the plan for its employees with the consent of Biogen,
Inc. upon such terms and conditions as Biogen, Inc. determines.

  2.7    ERISA means the Employee Retirement Income Security Act of 1974, as 
amended from time to time, or any successor statute enacted in its place.

  2.8    PARTICIPANT means an employee or former employee whose participation in
the plan has begun and has not yet ended.

  2.9    PLAN means the Biogen Savings Plan, as set forth in this plan 
instrument, and as it may be amended from time to time.

  2.10   PLAN ADMINISTRATOR means the person, committee or entity appointed by 
Biogen or otherwise designated under Article 13 to administer the plan.

  2.11   PLAN YEAR means the 12-month period beginning on each January 1 during 
the continuance of the plan.

  2.12   TRUST AGREEMENT means the instrument executed by Biogen and the 
trustee, as amended from time to time, fixing the rights and responsibilities of
each party with respect to the holding, investment and administration of the
trust fund.

  2.13   TRUST FUND means the property held by the trustee for the purposes of 
the plan.

  2.14   TRUSTEE means the person, individual or corporate, serving as sole 
trustee, or the persons serving as co-trustees, at any time under the terms of
the trust agreement.

                                    ARTICLE 3
                                  PLAN SERVICE

  3.1    PLAN SERVICE. Plan service of an employee means the sum of the 
following: (a) any period of his employment on and after January 1, 1987,
whether or not continuous; and (b) each period, if any, between a termination of
his employment on and after January 1, 1987, and his earliest subsequent
reemployment, but only if such reemployment occurs within one year after such
termination of employment.

         To determine an employee's years of plan service, all periods of his
plan service will be aggregated, and each 365 days thereof will be one year of
plan service.

  3.2    EMPLOYMENT DEFINED.



                                       3


<PAGE>   8

         (a)   ACTIVE SERVICE. Employment of a person means his active service
as an employee (determined under the regular personnel policies, practices and
classifications of his employer).


         (b)   CERTAIN ABSENCES. A period of absence from active service will be
considered part of an employee's employment if he receives compensation from his
employer for such period or if such period falls in one of the following
categories (whether or not he receives compensation for such period):


               (i)    absence for military service for which his reemployment 
rights are protected by law; provided (but only for that part of the absence
which exceeds one year in length) he returns to active service as an employee
within the period when his reemployment rights are protected by law (or within
such longer period as his employer in its discretion permits); and

               (ii)   leave of absence due to sickness, accident, disability or 
other reason, for the period authorized by the employer, provided (but only for
that part of such leave of absence which exceeds one year in length) he returns
to active service with his employer at the end of such period of authorized
absence.

  3.3    SERVICE WITH AFFILIATED COMPANIES. An employee's plan service with an 
affiliated company will be counted for purposes of this plan.

  3.4    AFFILIATED COMPANY DEFINED. Affiliated company means any corporation
(foreign or domestic) which is a member of a controlled group of corporations
(as defined in Code Section 414(b)) which includes the employer; any trade or
business (whether or not incorporated) which is under common control (as defined
in Code Section 414(c)) with the employer; any organization (whether or not
incorporated) which is a member of an affiliated service group (as defined in
Code Section 414(m)) which includes the employer; and any other entity required
to be aggregated with the employer pursuant to regulations under Code 
Section 414(o).

                                    ARTICLE 4
                                  PARTICIPATION

  4.1    ELIGIBLE CLASS.

         (a)   GENERAL RULE. Each REGULAR employee (DETERMINED UNDER THE REGULAR
PERSONNEL POLICIES, PRACTICES AND CLASSIFICATIONS OF THE EMPLOYER) is in the
class eligible to participate in the plan unless he is (a) a student (whether
high school, college or graduate) employed on a temporary or casual basis, (b)
except as provided iin subsection (b) below, an employee of a non-U.S.
subsidiary or division of an employer or (c) a member of a collective bargaining
unit.

         (b)   ELIGIBILITY OF EMPLOYEES ON TEMPORARY FOREIGN ASSIGNMENT. An
otherwise eligible employee (under subsection (a) above who is temporarily
assigned by the employer to work for the employer, an affiliated company, or on
a joint venture with the employer outside of the United States with the intent
on teh part of the employer that such employee will return to work as an
eligible employee of the employer in the United States will be treated as an
employee in the class eligible to participate in the plan during such temporary
assignment. Accordingly, such employee will be eligible to make savings deposits
in accordance with the plan and to have employer matching contributions made
with respect to his basic savings deposits as provided in the plan (provided,
however, that any matching employer contributions that would otherwise be made
while the eligible employee is on temporary foreign assignment will be reduced
but not below zero -- to reflect any vested employer contributions on behalf of
such employee to a defined contribution program in which such employee
participates as a result of his temporary foreign assignment.




                                       4

<PAGE>   9

  4.2    PARTICIPATION. Participation in the plan is voluntary and no employee 
will be required to participate. Each employee in the eligible class who has
attained age 21 will be eligible to make savings deposits starting effective as
of any entry date. The plan administrator, pursuant to its authority under
Article 13, may adopt rules and procedures regarding the exact date when savings
deposits will commence and what forms, if any, participants must complete and
file with the plan administrator as prerequisites for such savings deposits to
commence. Each employee who has met the eligibility requirements of this section
will become a participant when a savings deposit by such employee is first
credited to his account.

         (a)   Each January 1, April 1, July 1, and October 1 is an entry date.

  4.3    END OF PARTICIPATION. A participant's active participation in the plan 
will end upon the suspension of his savings deposits under Section 0 or the
termination of his service as an employee in the eligible class for any reason.
His participation will end when he has no further interest under the plan.

  4.4    REENTRY OF FORMER ACTIVE PARTICIPANT. A former active participant who 
suspends his savings deposits may resume active participation under Section 5.3.
A former active participant who terminates his service as an employee in the
eligible class and who later returns to service as an employee in the eligible
class will be eligible to make savings deposits which, subject to the applicable
rules and procedures of the plan administrator, may start EFFECTIVE AS OF HIS
DATE OF REHIRE OR any subsequent entry date. He will be an active participant
again when a savings deposit by such employee is first credited to his account.


                                    ARTICLE 5
                        SAVINGS DEPOSITS BY PARTICIPANTS

  5.1    SAVINGS DEPOSITS.

         (a)   SAVINGS DEPOSITS BY PARTICIPANTS. Subject to the various 
limitations imposed by the Plan, each employee described in Section 0 may make
savings deposits to the plan in a whole percentage of his applicable
compensation. Reasonable minimum or maximum percentages by which participants
(or a particular class of participants such as highly-compensated participants)
may reduce applicable compensation may be established by the plan administrator.
An eligible employee may reduce his applicable compensation by such amount as he
elects, provided that his savings deposits do not exceed the limits of Section 0
or result in a violation of the limitations of Section 0 or violate any rules of
the plan administrator. Such amounts are referred to herein as the participant's
SAVINGS DEPOSITS.

               A participant's savings deposits up to six percent of his
applicable compensation are referred to as his BASIC SAVINGS DEPOSITS and any
savings deposits above this amount are referred to as his SUPPLEMENTAL SAVINGS
DEPOSITS.

         (b)   SIGN-UP PROCEDURE FOR SAVINGS DEPOSITS.  The plan administrator, 
pursuant to Article 13, may adopt procedures and forms which participants must
complete and file on a timely basis, as determined by the plan administrator, as
prerequisites to commence savings deposits. The plan administrator may prescribe
any and all notice requirements, deadlines, time periods, and election rules as
it deems appropriate to administer participant sign-up requirements.
Participants will be provided information regarding all sign-up requirements and
procedures by the plan administrator.

         (c)   CHARACTER OF SAVINGS DEPOSITS. For purposes of Section 414(h) of 
the Code, it is specifically provided that savings deposits are employer
contributions.



                                       5


<PAGE>   10

  5.2    401(k) LIMITS.

         (a)   LIMITATIONS. A participant's savings deposits for a plan year may
not exceed the limits of subsection (b) below or result in a violation of
Section 0. Also, a participant's savings deposits (when combined with any other
elective deferrals - as defined in regulations under Code Section 402(g) - under
any other plan or arrangement maintained by Biogen or any employer in a
controlled group with Biogen under Code Section 414(b) or (c), an affiliated
service group with Biogen under Code Section 414(m) or otherwise aggregated with
Biogen under Code Section 414(o)) in any calendar year may not exceed $9,500 (as
adjusted periodically in accordance with Code Section 402(g)).

         (b)   HIGH-LOW TESTS.

               Savings deposits by highly-compensated employees must not exceed
amounts such that, as of the last day of each plan year, the average of the
individual deferral percentages of the highly-compensated employees (the
"highly-compensated deferral percentage" or "HDP") for such plan year does not
exceed the average of the individual deferral percentages of lower-compensated
employees (the "lower-compensated deferral percentage" or "LDP" for the
preceding plan year by more than the amount specified under the following table.
Notwithstanding the preceding sentence, if elected by Biogen (which election,
once made, may not be changed without the approval of the Secretary of the
Treasury), the maximum HDP for a plan year may be determined with reference to
the LDP for that plan year (rather than the preceding plan year).

                  If LDP is:                                HDP may not exceed:
                  ----------                                -------------------

                  Less than 2%                              2 times LDP

                  At least 2% but                           2% more than LDP
                  less than 8%

                  8% or higher                              1.25 times LDP

Compliance with the foregoing requirement will be determined after taking into
account any adjustments made to comply with the Code Section 415 limits under
Section 7.6 below.

         (c)   DEFINITIONS. For purposes of subsection (b) above, the following 
definitions will apply.

               (i)    The DEFERRAL PERCENTAGE of a participant for a plan year 
means his basic and supplemental savings deposits for such year computed as a
percentage of his applicable compensation for such year. If an employee is
eligible to be a participant under Section 0 but has not elected to make savings
deposits, he will nevertheless be taken into account and his deferral percentage
will be zero. In the plan administrator's discretion, savings deposits of a
participant in the lower paid group will not be included when determining his
deferral percentage to the extent that the requirements of (b) above are met
without taking such contributions into account.

               (ii)   HIGHLY-COMPENSATED EMPLOYEES DEFINED. An employee is 
considered a HIGHLY-COMPENSATED EMPLOYEE for a plan year if he or she:

                      (A) owns (or is considered to own within the meaning of
Code Section 318) more than 5%of the outstanding stock of an employer at any
time during the plan year or the preceding plan year;

                      (B) receives annual compensation from the employer in
excess of $80,000 (before any reductions in his compensation in accordance with
a salary reduction agreement) during the preceding plan year.



                                       6


<PAGE>   11

                      However, at the election of the plan administrator, an
employee will be considered a highly-compensated employee under subsection (B)
above only if he or she also was a member of the top-paid group of employees for
the preceding plan year. If the plan administrator so elects, the "top-paid
group of employees" is the group consisting of the highest-paid 20% of employees
of the employer for such preceding plan year. In determining the number of
employees in the top-paid group, employees specified in Code Section 414(q)(5)
will be excluded. The $80,000 amount in (B) above will be adjusted for
cost-of-living increases in accordance with Code Section 414(q).

               (iii)  If an eligible employee under Section 0 is not a 
highly-compensated employee for a plan year, then he is a lower-compensated
employee for such year.

         (d)   MONITORING AND ADJUSTING EMPLOYEE SAVINGS DEPOSITS . The plan
administrator will monitor (or will arrange with the employer or a recordkeeper
or service provider to monitor) participants' savings deposits elections and
deferral percentages and will adjust the amount of savings deposits elected or
made by highly-compensated employees to the extent necessary to satisfy the
requirements of subsection (b) above. Any such adjustments may be made during a
plan year in accordance with the plan administrator's procedures. If following
the end of a plan year such requirements are not satisfied, the plan
administrator will determine the maximum permitted HDP and will determine the
amount needed to reduce the deferral percentage of each highly compensated
employee whose deferral percentage is above such maximum to the maximum. After
the aggregate dollar amount of all such reductions in the amount of savings
deposits for a plan year by highly-compensated employees to satisfy such
subsection has been determined, the necessary reductions will be accomplished by
reducing the savings deposits of the participant (or participants) in the
highly-compensated group with the highest dollar amount of savings deposits
first, next the participant (or participants) in such group with the second
highest dollar amount of savings deposits, and so on until adjustments equal to
such aggregate dollar amount have been made.

         Such adjustments OR REDUCTIONS will be made by the plan administrator 
at such time or times as the plan administrator determines, and will be made in
accordance with procedures established by the plan administrator. If savings
deposits by a highly-compensated employee for a plan year exceed the amount
permitted under subsection (b) above, the necessary reduction amount plus
earnings on such amount (as determined in accordance with applicable regulations
under Code Section 401(k) but without any earnings for periods after the end of
the relevant plan year) will be paid to the affected participant. Such payment
will be made no later than the end of the succeeding plan year.

The amount of earnings (or losses) to be distributed with a participant's excess
savings deposits will be determined under Section 8.3 with the following special
rules: (i) payments to correct excess savings deposits will be paid from each
separate investment fund under Section 8.1 in which the participant's
supplemental and (if necessary) basic savings deposits accounts are invested in
accordance with the plan's ordering rules for withdrawals (or pro rata from each
investment fund if there are no ordering rules); and (ii) the amount of earnings
(or losses) will be the earnings or losses of such investment fund for the plan
year of the correcting distribution multiplied by a fraction whose numerator is
the amount of excess savings deposits being distributed and whose denominator is
the total of (i) the balance in such investment fund(s) at the beginning of such
plan year plus (ii) the total of the contributions allocated to such investment
fund(s) during such year.

         (e)   415 LIMITS. In addition, the plan administrator will monitor 
participants' savings deposits to insure that the annual additions to each
participant's accounts are within the limits specified in Section 0. If
additional savings deposits by a participant would cause a violation of such
limits, his savings deposits will cease immediately.


  5.3    CHANGES IN SAVINGS DEPOSITS.



                                       7


<PAGE>   12

         (a)   INCREASE OR REDUCTION. A participant making savings deposits may
increase or reduce the rate of his savings deposits to any higher or lower rate
permitted under Section 5.1(a) provided he satisfies such rules and procedures
the plan administrator adopts for changes in participant savings deposits. The
effective date of the new rate will be determined by rules and procedures
adopted by the plan administrator.

         (b)   SUSPENSION. A participant may suspend his savings deposits
provided he satisfies such rules and procedures the plan administrator adopts
for suspensions of participant savings deposits. The suspension of savings
deposits will become effective as soon as practicable upon fulfilling the
requirements established by the Plan administrator.

         (c)   RESUMPTION. Subject to Section 4.4 with respect to participants 
described therein, a participant who suspended his savings deposits under
subsection (b) above may resume such deposits provided he satisfies such rules
and procedures the plan administrator adopts for resumptions of savings
deposits. The effective date of such resumption will be determined in accordance
with the plan administrator's rules and procedures.

         (d)   PLAN ADMINISTRATOR RULES. The plan administrator may establish 
such rules and procedures for savings deposits as the plan administrator deems
necessary for the efficient administration of the plan. The plan administrator's
rules may provide that any commencement or change in a participant's savings
deposits will be effective on the first day of the payroll period, may require a
specified reasonable minimum period of advance notice prior to a change in
savings deposits, and/or may provide for a minimum period of suspension prior to
the resumption of savings deposits by a participant.

  5.4    COLLECTION OF SAVINGS DEPOSITS. The employer will collect participants'
savings deposits through payroll or other procedures and will deliver the
amounts collected to the trustee as soon as practicable after collection (but in
no event later than the deadline specified in any applicable regulations of the
Department of Labor under ERISA).

  5.5    ROLLOVER CONTRIBUTIONS.

         (a)   With the approval of the plan administrator, an employee in the
eligible class (whether or not a participant hereunder) may make a rollover or a
direct rollover to the plan (or cause to be transferred to the plan directly
from a qualified trust, qualified annuity plan, individual retirement account or
individual retirement annuity) cash or property in an amount which constitutes
(i) an eligible rollover distribution (as defined in Code Section 402(c)(4) or
Code Section 403(a)(4), or (ii) a rollover contribution (as defined in Code
Section 408(d)(3)). However, no rollover to this plan of accumulated deductible
employee contributions made under another plan will be permitted, and a direct
or indirect transfer to this plan from another qualified plan will not be
permitted if such transfer would subject this plan to the qualified joint and
survivor rules of Code Section 401(a)(11).

         (b)   The employers, the plan administrator and the trustee have no 
responsibility for determining the propriety of, proper amount or time of, or
status as a tax-free transaction of any transfer under subsection (a) above.

         (c)   If an employee who is not yet a participant makes a rollover 
contribution under subsection (a) above, he will be considered to be a
participant with respect to such contribution only. He will not be a participant
for any other purpose of the plan until he completes the requirements for
participation under Article 0.

         (d)   A rollover will be credited to a separate rollover account in the
name of the employee making such rollover contribution.

         (e)   The plan administrator in its discretion may direct the return to
the employee (or the retransfer to another trustee or custodian designated by
the employee) of any rollover contribution to the extent the plan administrator
determines that such return may be necessary to insure the continued
qualification of this plan under Section 401(a) of the Code or that the holding
of such rollover contribution would be administratively burdensome.



                                       8
<PAGE>   13

                                    ARTICLE 6
                             EMPLOYER CONTRIBUTIONS

  6.1    MATCHING EMPLOYER CONTRIBUTIONS. Periodically each employer will make 
a matching contribution on behalf of each participant employed by such employer
who makes basic savings deposits. Subject to the limitations of Section 0, the
employer's matching contribution will be equal to 25% of the participant's basic
savings deposits. Matching contributions will be reduced by any forfeitures
under Section 0.

  6.2    401(m) LIMITS.

         (a)   GENERAL LIMITS. Matching employer contributions on behalf of
highly-compensated employees must not exceed such amounts that, as of the last
day of each plan year, the average of the individual matching contribution
percentages of the highly-compensated employees (the "highly-compensated
matching contributions percentage" or "HMCP" may not exceed the average of the
individual matching contribution percentages of the lower-compensated employees
(the "lower-compensated matching contributions percentage" or "LMCP") by more
than the amount specified in the following table.

                      If LMCP is:                    HMCP may not exceed:
                      -----------                    --------------------

                      Less than 2%                   2 times LMCP

                      At least 2% but                2% more than LMCP
                      less than 8%

                      8% or higher                   1.25 times LMCP

         Compliance with the foregoing requirement will be determined after
taking into account any adjustments made to comply with the Code Section 415
limits under Section 7.6 below.

         Highly-compensated and lower-compensated employees are defined in
Section 5.2(c).

         The plan administrator will monitor and adjust matching contribution 
percentages of highly-compensated employees to the extent necessary to satisfy
this subsection. Such monitoring and adjustments will be accomplished under
procedures similar to those specified in Sections 5.2(b) and (d).

         (b)   MATCHING CONTRIBUTION PERCENTAGE. For purposes of this section,
the MATCHING CONTRIBUTION PERCENTAGE of a participant for a plan year means the
employer matching contribution on his behalf for such year, computed as a
percentage of his applicable compensation for such year. The plan administrator
may in its discretion include savings deposits of a participant in the lower
compensated group in determining his matching contribution percentage, to the
extent that such savings deposits are not used in determining his deferral
percentage under Section 5.2(c)(i).

         (c)   COMBINED 401(k) AND 401(m) LIMITS. The sum of the HDP plus the 
HMCP may not exceed the sum of the following:

               (i)    125 percent of the greater of the LDP (under Section 5.2) 
and the LMCP (under this Section 6.2); and



                                       9


<PAGE>   14

               (ii)   two percentage points more than the smaller of such LDP or
LMCP, but in no event more than twice the smaller of such LDP or LMCP.

                      If the sum of the HDP and the HMCP determined as set forth
above exceed the limit specified in this subsection, the plan administrator will
reduce deferrals by participants who are highly-compensated employees or will
reduce matching contributions on behalf of such employees in accordance with
this section to the extent necessary to meet the requirements of this
subsection.

  6.3    FORM AND TIME OF CONTRIBUTION. The employers' contribution under
Section 0 will be paid to the trustee in employer stock or in cash to be
invested in Biogen stock. Such contributions will normally paid at periodic
intervals (not less frequently than quarterly) determined by the employers. In
any event, such contributions will be paid to the trustee no later than the due
date (including extensions) for filing the employers' federal income tax return
for such year.

  6.4    BIOGEN STOCK DEFINED. Biogen stock means the common stock, $.01 par
value, of Biogen. If the outstanding common stock of Biogen is changed into or
exchanged for a different kind of stock or other securities of Biogen or another
successor corporation by reason of any reorganization, merger, consolidation,
recapitalization, reclassification, change in par value or the like, then Biogen
stock means such other stock or security. For purposes of determining the number
of shares of Biogen stock to be allocated to a participant as a matching
employer contribution, Biogen stock will be valued as of the most recent
practicable business day preceding an employer matching contribution based on
the average of the high and low sales prices of such stock on such business day
as reported on the NASDAQ National Market System. For purposes of participant
account statements, Biogen stock will be valued at the closing price on the last
business day of the period covered by such statement, as reported on the NASDAQ
National Market System. For purposes of paying the value of a fractional share
under Section 10.7, Biogen stock will be valued at the closing price on the most
recent practicable business day preceding such payment, as reported on the
NASDAQ National Market System.

  6.5    RETURN OF CONTRIBUTION MADE IN ERROR OR NOT DEDUCTIBLE. Unless 
specifically identified as a non-deductible contribution, each matching employer
contribution hereunder is specifically conditioned upon its deductibility on the
federal income tax return covering the employer making the contribution. If all
or part of a matching employer contribution is made because of a mistake of fact
or if the deduction under Code Section 404 of any portion of a matching employer
contribution is disallowed, the amount contributed because of a mistake of fact
or the amount for which the deduction is disallowed will be returned to the
contributing employer if demand therefor is made within the time allowed by law.


                                    ARTICLE 7
                              ACCOUNTS AND CREDITS

  7.1    ESTABLISHMENT OF ACCOUNTS.  The plan administrator will establish and 
maintain in the name of each participant such of the following accounts as are
appropriate for the participant:

         (a)   basic savings deposits account;

         (b)   supplemental savings deposits account;

         (c)   matching contributions account; and

         (d)   rollover account.



                                       10


<PAGE>   15

Credits and charges to such accounts will be made as provided in the plan.

  7.2    CREDITING PARTICIPANTS' SAVINGS DEPOSITS. Basic and supplemental 
savings deposits made by a participant will be credited to such participant's
basic or supplemental savings deposits account, as appropriate, when received
and processed by the trustee.

  7.3    CREDITING MATCHING CONTRIBUTIONS AND FORFEITURES. Matching employer
contributions on behalf of a participant and any forfeitures to be credited to
such participant under Section 6.1 will be credited to such participant's
matching contributions account when received and processed by the trustee. To
the extent that forfeitures during any period exceed the amount to be credited
under the preceding sentence during such period, the forfeitures will be carried
over and credited as a matching contribution for the next processing period.

  7.4    CREDITING ROLLOVERS. Rollovers by or on behalf of an eligible employee 
will be credited to the employee's rollover account when received and processed
by the trustee following receipt thereof by the trustee.

  7.5    CHARGES TO ACCOUNTS. Any amount distributed, paid, withdrawn or 
transferred from an account will be a charge against such account as of the date
of distribution, payment, withdrawal or transfer. All distributions, payments,
withdrawals or transfers from an account will be based upon the amount in such
account as of the valuation date most recently preceding the date of such
distribution, payment, withdrawal or transfer.

  7.6    MAXIMUM ADDITIONS.

         (a)   The annual additions to a participant's accounts for any plan
year (which will be the LIMITATION YEAR for purposes of Code Section 415) may
not exceed the lesser of (i) $30,000 as adjusted periodically for cost-of-living
changes in accordance with Code Section 415 and regulations thereunder, or (ii)
25 percent of his total compensation for such year. For purposes of this
section, TOTAL COMPENSATION means a participant's total non-deferred
compensation from his employer for a plan year, as defined in Code Section 415
and regulations thereunder.

         (b)   If the annual additions to a participant's accounts would exceed 
the limitations of subsection (a) above, the savings deposits made on his behalf
for such year which would be credited to his accounts but for the limitations of
subsection (a) will be returned to him, but only to the extent necessary to
comply with subsection (a). Any amount returned to a participant under this
subsection (b) will include interest or earnings on such amount as provided in
regulations under Code Section 415.

         (c)   For purposes of this Section 0, ANNUAL ADDITIONS to a 
participant's accounts for any plan year means the sum of the following amounts
credited to his accounts for such year: (i) savings deposits, and (ii) matching
employer contributions and forfeitures.

         (d)   For any plan year, the sum of a participant's defined 
contribution plan fraction and his defined benefit plan fraction may not exceed
one, as follows:

               (i)   His DEFINED CONTRIBUTION PLAN FRACTION for any plan year is
  the fraction (A) whose numerator is the sum of annual additions (as defined in
  Code Section 415(c)(2)) to his accounts under all qualified defined
  contribution plans maintained by Biogen (or any other employer that is
  included in a controlled group or under common control with Biogen within the
  meaning of Code Sections 414(b) and (c) and 415(h)) as of the close of such
  plan year, and (B) whose denominator is the sum of the lesser of the following
  amounts determined for such year and for each prior year of plan service with
  his employer: the product of 1.25 (1.0 if the plan is top-heavy) and the
  dollar limitation in effect for such year, or the product of 1.4 and 25
  percent of the participant's compensation for such year.



                                       11


<PAGE>   16

               (ii)   His DEFINED BENEFIT PLAN FRACTION for any plan year is a
  fraction (A) whose numerator is his aggregate projected annual benefit under
  all defined benefit plans sponsored by Biogen (or any other employer that is
  included in a controlled group or under common control with Biogen within the
  meaning of Code Sections 414(b) and (c) and 415(h)) as of the close of such
  plan year, and (B) whose denominator is the lesser of the product of 1.25 (1.0
  if the plan is top-heavy) and the dollar limitation in effect under Section
  415(b)(1)(A) of the Code, or the product of 1.4 and the participant's highest
  average compensation as determined under Section 415(b)(1)(B) of the Code. For
  this purpose, the PROJECTED ANNUAL BENEFIT of a participant means the total
  normal retirement benefit to which he would be entitled on the assumptions
  that his employment continues until his normal retirement date and his annual
  earnings and all other relevant factors remain the same for all future years
  as in the year when the projection is made.

               (iii)  If the sum of such fractions would exceed one without the 
  application of this section, his benefit under the defined benefit plan or
  plans will be reduced to a benefit that will produce a defined benefit plan
  fraction and a defined contribution plan fraction that equal one.

               (iv)   The provisions of this subsection (d) will not apply to 
  any limitation year or plan year when combined defined contribution and
  defined benefit plan limitations are not applicable under Code Section 415(e).

                                    ARTICLE 8
              INVESTMENT FUNDS AND CREDITING INVESTMENT EXPERIENCE

  8.1    INVESTMENT FUNDS. The plan administrator may direct the trustee to
establish two or more separate investment funds within the trust fund and to
invest each such separate investment fund in different types or categories of
assets (e.g., equities or fixed-income securities, or shares of specified mutual
funds) or in accordance with investment objectives specified by the plan
administrator. One such separate investment fund will consist of Biogen stock
(as defined in Section 0). The plan administrator may add or delete separate
investment funds in its discretion.

         The plan administrator will maintain records which reflect the portion
of each account of a participant that is invested in each separate investment
fund. The plan administrator may delegate such recordkeeping responsibilities or
may contract with a service provider (which may be the trustee or an affiliate
of the trustee) for recordkeeping services. The existence of such records and of
participants' accounts will not be deemed to give any person any right, title or
interest in or to any specific assets or part of the trust fund or any separate
investment fund.

  8.2    INVESTMENT DIRECTIONS AND TRANSFERS AMONG FUNDS.

         (a)   INVESTMENT OF MATCHING CONTRIBUTIONS. (i) Except as provided in 
subsection (ii) below, the matching contributions account of each participant
will be invested in the Biogen stock fund.

               (ii)  A participant who is at least age 55 and who has four or
  more years of plan service may elect to transfer all or any portion of the
  Biogen stock in his matching contributions account and to invest such amount
  in one or more of the separate investment funds available for the investment
  of basic and supplemental savings deposits accounts, and the participant may
  thereafter direct transfers among such separate investment funds in accordance
  with the rules specified in subsection (b) below. The manner and time of
  giving directions under this subsection (ii) will be governed by the rules
  specified in subsection (c) below. Once a participant has made an election to
  transfer under this subsection, he may not subsequently transfer any amounts
  from the separate investment funds back to the Biogen stock fund.



                                       12
<PAGE>   17


               Notwithstanding the preceding paragraph, a participant who is an
  officer or director of an employer (or any affiliate of an employer), or an
  owner of more than 10% of a class of equity securities of an employer, and who
  is subject to Section 16 of the 1934 Act, may make an election to transfer all
  or any portion of the Biogen stock in his matching contributions account and
  to invest such amount in one or more of the separate investment funds only
  during a quarterly period beginning on the third business day following
  Biogen's release of quarterly financial information to the public and ending
  on the 12th business day thereafter. In addition, such a participant, after
  making one such election, may not make another such election until the minimum
  period of time (if any) required by regulations under Section 16(b) of such
  Act has elapsed.

         (b)   INVESTMENT OF SAVINGS DEPOSITS AND ROLLOVERS. Each participant
may direct the separate investment fund or funds in which his basic and
supplemental savings deposits accounts and rollover account will be invested. A
participant may direct investment of these accounts on his behalf entirely in
one investment fund or in a combination of two or more of the investment funds.
In addition, the participant may direct transfers among the investment funds so
that these accounts are invested entirely in one investment fund or in a
combination of two or more of the investment funds.

         Notwithstanding the preceding paragraph, a participant may not invest 
his basic or supplemental savings deposits accounts or rollover account in the
Biogen stock fund. In addition, no other investment fund will have more than 10%
of its assets invested in Biogen stock or any other security of Biogen (or its
successors and affiliates) at any time.

               The participant will have sole responsibility for the investment
of his basic and supplemental savings deposits accounts, rollover account and,
to the extent provided for in subsection (a) (ii) above, matching contribution
account, and for transfers among the available investment funds, and no named
fiduciary or other person will have any liability for any loss or diminution in
value resulting from the participant's exercise of such investment
responsibility. It is intended that Section 404(c) of ERISA will apply to a
participant's exercise of investment responsibilities under this subsection.

         (c)   MANNER AND TIME OF GIVING DIRECTIONS. A participant's directions
governing the investment of his accounts under subsection (a) or (b) will be
given in accordance with such rules and procedures as the plan administrator may
from time to time establish and communicate to participants. A participant must
give such investment directions when he first authorizes savings deposits or
makes a rollover contribution. A participant may give new investment directions
changing the investment of his future savings deposits or rollovers or directing
transfers among the investment funds, subject in each case to subsection (b)
above.

  8.3    VALUATION OF ASSETS AND CREDITING INVESTMENT EXPERIENCE.

         (a)   IN GENERAL. As of each valuation date, the trustee will determine
the fair market value of the plan's assets, relying upon such evidence of value
as it deems appropriate. In performing such valuations, the trustee may include
any accrued but unpaid dividends, interest or other items, but will not include
any contributions or forfeitures to be allocated as of such valuation date as an
asset of any participant's account. Investment income and gains and losses of
the trust fund will be credited or debited to participants' accounts in
proportion to the balances in such accounts as of the preceding valuation date,
reduced by any charges against any such account since such preceding valuation.

         The last day of each plan year will be a valuation date, and the plan
administrator may from time to time designate other valuation dates for all
accounts or for specified accounts only (such as, by way of example, daily or
monthly valuation dates for accounts invested exclusively in shares of open-end
mutual funds and annual valuation dates for all other accounts).



                                       13


<PAGE>   18

         (b)   OPEN-END MUTUAL FUNDS. Notwithstanding subsection (a) above, in
the case of accounts that are invested exclusively in shares of open-end mutual
funds, the valuation of such accounts will be performed separately from the
valuation of the other accounts, and the value of such mutual fund shares as of
any valuation date will be the net asset value as of such date determined by the
mutual fund (with such adjustments, if any, as the mutual fund may make in
accordance with its normal procedures as described in its prospectus). Any
dividends or capital gains or other distributions on shares of a mutual fund
allocated to a participant's account will be reinvested in shares and fractional
shares of such mutual fund.

         (c)   BIOGEN STOCK.  Notwithstanding subsection (a) above, in the case 
of accounts that are invested exclusively in Biogen stock, the valuation of such
accounts will be performed separately from the valuation of the other accounts,
and the value of a share of Biogen stock as of any valuation date will be
determined in accordance with Section 0. Any cash dividends on shares of Biogen
stock will be reinvested in additional shares of Biogen stock.

         At any time when Biogen stock (or any other employer security held by
the plan or proposed to be purchased by the plan) is not readily tradable on an
established securities market, all valuations of Biogen stock (or other employer
security) will be performed by an independent appraiser (meeting the
requirements of Code Section 401(a)(28)(C)), and the trustee's determination of
the fair market value of Biogen stock (or such other employer security) will be
based upon a valuation report of such independent appraiser.

                                    ARTICLE 9
                              LOANS TO PARTICIPANTS

  9.1    LOANS TO PARTICIPANTS.

         (a)   AVAILABILITY OF LOANS. Upon application by a participant, the
trustee may be directed to make a loan to the participant from his savings
deposits account(s) and rollover account. Loans will be available to each
eligible participant on a reasonably equivalent basis under uniform,
nondiscriminatory borrowing rules. Such borrowing rules must be formulated and
administered so that the requirements of Code Section 72(p) for non-taxable
loans, the applicable Department of Labor regulations on plan loans, and the
following provisions of this section are satisfied. Any loan hereunder will bear
a reasonable rate of interest and will be evidenced by a promissory note signed
by the participant in such form as the plan administrator may require. The
amount of any such loan will be withdrawn from the participant's accounts and
the investment fund or funds in which the participant's accounts are invested in
accordance with the borrowing rules.

         (b)   BORROWING RULES. The borrowing rules may contain such
requirements pertaining to loans as are deemed necessary or desirable and which
are not specified herein. The borrowing rules may govern the procedures and
cut-off dates for applying for loans hereunder and the terms of such loans,
including (i) the number of loans that a participant may request in any year and
the number of loans that may be outstanding at any time to a participant, (ii)
any restrictions on reborrowing not stated in this section, (iii) the interest
rate in effect from time to time for loans or the method of ascertaining such
interest rate, and (iv) the repayment schedule for loans or the method for
determining the repayment schedule.

         (c)   AMOUNT OF LOANS.  The minimum loan amount is $1,000 or such
lesser amount as the borrowing rules may from time to time provide. The maximum
aggregate loan amount is based upon the vested balance in the participant's
accounts. No participant loan will exceed the smallest of (i) the amount in the
participant's savings deposits accounts and rollover account, (ii) one-half of
the participant's vested account balances, or (iii) $50,000 (reduced by the
highest outstanding loan balance to the participant during the 12 months
preceding the loan). For purposes of applying such limits, account values as of
the valuation date immediately preceding the date when the loan is made will be
used.



                                       14


<PAGE>   19

         (d)   MAXIMUM REPAYMENT PERIOD.

               (i)    OTHER THAN RESIDENTIAL LOANS. Except as provided in 
subsection (ii) below, the maximum term of a loan will be five years (provided
that, if provided in the borrowing rules, there may be a shorter repayment
period for small loans).

               (ii)   RESIDENTIAL LOANS. If a participant requests a loan for 
the acquisition or construction of his principal residence, maximum the
repayment period will be determined by reference to bank loans for the same
purpose but may not exceed 10 years.

         (e)   SECURITY FOR REPAYMENT. Each loan hereunder will be a 
participant-directed investment for the benefit of the participant requesting
such loan; accordingly, any default in the repayment of principal or interest of
any loan hereunder will reduce the amount available for distribution to such
participant (or his beneficiary). Thus, any loan hereunder will be effectively
and adequately secured by the vested amount in the participant's accounts. To
the extent provided in the borrowing rules, other security for repayment of a
loan may be required in any instance. A participant receiving a loan must
execute such instruments and must pay any fees for filings required to perfect
any security interest in the participant's accounts or other security.

         (f)   REPAYMENT. A participant may be required to execute an agreement
to repay the principal and interest of a loan through regular payroll deduction
payments from the participant's compensation. Back-up repayment procedures may
be established for participants who do not make payroll deduction repayment.
Except as otherwise may be permitted under Treasury regulations, any such
back-up procedures will provide for substantially level amortization payments
made quarterly or more frequently. Any loan hereunder may be prepaid, in
accordance with the note or other document evidencing the loan. If a
participant's service as an employee is terminated for any reason, the entire
unpaid principal and interest of any loan then outstanding to such participant
may become immediately due and payable in accordance with the note or other
document evidencing the loan.

         (g)   ACTION UPON DEFAULT. If a participant defaults on any payment of 
interest or principal of a loan hereunder or defaults upon any other obligation
relating to such loan, the plan administrator may take (or direct the trustee to
take) such action or actions as it determines to be necessary to protect the
interests of the plan. Such actions may include commencing legal proceedings
against the participant, or foreclosing on any security interest in the
participant's accounts or other security given in connection with a loan
hereunder; however, the plan administrator will not direct foreclosure on the
participant's savings deposits accounts at a time when the participant would not
be entitled to withdraw from such account under Section 0.

         (h)   DISTRIBUTION TO PARTICIPANT WITH LOAN. In the case of any 
participant with a loan outstanding hereunder, the amount available for
distribution to such participant (or his beneficiary) will consist of the
portion of his accounts invested in the investment funds of the trust fund. In
addition, the participant's note will be distributed to him (or his
beneficiary), and the trustee will report the value of the note for income tax
purposes as the amount of unpaid principal and interest due thereon at the date
of distribution.

  9.2    ACCOUNTING FOR LOANS.

         (a)   SOURCE OF LOAN. The plan administrator will establish procedures
and ordering rules for liquidating the participant's accounts to make a loan to
him.

         (b)   LOAN ACCOUNT. The plan administrator will establish and maintain
a loan account for each borrowing participant. The unpaid principal and accrued
but unpaid interest on the loans to a participant will be reflected for plan
accounting purposes in the participant's loan account. Repayments by the
participant will be credited to his loan account, and will then be transferred
from his loan account to the participant's other accounts.




                                       15


<PAGE>   20

                                   ARTICLE 10
                         WITHDRAWALS AND DISTRIBUTIONS

  10.1         IN-SERVICE WITHDRAWALS FROM SAVINGS DEPOSITS.

         (a)   APPLICATION. A participant may apply to the plan administrator
for a withdrawal from his basic and supplemental savings deposits accounts.
Except as provided in subsection (d) below, such withdrawals will be available
only in cases of financial hardship. The participant will file an application
setting forth the specific immediate and heavy financial needs prompting his
request and the amount needed to meet such immediate financial need. The minimum
withdrawal amount is $100, and the maximum withdrawal amount is the lesser of
(i) the total amount in the participant's basic and supplemental savings
deposits (excluding any interest or earnings on such savings depositsCother than
any such interest or earnings in the participant=s basic and supplemental
savings deposits accounts as of December 31, 1988 to the extent identifiable in
the accounting records of the plan) or (ii) the amount needed to alleviate his
financial hardship (including the amount reasonably expected to be needed for
income taxes and penalties payable on the amount withdrawn).

         (b)   FINANCIAL HARDSHIP DEFINED. Financial hardship means that a 
participant has an immediate and heavy financial need and that the withdrawal is
necessary to meet the need.

               (i)   IMMEDIATE AND HEAVY FINANCIAL NEED. A withdrawal for an
         immediate and heavy financial need must be occasioned by: (A) medical
         expenses incurred or needed by the participant or his spouse or any of
         his dependents; (B) tuition and related educational fees for the next
         12 months of post-secondary education for the participant, his spouse,
         child or dependent; (C) purchase of the participant's principal
         residence (not including mortgage payments); (D) rent or mortgage
         payments to prevent the participant's eviction from or the foreclosure
         of the mortgage on his principal residence; (E) such other event or
         circumstance as the Internal Revenue Service permits; or (F) any other
         extraordinary personal need which the plan administrator in its sole
         discretion may determine under uniform rules of general application.
         Financial need under (A) through (E) above will be deemed to be a
         financial hardship without evidence of the participant's other
         financial resources.

               (ii)  EVIDENCE OF NEED AND NECESSITY.  A participant must 
establish to the plan administrator's satisfaction both that the participant has
an immediate and heavy financial need and that the withdrawal is necessary to
meet the need. The participant may elect to establish need under the deemed
method referenced in the last sentence of subsection (i) above. If the
participant does not elect to use the deemed method, the participant in his
hardship withdrawal application must show facts and circumstances amounting to
immediate and heavy financial need. In addition, the participant must show that
the participant cannot meet the need from other reasonably available financial
resources.

               (iii) PLAN ADMINISTRATOR DETERMINATIONS. A participant's
         application for a hardship withdrawal may be in writing or transmitted
         by electronic means, on such form and containing such information (or
         other evidence or materials establishing the participant's financial
         hardship) as the plan administrator may require. In determining whether
         a participant has a financial hardship, the plan administrator may
         reasonably rely upon the participant's representations, including
         representations concerning his inability to meet the need through other
         resources such as insurance proceeds, reasonable liquidation of other
         assets, distributions or non-taxable loans from other plans, or
         borrowing from other sources on reasonable terms, or through cessation
         of savings deposits hereunder. The plan administrator's determination
         of the existence of and the amount needed to meet a financial hardship
         will be binding on the participant.

         (c)   PAYMENT. If approved by the plan administrator, a hardship
withdrawal will be paid to the participant as soon as practicable after the
hardship withdrawal is approved by the plan administrator. Any such withdrawal
will 



                                       16


<PAGE>   21

be paid first from the participant's supplemental savings deposits account and
next from his basic savings deposits account.
 
         (d)   EXCEPTIONS. The requirement for financial hardship will not apply
to withdrawals requested by a participant in the following situations: (i) after
the participant has reached age 59 1/2, or (ii) to remove savings deposits
during a calendar year which, when added to savings deposits under Code Section
401(k) or salary reduction contributions under Code Section 403(b) during the
same year to another plan, exceed the limits on elective deferrals under Code
Section 402(g). A request to remove excess savings deposits under (ii) above
must be filed with the plan administrator no later than the March 1 following
such calendar year. Such a withdrawal will be paid to the participant as soon as
practicable after the filing of his request and will include income and
investment gain or loss allocable to such withdrawn amount.

  10.2   IN-SERVICE WITHDRAWALS FROM ROLLOVER ACCOUNT.

         (a)   AMOUNT. A participant may elect to withdraw from his rollover 
account any amount he specifies from a minimum of $1,000 (or the amount in his
rollover account if less) to a maximum of the total amount in that account. A
withdrawal request under this section will be made in accordance with applicable
rules and procedures adopted by the plan administrator.

         (b)   PAYMENT. Such an in-service withdrawal by a participant will be 
paid to him as soon as practicable after the participant files the form
requesting the withdrawal.

  10.3   DISTRIBUTION UPON RETIREMENT OR DISABILITY.

         (a)   AMOUNT AND TIME OF DISTRIBUTION. A participant who retires or who
terminates service because of disability will receive the total amount in his
accounts. Subject to Section 0, distribution of his account will be made as soon
as practicable after his retirement or disability.

         For plan years beginning January 1, 1997, and thereafter a participant
may elect to receive a distribution on the April 1 of the year following the
calendar year in which the participant reaches age 70-1/2, or, if still an
employee of Biogen, the participant may elect to defer receiving distributions
until he/she retires from the employer. Such elections will be made in
accordance with rules and procedures of the plan administrator. Notwithstanding
the preceding sentence, in the case of a participant who is a 5% or greater
stockholder of Biogen, distribution must be made or begin no later than the
April 1 of the year following the calendar year in which the participant reaches
age 70 1/2.

         (b)   RETIREMENT AND DISABILITY DEFINED. For purposes of this plan, 
RETIREMENT means a participant's termination of employment on or after his 55th
birthday. DISABILITY means a participant's inability to perform the normal
duties of his position with his employer because of a physical or mental
impairment.

  10.4   DISTRIBUTION UPON TERMINATION OF EMPLOYMENT.

         (a)   AMOUNT AND TIME OF DISTRIBUTION. A participant who terminates
employment for any reason other than retirement, disability or death will
receive the vested portion of his accounts. Subject to Section 0, his vested
accounts will be payable as soon as practicable after his termination of
employment.

         (b)   VESTED INTEREST. A participant will be fully vested in his basic 
and supplemental savings deposits accounts and rollover account at all times.



                                       17
<PAGE>   22

               A participant who is an active employee will be fully vested in 
his matching contribution account on his 55th birthday. Before that date, such a
participant will have a vested interest in that percentage of his matching
contribution account specified in the following table based upon his number of
years of plan service:


                       Years of
                     Plan Service                      Vested Percentage
                     ------------                      -----------------

                      Less than 1                              0%
                           1                                  25%
                           2                                  50%
                           3                                  75%
                       4 or more                             100%

         (c)   FORFEITURE OF NON-VESTED INTEREST. The non-vested portion of a
terminated participant's matching contribution account, if any, will be
forfeited by him on the day after his termination of employment. If a terminated
participant whose account was wholly or partially forfeited returns to
employment with an employer within six years of his termination, the amount
forfeited will be restored to his account (out of current forfeitures to the
extent available) as soon as practicable after the date of his return. The
dollar amount restored will equal the cash value of the shares forfeited on the
date of forfeiture and will be reinvested in Biogen stock at the market value in
effect on the date of restoration.

  10.5   RIGHT TO DEFER. The plan administrator will notify a participant of his
right to receive payment of his vested account balance as soon as practicable
after his retirement, disability or other termination of employment.
Notwithstanding the foregoing provisions of this article, if his vested account
balance exceeds $3,500, the participant will be given the right to defer payment
of his accounts to a later date.

         Distribution of a participant's accounts may be deferred to a date no 
later than the April 1 following the year in which such participant reaches age
70 1/2. A participant who deferred payment of his accounts may accelerate such
payment by filing a notice with the plan administrator specifying the earlier
distribution date.

  10.6   DISTRIBUTION UPON DEATH OF PARTICIPANT.

         (a)   IN GENERAL. If a participant dies during employment (or after 
retirement but before distribution of his accounts), his beneficiary will
receive the total amount in his accounts. Distribution of such amount will be
made as soon as practicable after the date the plan administrator receives such
evidence of the participant's death and the right of any beneficiary to receive
such payment as it deems necessary, but in no event later than the end of the
year following the calendar year containing participant's date of death.

         If a participant dies following termination of employment but before
the distribution of his vested interest, his beneficiary will receive the amount
owing to the participant under Section 0. Payment to the beneficiary will be
made in accordance with the last sentence of the preceding paragraph.

         (b)   DESIGNATION OF BENEFICIARY. A participant may designate one or 
more beneficiaries to receive any distribution payable under subsection (a)
above and may revoke or change such a designation at any time. If the
participant names two or more beneficiaries, distribution to them will be in
such proportions as the participant designates or, if the participant does not
so designate, in equal shares. Any designation of beneficiary will be in writing
on such form as the plan administrator may prescribe and will be effective upon
filing with the plan administrator.




                                       18


<PAGE>   23

         Notwithstanding the preceding paragraph, the sole beneficiary of a
participant who is married immediately before his death will be the
participant's surviving spouse unless the participant designated and the spouse
consented in writing to such designation of another person as beneficiary. The
spouse's consent must acknowledge the effect of such consent and be witnessed by
a plan representative or a notary public.

         (c)   NO DESIGNATION. Any portion of a distribution payable upon the 
death of a participant which is not disposed of by a designation of beneficiary
under subsection (b) above, for any reason whatsoever, will be paid to the
participant's surviving spouse if living at his death, otherwise equally to the
participant's natural and adopted children (and the issue of a deceased child by
right of representation), otherwise to the participant's estate.

         (d)   PAYMENT UNDER PRIOR DESIGNATION. The plan administrator may 
direct payment in accordance with a prior designation of beneficiary (and will
be fully protected in so doing) if such direction (i) is given before a later
designation is received, or (ii) is due to the plan administrator's inability to
verify the authenticity of a later designation. Such a distribution will
discharge all liability therefor under the plan.

  10.7   MANNER OF PAYMENT. A participant's savings deposits account and 
rollover account will be distributed in cash. A participant's matching
contributions account will be distributed in whole shares of employer stock,
with the value of any fractional share paid in cash, or, if applicable, the
diversified portion account will be distributed in cash.

         The amount payable will be distributed to the participant (or to his
beneficiary or beneficiaries in the event of his death) in one lump sum payment.

  10.8   DIRECT ROLLOVERS.

         (a)   APPLICABILITY OF THIS SECTION. Notwithstanding any provision of 
the plan to the contrary that would otherwise limit a distributee's election
under this section, a distributee may elect, at the time and in the manner
prescribed by the plan administrator, to have any portion of an eligible
rollover distribution paid directly to an eligible retirement plan specified by
the distributee in a direct rollover (a distributee may designate only one
eligible retirement plan to receive such a direct payment).

         (b)   DEFINITIONS. The following definitions apply for purposes of this
section.

               (i)  ELIGIBLE ROLLOVER DISTRIBUTION means any distribution of all
  or any portion of the balance to the credit of the distributee under this
  plan, except that an eligible rollover distribution does not include: any
  distribution that is one of a series of substantially equal periodic payments
  (not less frequently than annually) made for the life (or life expectancy) of
  the distributee or the joint lives (or joint life expectancies) of the
  distributee and the distributee's designated beneficiary, or for a specified
  period of ten years or more; any distribution to the extent such distribution
  is required under Code Section 401(a)(9); and the portion of any distribution
  that is not includible in gross income (determined without regard to the
  exclusion of net unrealized appreciation with respect to employer securities).
  If the distributee receives a distribution from his matching contributions
  account in the form of Biogen stock and a cash distribution from his matching
  contributions account (the value of any fractional share of Biogen stock) or
  any other account, the cash distribution may be directly rolled over to an
  eligible retirement plan if such cash distribution qualifies as an eligible
  rollover distribution, and the stock distribution may be retained by the
  distributee. Alternatively, the entire distribution (cash and stock) may be
  directly rolled over to an eligible retirement plan by the distributee.
  However, an eligible rollover distribution will not include any distribution
  that is below any threshold amount established by applicable regulations
  (including any threshold amount for direct rollovers when only a portion of a
  distributee's distribution is being rolled over).




                                       19


<PAGE>   24

               In general, subject to any thresholds established under the last
sentence of the preceding paragraph, as long as Section 0 provides for
distributions to participants or beneficiaries only in the form of a lump sum
payment, all distributions from the plan will be eligible rollover
distributions.

               (ii)  ELIGIBLE RETIREMENT PLAN means an individual retirement
  account described in Code Section 408(a), an individual retirement annuity
  described in Code Section 408(b), an annuity plan described in Code Section
  403(a), or a qualified trust described in Code Section 401(a), that will
  accept the distributee's eligible rollover distribution. However, in the case
  of an eligible rollover distribution to the surviving spouse, an eligible
  retirement plan is an individual retirement account or individual retirement
  annuity.

               (iii) DISTRIBUTEE means an employee or former employee. In 
  addition, an employee's or former employee's surviving spouse and an
  employee's or former employee's spouse or former spouse who is an alternate
  payee under a qualified domestic relations order, as defined in Code Section
  414(p), is a distributee with regard to the interest of the spouse or former
  spouse.

               (iv)  DIRECT ROLLOVER means payment by this plan to the eligible 
retirement plan specified by the distributee, with such payment being made in
any manner permitted by applicable regulations.

  10.9   REHIRE BEFORE DISTRIBUTION. If a former active participant is rehired 
before distribution of his accounts has been made, such distribution will be
deferred until his subsequent termination of employment.

                                   ARTICLE 11
                    AMENDMENT, MERGER AND TERMINATION OF PLAN

  11.1   AMENDMENT OF PLAN. At any time and from time to time, Biogen may amend
or modify any or all of the provisions of the plan without the consent of any
person, provided that no amendment will reduce any participant's nonforfeitable
account balance as of the date such amendment is adopted (or its effective date
if later), and provided further that no amendment will permit any part of the
trust fund to revert to the employers or be used for or diverted to purposes
other than for the exclusive benefit of participants or their beneficiaries,
except as provided in Section 0. In addition, the plan administrator may,
subject to the provisos contained in the preceding sentence, amend or modify any
or all of the provisions of the plan other than an amendment that would
materially affect the benefits of participants or that would materially increase
the cost of maintaining the plan to the employers.

  11.2   MERGER OF PLANS. A merger or consolidation with, or transfer of assets 
or liabilities to, any other plan will be permitted only if the benefit each
participant would receive if such plan were terminated immediately after the
merger, consolidation or transfer is not less than the benefit he would have
received if this plan had terminated immediately before the merger,
consolidation or transfer.

  11.3   TERMINATION. Biogen has established the plan and the employers are 
maintaining the plan with the bona fide expectation and intention that they will
be able to continue the plan and contributions thereto indefinitely, but they
will not be under any obligation or liability whatsoever to continue
contributions or maintain the plan for any particular length of time.
Notwithstanding any other provision hereof, an employer in its discretion may
discontinue contributions to the plan indefinitely or temporarily and Biogen may
terminate this plan at any time. There will be no liability to any participant,
beneficiary or other person as a result of any such discontinuance or
termination.

         An employer's failure to make contributions in any year or years will
not operate to terminate the plan in the absence of formal action by Biogen to
terminate the plan.

  11.4   EFFECT OF TERMINATION. Upon complete discontinuance of contributions or
termination or partial termination of the plan, the accounts of affected
participants will become nonforfeitable. After termination of the 



                                       20



<PAGE>   25

plan, no employee will become a participant and no further savings deposits or
contributions will be made hereunder on behalf of participants.

         The trustee will continue to hold the assets of the trust fund for 
distribution as directed by the plan administrator. The plan administrator will
determine whether to direct the trustee to disburse the plan's assets as
immediate benefit payments, to retain and disburse them in the future, or to
follow any other procedure which it deems advisable.

                                   ARTICLE 12
                                NAMED FIDUCIARIES

  12.1   IDENTITY OF NAMED FIDUCIARIES.

         (a)   NAMED FIDUCIARIES. Biogen, the trustee, any investment manager
appointed by Biogen, and the plan administrator will be the named fiduciaries
under the plan and will control and manage the plan and its assets to the extent
and in the manner indicated in this article and in the trust agreement. Any
responsibility assigned to a named fiduciary will not be deemed to be a duty of
a "fiduciary" (as defined in ERISA) solely because of such assignment.

         (b)   PLAN ADMINISTRATOR. Biogen will be the "plan administrator" as 
defined in ERISA.

  12.2   RESPONSIBILITIES AND AUTHORITY OF PLAN ADMINISTRATOR. The plan 
administrator will control and manage the operation and administration of the
plan except to the extent that such responsibilities are specifically assigned
hereunder to Biogen, to an investment manager or to the trustee. The
responsibilities and authority of the plan administrator are set forth in detail
in various articles of this plan and primarily in Article 0.

  12.3   RESPONSIBILITIES AND AUTHORITY OF TRUSTEE. The trustee will manage and
control the assets of the plan, except to the extent that such responsibilities
are specifically assigned hereunder or under the trust agreement to Biogen or
the plan administrator, or are delegated to one or more investment managers by
Biogen, or are performed by a participant with respect to the investment of his
accounts among the investment funds in the trust fund or with respect to his
taking a loan from his accounts in accordance with the plan. The
responsibilities and authority of the trustee are set forth in detail primarily
in the trust agreement.

  12.4   RESPONSIBILITIES OF BIOGEN. Biogen will have the following 
responsibilities and authority with respect to control and management of the
plan and its assets:

         (a)   to amend or terminate the plan;

         (b)   to merge or consolidate the plan with, or transfer all or part of
the assets or liabilities to, any other plan;

         (c)   to appoint, remove and replace the trustee and the plan
administrator and to monitor their performances;

         (d)   to appoint, remove and replace one or more investment managers,
or to refrain from such appointments, and to monitor their performances;

         (e)   to communicate such information to the plan administrator,
trustee and investment managers as they may need for the proper performance of 
their duties; and

         (f)   to perform such additional duties as are imposed by the plan or
by law.



                                       21


<PAGE>   26

  The responsibilities and authority of Biogen are set forth in further detail
in the various articles of the plan and in the trust agreement.

  12.5   RESPONSIBILITIES NOT SHARED. Except as otherwise provided herein or 
required by law, each named fiduciary will have only those responsibilities that
are specifically assigned to it hereunder, and no named fiduciary will incur
liability because of improper performance or nonperformance of responsibilities
assigned to another named fiduciary.

  12.6   DUAL FIDUCIARY CAPACITY PERMITTED. Any person or group of persons may 
serve in more than one fiduciary capacity, including service both as trustee and
plan administrator.

  12.7   ACTIONS BY BIOGEN. Wherever the plan specifies that Biogen is required 
or permitted to take any action, such action will be taken by its board of
directors, or by a duly authorized committee thereof, or by one or more
directors, officers, employees or other persons duly authorized to do so by the
board of directors.

  12.8   PROCEDURE FOR ALLOCATION AND DELEGATION OF RESPONSIBILITIES. The plan 
administrator or the members of the board of directors of Biogen or of a
committee of such board may allocate their responsibilities among themselves in
any reasonable manner and may delegate any of their responsibilities to any
other person or persons by so specifying in a written instrument. No plan
administrator or director will be liable for the improper discharge or
nonperformance of any responsibility so allocated or delegated to another person
except to the extent liability is imposed by law.

  12.9   ADVICE. A named fiduciary may employ or retain such attorneys, 
accountants, investment advisors, consultants, specialists and other persons or
firms as it deems necessary or desirable to advise or assist it in the
performance of its duties. Unless otherwise provided by law, the fiduciary will
be fully protected with respect to any action taken or omitted by him or it in
reliance upon any such person or firm rendered within his or its area of
expertise.

  12.10  INDEMNIFICATION. To the extent permitted by law and not prohibited by
Biogen's charter and by-laws, Biogen will indemnify and hold harmless every
individual serving as a fiduciary (whether a named fiduciary or otherwise), and
the estate of such an individual if he is deceased, from and against all claims,
loss, damages, liability, and reasonable costs and expenses, incurred in
carrying out his fiduciary responsibilities, unless due to the gross negligence,
bad faith or willful misconduct of such individual; provided that counsel fees
and amounts paid in settlement must be approved by Biogen and provided further
that this Section 0 will not apply to any claim, loss, damages, liability, or
costs and expenses which are covered by a liability insurance policy maintained
by Biogen, or by the plan or by an individual fiduciary. The preceding sentence
will not apply to a corporate trustee, an insurance company, an investment
manager or outside service provider (or to an employee of any of the foregoing)
unless Biogen otherwise specifies in writing.



                                       22



<PAGE>   27

                                   ARTICLE 13
                             THE PLAN ADMINISTRATOR

  13.1   APPOINTMENT. Biogen will appoint a plan administrator to perform the 
duties of the plan administrator in accordance with the plan.

         The plan administrator may be an individual or the plan administrator
may be a committee. The individual plan administrator or the committee members
may, but need not, be plan participants or employees or officers of Biogen. If a
committee, the number of persons serving on the committee at any time will be
determined by Biogen, and may be changed from time to time by Biogen. Biogen may
remove any person serving as plan administrator or committee member at any time,
with or without cause, by filing written notice of his removal with such person
and the trustee. A person serving as plan administrator or committee member may
resign at any time by filing his written resignation with Biogen and the
trustee. If a person who is serving as plan administrator or committee member is
an employee of Biogen, such person's appointment as plan administrator or
committee member will automatically terminate when such person is no longer an
employee of Biogen. A vacancy, however arising, will be filled by Biogen.

         In the event Biogen does not designate an individual as plan
administrator or appoint a committee pursuant to this section, the Director of
Human Resources (or the official performing the duties thereof) will be the plan
administrator hereunder shall have all the same functions of the plan
administrator.

  13.2   NOTICE TO TRUSTEE. Biogen will notify the trustee in writing of the 
appointment of each person serving as plan administrator or committee member,
and the trustee may assume such appointment continues in effect until written
notice to the contrary is given by Biogen.

  13.3   ADMINISTRATION OF PLAN. The plan administrator will have all powers and
authority necessary or appropriate to carry out its responsibilities with
respect to the operation and administration of the plan. It will interpret and
apply all plan provisions and may correct any defect, supply any omission or
reconcile any inconsistency or ambiguity in such manner as it deems advisable.
It will have discretion to make all final determinations concerning eligibility,
benefits and rights hereunder, and all other matters concerning plan
administration and interpretation. All determinations and actions of the plan
administrator will be conclusive and binding upon all persons, except as
otherwise provided herein or by law, and except that the plan administrator may
revoke or modify a determination or action previously made in error. Any action
or omission by the plan administrator will be subject to review (by a court or
otherwise) only for an abuse of discretion. The plan administrator will exercise
all powers and authority given to it in a nondiscriminatory manner, and will
apply uniform administrative rules of general application in order to assure
similar treatment of persons in similar circumstances.

  13.4   REPORTING AND DISCLOSURE. The plan administrator, acting as agent of 
Biogen in its capacity as ERISA plan administrator, will prepare, file, submit,
distribute or make available any plan descriptions, reports, statements, forms
or other information to any government agency, employee, former employee, or
beneficiary as may be required by law or by the plan.

  13.5   RECORDS. The plan administrator will record its acts and decisions, 
and keep all data, records, books of account and instruments pertaining to plan
administration, which will be subject to inspection or audit by Biogen at any
time. The employers will supply all information required by the plan
administrator to administer the plan, and the plan administrator may rely upon
the accuracy of such information.

  13.6   COMPENSATION AND EXPENSES. The plan administrator will serve without 
compensation unless otherwise determined by Biogen, provided that in no event
will an employee of an employer be compensated for his services as 




                                       23


<PAGE>   28

a plan administrator. All reasonable expenses of administering the plan will be
paid out of the trust fund unless paid by the employers at the option of Biogen
(with each employer bearing such share of the expenses that Biogen specifies).
Such expenses include the compensation of all persons employed or retained by
the plan administrator, premiums for bonds and insurance protecting the plan or
trust fund and required by law or deemed advisable by the plan administrator,
and all other costs of plan administration.

  13.7   DECISIONS, RULES AND REGULATIONS. When the plan administrator is a 
committee, any action or decision concurred in by a majority of the members,
either at a meeting or in writing without a meeting, will constitute an action
or decision of the plan administrator.

         No plan administrator may act or vote on any matter which relates
exclusively to himself.

         The plan administrator may adopt and amend such rules for the conduct
of its business and may adopt, promulgate and amend any and all such rules
relating to the administration of the plan, including, but not limited to, rules
relating to plan enrollment periods, forms and procedures, elections, notice
requirements, claims other rules and procedures it deems, with the discretionary
powers granted to it under Section 13.3 of this Article, advisable to administer
the plan.

  13.8   SECRETARY. If the plan administrator is a committee, the committee at 
its option may appoint any member or other person to serve as secretary, and may
remove him at any time. The committee will notify the trustee in writing of such
appointment, and the trustee may assume his appointment as secretary continues
until written notice to the contrary is given by the committee. The secretary,
or a majority of the committee members then in office, will have the authority
to execute all instruments or memoranda necessary or appropriate to carry out
the actions and decisions of the whole committee; and any person may rely upon
any instrument or memorandum so executed as evidence of the committee action or
decision indicated thereby.

  13.9   CLAIMS REVIEW PROCEDURE. Any request for benefits (the "CLAIM") by a
participant or his beneficiary (the "CLAIMANT") will be filed with the plan
administrator. Within a reasonable period after receipt of a claim, the plan
administrator will provide written notice to any claimant whose claim has been
wholly or partly denied, including: (a) the reasons for the denial, (b) the plan
provisions on which the denial is based, (c) any additional material or
information necessary to perfect the claim and the reasons it is necessary, and
(d) the plan's claims review procedure. A claimant will be given a full and fair
review by the plan administrator of the denial of his claim if he requests a
review in writing within 60 days after notification of the denial. The claimant
may review pertinent documents and may submit issues and comments orally, in
writing, or both. The plan administrator will render its decision on review
promptly and in writing and will include specific reasons for the decision and
reference to the plan provisions on which the decision is based.

                                   ARTICLE 14
                                  MISCELLANEOUS

  14.1   QUALIFIED DOMESTIC RELATIONS ORDERS.

         (a)   A qualified domestic relations order (QDRO) is a judgment, 
decree, or order which meets the requirements of Code Section 414(p). An
alternate payee is an individual named in the QDRO who is to receive some or all
of the participant's benefit.

         (b)   Upon receipt of any domestic relations order, the plan 
administrator will notify the participant involved and each alternate payee
under the order (and under any previous QDRO relating to the participant's
benefits). The plan administrator will determine whether the order is a QDRO and
will notify each affected individual of its determination. In general, subject
to the provisions of Code Section 414(p), the plan's claims 




                                       24



<PAGE>   29

procedure rules under Section 0 apply to this determination and any subsequent
determination relating to the order. To the extent permitted by law, the plan
administrator's determination that an order is or is not a QDRO is final. Any
subsequent change in this determination is applied only prospectively unless the
plan administrator rules otherwise.

         (c)   If an order is determined to be a QDRO, the provisions of the 
QDRO will take precedence over any conflicting provisions of the plan (including
Section 0 relating to non-alienation of benefits). To the extent provided in a
QDRO, a former spouse will be treated as the spouse or surviving spouse of a
participant for purposes of the death benefit provisions of Section 0 and any
other relevant provision of the plan. The plan administrator may carry out the
requirements of a QDRO and may make distribution to an alternate payee in
accordance with a QDRO regardless of the age of the participant and regardless
of whether the participant himself would be eligible to receive a distribution
at such time.

  14.2   NONALIENATION OF BENEFITS. No benefit, right or interest hereunder of 
any person will be subject to anticipation, alienation, sale, transfer,
assignment, pledge, encumbrance or charge, or to seizure, attachment or other
legal, equitable or other process, or be liable for, or subject to, the debts,
liabilities or other obligations of such person, except that the plan
administrator may prescribe rules for the payment of benefits in accordance with
a qualified domestic relations order as defined in Section 0.

  14.3   PAYMENT TO MINORS AND INCOMPETENTS. If the plan administrator deems any
person incapable of giving a binding receipt for benefit payments because of his
minority, illness, infirmity or other incapacity, it may direct payment directly
for the benefit of such person, or to any person selected by the plan
administrator to disburse it. Such payment, to the extent thereof, will
discharge all liability for such payment under the plan.

  14.4   CURRENT ADDRESS OF PAYEE; UNCLAIMED PAYMENTS. Any person entitled to 
benefits is responsible for keeping the plan administrator informed of his
current address at all times. The plan administrator, trustee and the employers
have no obligation to locate such person, and will be fully protected if all
payments and communications are mailed to his last known address as shown on the
records of his employer, or are withheld pending receipt of proof of his current
address and proof that he is alive.

         If a communication from the plan has been mailed to the participant's 
or beneficiary's last known address and is returned with no forwarding address
or if the check for a plan distribution is not cashed, and if the participant
(or his beneficiary) makes no claim for such benefit, the participant's benefit
will be forfeited and reallocated to other participants when a period of three
years less one day has elapsed since such communication or benefit payment check
was mailed. If the participant (or his beneficiary) subsequently makes a claim
for such benefit, the amount that was forfeited (without investment earnings or
gains or losses) will be restored and such amount will be distributed to the
participant (or beneficiary). Such restoration will be made first using any
available forfeitures and second (to the extent necessary) using a contribution
by the participant's employer.

  14.5   DISPUTES OVER ENTITLEMENT TO BENEFITS. If two or more persons claim 
entitlement to payment of the same benefit hereunder, the plan administrator in
its discretion may withhold payment of such benefit until the dispute has been
determined by a court of competent jurisdiction or has been settled by the
persons concerned.

  14.6   PAYMENT OF BENEFITS. Unless he elects otherwise (but subject to the 
requirements of Section 0), a participant's benefit payments under the plan will
begin no later than 60 days after the close of the plan year in which the latest
of the following dates occurs: (a) the date he terminates service with his
employer; (b) his 65th birthday; and (c) the tenth anniversary of the year in
which he began participating in the plan.



                                       25


<PAGE>   30

  14.7   TOP-HEAVY PLAN PROVISIONS.

         (a)   APPLICABILITY OF SECTION. This section is included in the plan 
to meet the requirements of Code Section 416, and the provisions of this section
will be operative only if, when and to the extent that Code Section 416 applies
to the plan. At such time as the requirements of Code Section 416 apply to the
plan because the plan is top-heavy as defined in subsection (b)(i) below, the
provisions of this section will apply and will govern over any contrary
provision of the plan.

         (b)   DEFINITIONS.

               (i)    The plan will be TOP-HEAVY for a plan year if, as of the 
determination date, the sum of the aggregate amount in the accounts of
participants who are key employees exceeds 60 percent of such amount determined
for all participants in this plan.

                      Notwithstanding the preceding paragraph, if the plan is
  included within a required or permissive aggregation group, the plan will be
  top heavy for a plan year if, as of the determination date, the sum of (A) the
  aggregate amount in the accounts of participants who are key employees
  (including all defined contribution plans within such group) and (B) the
  aggregate present value of cumulative accrued benefits of participants who are
  key employees (including all defined benefit plans within such group), exceeds
  60 percent of such amount determined for all participants in all such plans.

                      In determining the amounts in participants' accounts and
  present values of accrued benefits under the preceding two paragraphs, (V) the
  present value of accrued benefits will be based on the actuarial assumptions
  used to determine the minimum funding requirements of Code Section 412(b); if
  there is more than one defined benefit plan in the aggregation group, each
  plan will use the same actuarial assumptions for purposes of the top heavy
  test, as determined by the actuary; (W) distributions made during the five
  years ending on the determination date will be taken into account; (X)
  rollover contributions after December 31, 1983, will be taken into account
  only to the extent provided in regulations under Code Section 416(g)(4)(A);
  (Y) account balances and accrued benefit values of a person who was but no
  longer is a key employee will be disregarded; and (Z) account balances and
  accrued benefit values of any individual who has not performed any services
  for an employer (other than benefits under the plan) at any time during the
  five years ending on the determination date will be disregarded.

               (ii)   The DETERMINATION DATE for purposes of determining whether
the plan is top-heavy under subsection (i) for a particular plan year is the
last day of the preceding plan year. In the case of the first plan year, the
determination date is the last day of that year.


               (iii)  A KEY EMPLOYEE is any employee or former employee 
(including a beneficiary of such an employee) who at any time during the plan
year or any of the four preceding plan years was:

                      (A) an officer of an employer having annual compensation
         greater than 50% of the amount in effect under Section 415(b)(1)(A) of
         the Code for such plan year (but no more than the lesser of 50
         employees or 10% of all employees will be taken into account under this
         subsection (A) as key employees);

                      (B) one of the ten employees owning (or considered s
         owning within the meaning of Code Section 318) the largest interests in
         his employer (or any direct or indirect parent of his employer) but
         only if such employee's compensation for such plan year exceeds the
         amount specified in Code Section 415(c)(1)(A). For purposes of the
         preceding sentence, if two employees have the same interest in an
         employer, the employee having greater annual compensation from the
         employers shall be treated as having a larger interest;



                                       26


<PAGE>   31

                      (C) a person owning (or considered as owning within the
         meaning of Code Section 318) more than 5% of the outstanding stock of
         his employer (or any direct or indirect parent of his employer) or
         stock possessing more than 5% of the total combined voting power of all
         such stock; or

                      (D) a person who has annual compensation from his employer
         of more than $150,000 and who would be described in subsection (C)
         above if 1% were substituted for 5%.

  For purposes of applying Code Section 318 to the provisions of this subsection
  (iii), subparagraph (C) of Code Section 318(a)(2) will be applied by
  substituting "five percent" for "50 percent". In addition, the rules of Code
  Section 414 (b), (c) and (m) will not apply for purposes of determining
  ownership under subsections (C) and (D) above.

               (iv)   A NON-KEY EMPLOYEE is any employee in the plan (including 
  a beneficiary of such an employee) who is not a key employee under subsection
  (iii) above.

               (v)    A REQUIRED AGGREGATION GROUP includes all qualified plans
  of the employers in which a key employee participates and each other qualified
  plan of the employers that enables any of such plans to meet the requirements
  of Section 401(a)(4) or Section 410 of the Code. A PERMISSIVE AGGREGATION
  GROUP includes (in addition to plans in a required aggregation group) any plan
  which Biogen designates for inclusion provided that inclusion of such plan
  does not cause the group to fail the requirements of Section 401(a)(4) or
  Section 410 of the Code.

         (c)   MINIMUM CONTRIBUTION. For any plan year in which the plan is
top-heavy, the employers will make a minimum contribution on behalf of each
non-key employee who has satisfied the requirements of Section 0 (and who is
therefore eligible to make savings deposits) equal to 3% of his total
compensation (as defined in Section 0), or 4% of his total compensation if
necessary to prevent any reduction in the amount of annual additions to the
accounts of a key employee or accrued benefits of a key employee under Code
Section 415(e) as modified by Code Section 416(h). However, the minimum
contribution called for under the preceding sentence will not exceed the
contribution (determined as a percentage of his total compensation) for such
plan year under this plan (and any other defined contribution plan included in
an aggregation group with this plan) on behalf of the key employee for whom such
contribution is the highest. Also, such minimum contribution will take into
account any savings deposits and matching contributions by such employee during
such plan year, and will be reduced as permitted under regulations under Code
Section 416 to reflect contributions on behalf of or benefits accrued by such
non-key employee under any other plan maintained by the employers.

  14.8   RULES OF CONSTRUCTION.

         (a)   A word or phrase defined or explained in any section or article 
has the same meaning throughout the plan unless the context indicates otherwise.

         (b)   Where the context so requires, the masculine includes the 
feminine, the singular includes the plural, and the plural includes the
singular.

         (c)   Unless the context indicates otherwise, the words "herein", 
"hereof", "hereunder", and words of similar import refer to the plan as a whole
and not only to the section in which they appear.

  14.9   TEXT CONTROLS. Headings and titles are for convenience only, and the 
text will control in all matters.

  14.10  APPLICABLE STATE LAW. To the extent that state law applies, the 
provisions of the plan will be construed, enforced and administered according to
the laws of the Commonwealth of Massachusetts.



                                       27


<PAGE>   32

  14.11  PAPERLESS ADMINISTRATION. The plan administrator may establish 
procedures whereby an electronic or voice recognized authorization or election
or election made by a participant may be acceptable under the plan in lieu of
filing a written form as may otherwise be required under the plan. In such
event, any reference in the plan to a written form shall be deemed to include
such other authorization or election.

  14.12  CORRECTION OF MISTAKES IN PLAN OPERATION. If as a result of a mistake
in plan operation or administration (including by way of illustration and not by
way of limitation, the omission of an employee who should have become a
participant, the inclusion of an employee as a participant who should not have
become a participant, the crediting of the wrong amount to a participant's
accounts, and similar mistakes), the plan administrator may take such steps as
the plan administrator determines are necessary or proper to correct the mistake
(i.e., to put the affected participant(s) in the same position he would have
been in if the mistake had never occurred). In so doing, the plan administrator
may apply a correction methodology promulgated in any program of the IRS such as
the VCR, SVP or APSRC programs.

  14.13  VETERANS' RIGHTS. Notwithstanding any provision of this plan to the 
contrary, contributions and service credit with respect to qualified military
service will be provided in accordance with Code Section 41(u).

         Executed on November 17, 1997, to be effective as of January 1, 1997

                                         BIOGEN, INC.


                                         By: /s/ Frank A. Burke, Jr.
                                             ----------------------------------
                                             Frank A. Burke, Jr.
                                             Vice President - Human Resources





                                       28
<PAGE>   33




                                   APPENDIX A

                                 PLAN AMENDMENTS

    1.   The plan was amended by the First Amendment effective as of January 1,
         1987.

    2.   The plan was amended by the Second Amendment effective as of 
         December 30, 1989.

    3.   The plan was amended by the Third Amendment effective generally as of
         January 1, 1989.

    4.   The plan was amended by the Fourth Amendment effective as of 
         December 6, 1991.

    5.   The plan was amended by the Fifth Amendment effective generally as of
         April 1, 1994.

    6.   The plan was amended by the Sixth Amendment effective generally as of
         May 1, 1995.

    7.   The plan was amended by the Seventh Amendment effective as of 
         January 1, 1996

    8.   The plan was amended and restated in its entirety by the Eighth
         Amendment, effective as of January 1, 1997.



                                       29




<PAGE>   34


                            BIOGEN, INC. SAVINGS PLAN

                                 NINTH AMENDMENT


        The Biogen Savings Plan, as amended and restated by the Eighth
Amendment, is hereby further amended, effective as of the effective date
specified in Part B below, as follows:

        PART A:  PLAN AMENDMENTS

         1.    The first sentence of Section 6.3 is amended by deleting the
               words "employer stock" and by inserting in their place the words
               "Biogen stock".

         2.    The last three sentences of Section 6.4 are deleted and the
               following sentence is inserted in their place: "If an employer
               contribution is made in the form of Biogen stock under Section
               6.3, for purposes of determining the amount of such employer
               contribution, such Biogen stock will be valued at its fair market
               value at the time of contribution to the trust fund."

        3.     The second sentence of the first paragraph of Section 8.1 is
               amended by changing the period to a comma, and by adding
               thereafter the words "plus such shares of a money market mutual
               fund (or similar cash equivalent investment) as are necessary to
               facilitate the operation of such Biogen stock investment fund."

        4.     The first sentence of subsection 8.2(a)(ii) is amended in its
               entirety to read as follows: "A participant who is at least age
               55 and who has four or more years of plan service may elect to
               transfer all or any portion of his matching contributions account
               invested in the Biogen stock fund to one or more of the other
               separate investment funds available for the investment of basic
               and supplemental savings deposits accounts, and the participant
               may thereafter direct transfers of his matching contributions
               account among the investment funds available under the plan in
               accordance with the rules specified in subsection (b) below."

               Subsection 8.2(a)(ii) is further amended by deleting the last
               sentence of the first paragraph and by deleting the second
               paragraph.

        5.     Subsection 8.2(b) is amended by deleting the second paragraph.


<PAGE>   35

        6.     The last sentence of subsection 8.2(c) is amended in its entirety
               to read as follows: "A participant may give new investment
               directions, changing the investment of his future savings
               deposits or rollovers or future matching employer contributions
               on the participant's behalf, or directing transfers among the
               investment funds, subject in each case to subsections (a) and (b)
               above."

        7.     The first sentence of subsection 8.3(c) is amended in its
               entirety to read as follows: "Notwithstanding subsection (a)
               above, in the case of accounts that are invested in the Biogen
               stock fund, the valuation of such accounts will be performed
               separately from the valuation of the other accounts, and the
               value of a share of Biogen stock as of any valuation date will be
               its fair market value as determined in good faith by the trustee
               (subject to the following paragraph)."

        8.     The last sentence of subsection 10.4(c) is amended in its
               entirety to read as follows: "In the case of the Biogen stock
               fund, the dollar amount restored will equal the value of the fund
               units forfeited on the date of forfeiture and such amount will be
               reinvested in fund units at their fair market value in effect on
               the date of reinvestment; no particular number of fund units (or
               shares of Biogen stock) will be restored."

        9.     Section 10.7 is amended in its entirety to read as follows:

               "10.7 MANNER OF PAYMENT. Payments to a participant under the plan
               will be distributed in cash. Notwithstanding the preceding
               sentence, at a participant's election, the portion of the
               participant's accounts invested in the Biogen stock fund will be
               paid to him or her in the form of whole shares of Biogen stock
               (with the value of any fractional share paid in cash). The number
               of shares of Biogen stock paid will be based upon the fair market
               value of a share of Biogen stock determined by the trustee in
               good faith (subject to the second paragraph of Section 8.3(c)) as
               of the most recent practicable date preceding payment."

        10.    The second sentence of subsection 10.8(b)(i) is amended in its
               entirety to read as follows: "If the distributee receives a
               distribution in the form of Biogen stock and cash, the cash
               distribution may be directly rolled over to an eligible
               retirement plan if such cash distribution qualifies as an
               eligible rollover distribution, and the stock distribution may be
               retained by the distributee."

        11.    Appendix A is amended by adding the following new paragraph:

               "9.  The plan was amended by the Ninth Amendment, effective as 
               stated therein."



                                       2



<PAGE>   36

        PART B:  EFFECTIVE DATE

               The amendments in this Ninth Amendment will be effective as of
               such date as the plan administrator specifies, which shall not be
               earlier than the later of January 1, 1998 or the effective date
               of the registration of Biogen stock under the plan, as and to the
               extent required under the Securities Act of 1933, as amended, and
               regulations and rulings of the Securities and Exchange Commission
               thereunder, using Form S-8 (or such other form as may be
               required).

        Executed on December 12, 1997, to be effective as stated above.

                                             BIOGEN, INC.


                                             By: /s/ Frank A. Burke, Jr.
                                                 -------------------------------
                                                 Frank A. Burke, Jr.
                                                 Vice President-Human Resources






                                       3

<PAGE>   1


                                                                     Exhibit 4.6






                                 TRUST AGREEMENT

                                     Between

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


                                  BIOGEN, INC.

                                       And

                        FIDELITY MANAGEMENT TRUST COMPANY

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


                               BIOGEN SAVINGS PLAN

                                      TRUST












                          Effective as of April 1, 1994






                                        1

<PAGE>   2


                             TABLE OF CONTENTS

 SECTION                                                               PAGE
 -------                                                               ----

 1 TRUST.................................................................2


 2 EXCLUSIVE BENEFIT AND REVERSION OF SPONSOR CONTRIBUTIONS..............2


 3 DISBURSEMENTS.........................................................2
   (a) Directions from Administrator
   (b) Limitations

 4 INVESTMENT OF TRUST...................................................3
   (a) Selection of Investment Options
   (b) Available Investment Options
   (c) Participant Direction
   (d) Mutual Funds
   (e) Sponsor Stock
   (f) Notes
   (g) Reliance of Trustee Directions
   (h) Trustee Powers

 5 RECORDKEEPING AND ADMINISTRATIVE SERVICES TO BE PERFORMED............11
   (a) General
   (b) Accounts
   (c) Inspection and Audit
   (d) Effect of Plan Amendment
   (e) Returns, Reports and Information

 6 COMPENSATION AND EXPENSES............................................12

 7 DIRECTIONS AND INDEMNIFICATION.......................................13
   (a) Identity of Administrator and Named Fiduciary
   (b) Directions from Administrator
   (c) Directions from Named Fiduciary
   (d) Co-Fiduciary Liability
   (e) Indemnification
   (f) Survival

 8 RESIGNATION OR REMOVAL OF TRUSTEE....................................14
   (a) Resignation
   (b) Removal



                                       -i-

                                        2


<PAGE>   3

                             TABLE OF CONTENTS
                                (CONTINUED)

 SECTION                                                               PAGE
 -------                                                               ----



  9 SUCCESSOR TRUSTEE...................................................14

    (a) Appointment
    (b) Acceptance
    (c) Corporate Action

 10 TERMINATION.........................................................15

 11 RESIGNATION, REMOVAL, AND TERMINATION NOTICES.......................15

 12 DURATION............................................................16

 13 AMENDMENT OR MODIFICATION...........................................16

 14 GENERAL.............................................................16

    (a) Performance by Trustee, its Agents or Affiliates
    (b) Entire Agreement
    (c) Waiver
    (d) Successors and Assigns
    (e) Partial Invalidity
    (f) Section Headings

 15 GOVERNING LAW.......................................................17

    (a) Massachusetts Law Controls
    (b) Trust Agreement Controls

 Schedules
 ---------

    A. Recordkeeping and Administrative Services
    B. Fee Schedule
    C. Investment Options
    D. Sponsor's Authorization Letter
    E. Named Fiduciary's Authorization Letter
    F. IRS Detamination Letter or Opinion of Counsel
    G. Telephone Exchange Procedures


                                      -ii-


                                       3
<PAGE>   4



     TRUST AGREEMENT, effective as of the first day of April, 1994, between
 BIOGEN, INC., a Massachusetts corporation, having an offce at Fourteen
 Cambridge Center, Cambridge, MA 02142 (the "SPONSOR"), and FIDELITY MANAGEMENT
 TRUST COMPANY, a Massachusetts trust company, having an off'ce at 82 Devonshire
 Street, Boston, Massachusetts 02109 (the "TRUSTEE").

                                   WITNESSETH:

     WHEREAS, the Sponsor is the sponsor of the Biogen Savings Plan (the
 "PLAN"); and

     WHEREAS, the Sponsor has established a trust to hold and invest plan assets
 under the Plan for the exclusive benefit of participants in the Plan and their
 beneficiaries; and

     WHEREAS, pursuant to the provisions of the existing trust agreement, the
 Sponsor wishes to replace the incumbent trustee and to appoint the Trustee
 named above as successor trustee, and to amend and restate the existing trust
 agreement with the terms of this instrument, all effective as of April 1, 1994;
 and

     WHEREAS, the Sponsor (the "NAMED FIDUCIARY") is a named fiduciary of the
 Plan (within the meaning of section 402(a) of the Employee Retirement Income
 Security Act of 1974, as amended (ERISA)); and

     WHEREAS, the Trustee is willing to hold and invest the aforesaid plan
 assets in trust among several investment options selected by the Named
 Fiduciary; and

     WHEREAS, the Sponsor wishes to have the Trustee perform certain ministerial
 recordkeeping and administrative functions under the Plan; and

     WHEREAS, the Sponsor (the "ADMINISTRATOR") is the administrator of the Plan
 (within the meaning of section 3(16)(A) of ERISA); and

     WHEREAS, the Trustee is willing to perform recordkeeping and administrative
 services 



                                       4



<PAGE>   5

 for the Plan if the services are purely ministerial in nature and are provided
 within a framework of plan provisions, guidelines and interpretations conveyed
 in writing to the Trustee by the Administrator.

     NOW, THEREFORE, in consideration of the foregoing premises and the mutual
 covenants and agreements set forth below, the Sponsor and the Trustee agree as
 follows:

 SECTION 1. TRUST. The Sponsor hereby establishes the Biogen Savings Plan Trust
 (the "Trust"), with the Trustee. The Trust shall consist of an initial
 contribution of money and other property acceptable to the Trustee in its sole
 discretion transferred from the predecessor trustee under the Plan, such
 additional sums of money and Sponsor Stock (hereinafter defined) as shall from
 time to time be delivered to the Trustee under the Plan, all investments made
 therewith and proceeds thereof, and all earnings and profits thereon, less the
 payments that are made by the Trustee as provided herein, without distinction
 between principal and income. The Trustee hereby accepts the Trust on the terms
 and conditions set forth in this Agreement. In accepting this Trust, the
 Trustee shall be accountable for the assets received by it, subject to the
 terms and conditions of this Agreement. Trustee shall render services hereunder
 in accordance with applicable professional standards.

 SECTION 2. EXCLUSIVE BENEFIT AND REVERSION OF SPONSOR CONTRIBUTIONS. Except as
 provided under applicable law and the Plan, no part of the Trust may be used
 for, or diverted to, purposes other than the exclusive benefit of the
 participants in the Plan or their baneficiaries prior to the satisfaction of
 all liabilities under the Plan with respect to the participants and their
 baneficiaries.

 SECTION 3. DISBURSEIRNENTS.

         (a) DIRECTIONS FROM ADMINISTRATOR. The Trustee shall make disbursements
 in the amounts and in the manner that the Administrator directs from time to
 time in writing. The Trustee shall have no responsibility to ascertain any
 direction's compliance with the terms of the Plan or of any applicable law or
 the direction's effect for tax purposes or otherwise; nor shall the Trustee
 have any responsibility to see to the application of any disbursement.

         (b) LIMITATIONS. The Trustee shall not be required to make any
 disbursement in 




                                       5

<PAGE>   6

 excess of the net realizable value of the assets of the Trust at the time of
 the disbursement. The Trustee shall not be required to make any disbursement in
 cash unless the Administrator has provided a written direction as to the assets
 to be converted to cash for the purpose of making the disbursement.

 SECTION 4. INVESTMENT OF TRUST.

         (a) SELECTION OF INVESTMENT OPTIONS. The Trustee shall have no
 responsibility for the selection of investment options under the Trust and
 shall not render investment advice to any person in connection with the
 selection of such options.

         (b) AVAILABLE INVESTMENT OPTIONS. The Named Fiduciary shall direct the
 Trustee as to what investment options: (i) the Trust shall be invested in
 during the period beginning on the initial transfer of assets to the Trust and
 ending on the completion of the reconciliation of participant records
 (nparticipant recordkeeping reconciliation period,n) and (ii) the investment
 options in which Plan participants may invest in, subject to the following
 limitations. The Named Fiduciary may determine to offer as investment options
 only (i) securities issued by the investment companies advised by Fidelity
 Management & Research Company ("Mutual Funds"), (ii) equity securities issued
 by the Sponsor or an aff~liate which are publicly-traded and which are
 "qualifying employer securities" within the meaning of section 407(d)(5) of
 ERISA ("Sponsor Stock"), (iii) notes evidencing loans to Plan participants in
 accordance with the terms of the Plan, (iv) guaranteed investment contracts
 chosen by the Trustee and (v) collective investment funds maintained by the
 Trustee for qualified plans; provided, however, that the Trustee shall be
 considered a fiduciary with investment discretion only with respect to Plan
 assets that are invested in guaranteed investment contracts chosen by the
 Trustee or in collective investment funds maintained by the Trustee for
 qualified plans. The investment options initially selected by the Named
 Fiduciary are identified on Schedules "A" and "C" attached hereto. The Named
 Fiduciary may change or add additional investment options with the consent of
 the Trustee and upon mutual amendment of this Trust Agreement and the Schedules
 thereto to reflect such additions.

         (c) PARTICIPANT DIRECTION. Subject to any limitations imposed by the
 Sponsor (which limitations shall be noted on Schedules "A" and "C" hereto),
 each Plan participant shall direct the Trustee in which investment option(s) to
 invest the assets in the participant's individual 



                                       6

<PAGE>   7

 accounts. Such directions may be made by Plan participants by use of the
 telephone exchange system maintained for such purposes by the Trustee or its
 agent, in accordance with written Telephone Exchange Guidelines attached hereto
 as Schedule "G". A participant shall be considered a named fiduciary of the
 Plan under ERISA for purposes of using the telephone exchange system to provide
 investment directions to the Trustee for the participant's individual account.
 In the event that the Trustee fails to receive a proper direction, the assets
 shall be invested in the securities of the Mutual Fund set forth for such
 purpose on Schedule "C", until the Trustee receives a proper direction.

         (d) MUTUAL FUNDS. The Sponsor hereby acknowledges that it has received
 from the Trustee a copy of the prospectus for each Mutual Fund selected by the
 Named Fiduciary as a Plan investment option. Trust investments in Mutual Funds
 shall be subject to the following limitations:

             (i) EXECUTION OF PURCHASES AND SALES. Purchases and sales of Mutual
 Funds (other than for Exchanges) shall be made on the date on which the Trustee
 receives from the Sponsor in good order all information and documentation
 necessary to accurately effect such purchases and sales (or in the case of a
 purchase, the subsequent date on which the Trustee has received a wire transfer
 of funds necessary to make such purchase). Exchanges of Mutual Funds shall be
 made in accordance with the Telephone Exchange Guidelines attached hereto as
 Schedule "G".

             (ii) VOTING. At the time of mailing of notice of each annual or
 special stockholders' meeting of any Mutual Fund, the Trustee shall send a copy
 of the notice and all proxy solicitation materials to each Plan participant who
 has shares of the Mutual Fund credited to the participant's accounts, together
 with a voting direction form for return to the Tmstee or its designee. The
 participant shall have the right to direct the Trustee as to the manner in
 which the Trustee is to vote the shares credited to the participant's accounts.
 The Trustee shall vote the shares as directed by the participant. The Trustee
 shall not vote shares for which it has received no directions from the
 participant. During the participant recordkeeping reconciliation period, the
 Sponsor shall have the right to direct the Trustee as to the manner in which
 the Trustee is to vote the shares of the Mutual Funds in the Trust. With
 respect to all rights other than the right to vote, the Trustee shall follow
 the directions of the participant and if no such directions are 




                                       7

<PAGE>   8

 received, the directions of the Named Fiduciary. Except as provided in the
 first sentence of this subsection (d)(ii), the Trustee shall have no duty to
 solicit directions from participants or the Sponsor.

         (e) SPONSOR STOCK. Trust investments in Sponsor Stock shall be
 subject to the following limitations:

              (i) ACQUISITION LIMIT. Pursuant to the Plan, the Trust shall be 
 invested in Sponsor Stock to the extent necessary to invest all employer
 matching contributions accounts in Sponsor Stock. In order to accomplish this,
 up to 99 percent of the Trust's assets may be invested in Sponsor Stock.

             (ii) FIDUCIARY DUTY OF NAMED FIDUCIARY. The Named Fiduciary shall
 monitor the suitability under the fiduciary duty rules of section 404(a)(1) of
 ERISA (as modified by section 404(a)(2) of ER1SA) of acquiring and holding
 Sponsor Stock. The Trustee shall not be liable for any loss, or by reason of
 any breach, which arises from the directions of the Named Fiduciary with
 respect to the acquisition and holding of Sponsor Stock, unless it is clear on
 their face that the actions to be taken under those directions would be
 prohibited by the foregoing fiduciary duty rules or would be contrary to the
 terms of the Plan or this Agreement.

            (iii) EXECUTION OF PURCHASES AND SALES.

                 (A) Purchases and sales of Sponsor Stock (other than for
 exchanges) shall be made on the open market on the date on which the Trustee
 receives from the Sponsor in good order all information and documentation
 necessary to accurately effect such purchases and sales (or, in the case of
 purchases, the subsequent date on which the Trustee has received a wire
 transfer of the funds necessary to make such purchases). Exchanges of Sponsor
 Stock are only permitted at the Sponsor's direction. Such general rules shall
 not apply in the following circumstances:

                     (1) If without fault on its own part, the Trustee is unable
 to determine the number of shares required to be purchased or sold on such day;
 or



                                       8


<PAGE>   9

                     (2) If the Trustee is unable to purchase or sell the total
 number of shares required to be purchased or sold on such day as a result of
 market conditions; or

                     (3) If the Trustee is prohibited by the Securities and
 Exchange Commission, the New York Stock Exchange, or any other regulatory body
 from purchasing or selling any or all of the shares required to be purchased or
 sold on such day.

 In the event of the occurrence of the circumstances described in (1), (2), or
 (3) above, the Trustee shall purchase or sell such shares as soon as possible
 thereafter and shall determine the price of such purchases or sales to be the
 average purchase or sales price of all such shares purchased or sold,
 respectively. The Trustee may follow directions from the Named Fiduciary to
 deviate from the above purchase and sale procedures provided that such
 direction is made in writing by the Named Fiduciary.

                 (B) USE OF AN AFFILIATED BROKER. The Sponsor hereby authorizes
 the Trustee to use Fidelity Brokerage Services, Inc. ("FBSI") to provide
 brokerage services in connection with any purchase or sale of Sponsor Stock in
 accordance with directions from Plan participants. FBSI shall execute such
 directions directly or through its aff'liate, National Financial Services
 Company ("NFSC"). The provision of brokerage services shall be subject to the
 following:

                     (i) As consideration for such brokerage services, the
 Sponsor agrees that FBSI shall be entitled to remuneration under this
 authorization provision in the amount of five cents ($.05) commission on each
 share of Sponsor Stock up to 10,000 shares in a singular transaction, four
 cents ($.04) commission on each share of Sponsor Stock from 10,001 to 20,000
 shares in a singular transaction, and three and one-half cents ($.035)
 commission on each share of Sponsor Stock in excess of 20,000 shares in a
 singular transaction. Any change in such remuneration may be made only by a
 signed agreement between Sponsor and Trustee.

                     (ii) Following the procedures set forth in Department of
 Labor Prohibited Transaction Class Exemption 86-128, the Trustee will provide
 the Sponsor with the following documents: (1) a description of FBSI's brokerage
 placement practices; (2) a copy of 



                                       9


<PAGE>   10

 PTCE 86-128; and (3) a form by which the Sponsor may terminate this
 authorization to use a broker aff'liated with the Trustee. The Trustee will
 provide the Sponsor with this termination form annually, as well as an annual
 report which summarizes all securities transaction-related charges incurred by
 the Plan, and the Plan's annualized turnover rate.

                   (iii) Any successor organization of FBSI, through
 reorganization, consolidation, merger or similar transactions, shall, upon
 consummation of such transaction, become the successor broker in accordance
 with the terms of this authorization provision.

                    (iv) The Trustee and FBSI shall continue to rely on this
 authorization provision until not)fied to the contrary. The Sponsor reserves
 the right to terminate this authorization upon sixty (60) days written notice
 to FBSI (or its successor) and the Trustee, in accordance with Section 11 of
 this Agreement.

                     (v) SECURITIES LAW REPORTS. The Named Fiduciary shall be
 responsible for filing all reports required under Federal or state securities
 laws with respect to the Trust's ownership of Sponsor Stock, including, without
 limitation, any reports required under section 13 or 16 of the Securities
 Exchange Act of 1934, and shall immediately notify the Trustee in writing of
 any requirement to stop purchases or sales of Sponsor Stock pending the filing
 of any report. The Trustee shall provide to the Named Fiduciary such
 information on the Trust's ownership of Sponsor Stock as the Named Fiduciary
 may reasonably request in order to comply with Federal or state securities
 laws.

                    (vi) VOTING AND TENDER OFFERS. Notwithstanding any other
 provision of this Agreement the provisions of this Section shall govern the
 voting and tendering of Sponsor Stock. The Sponsor, after consultation with the
 Trustee, shall provide and pay for all printing, mailing, tabulation and other
 costs associated with the voting and tendering of Sponsor Stock.

                 (A) VOTING.

                     (1) When the issuer of the Sponsor Stock prepares for any
 annual meeting, the Sponsor shall notify the Trustee thirty (30) days in
 advance of the intended 





                                       10

<PAGE>   11

 meeting date and shall cause a copy of all proxy solicitation materials to be
 sent to the Trustee. Based on these materials the Trustee shall prepare a
 voting instruction form. At the time of mailing of notice of each annual or
 special stockholders' meeting of the issuer of the Sponsor Stock, the Sponsor
 shall cause a copy of the notice and all proxy solicitation materials to be
 sent to each Plan participant, together with the foregoing voting instruction
 form to be returned to the Trustee or its designee. The form shall show the
 number of full and fractional shares of Sponsor Stock credited to the
 participant's accounts (as provided by the Trustee). The Sponsor shall provide
 the Trustee with a copy of any materials provided to the participants and shall
 certify to the Trustee that the materials have been mailed or otherwise sent to
 participants.

                     (2) Each participant shall have the right to direct the
 Trustee as to the manner in which the Trustee is to vote that number of shares
 of Sponsor Stock credited to the participant's accounts (both vested and
 unvested). Directions from a participant to the Trustee concerning the voting
 of Sponsor Stock shall be communicated in writing, or by mailgram or similar
 means. These directions shall be held in confidence by the Trustee and shall
 not be divulged to the Sponsor, or any off'cer or employee thereof, or any
 other person. Upon its receipt of the directions, the Trustee shall vote the
 shares of Sponsor Stock as directed by the participant. The Trustee shall not
 vote shares of Sponsor Stock credited to a participant's accounts for which it
 has received no directions from the participant.

                 (B) TENDER OFFERS.

                     (1) Upon commencement of a tender offer for any securities
 held in the Trust that are Sponsor Stock, the Sponsor shall notify each Plan
 participant of the tender offer and utilize its best efforts to timely
 distribute or cause to be distributed to the participant the same information
 that is distributed to shareholders of the issuer of Sponsor Stock in
 connection with the tender offer, and, after consulting with the Trustee, shall
 provide and pay for a means by which the participant may direct the Trustee
 whether or not to tender the Sponsor Stock credited to the participant's
 accounts (both vested and unvested). The Sponsor shall provide the Trustee with
 a copy of any material provided to the participants and shall certify to the
 Trustee that the materials have been mailed or otherwise sent to participants.



                                       11



<PAGE>   12

                     (2) Each participant shall have the right to direct the
 Trustee to tender or not to tender some or all of the shares of Sponsor Stock
 credited to the participant's accounts (both vested and unvested). Directions
 from a participant to the Trustee concerning the tender of Sponsor Stock shall
 be communicated in writing, or by mailgram or such similar means as is agreed
 upon by the Trustee and the Sponsor under the preceding paragraph. These
 directions shall be held in confidence by the Trustee and shall not be divulged
 to the Sponsor, or any officer or employee thereof, or any other person except
 to the extent that the consequences of such directions are reflected in reports
 regularly communicated to any such persons in the ordinary course of the
 performance of the Trustee's services hereunder. The Trustee shall tender or
 not tender shares of Sponsor Stock as directed by the participant. The Trustee
 shall not tender shares of Sponsor Stock credited to a participant's accounts
 for which it has received no directions from the participant.

                     (3) A participant who has directed the Trustee to tender
 some or all of the shares of Sponsor Stock credited to the participant's
 accounts may, at any time prior to the tender offer withdrawal date, direct the
 Trustee to withdraw some or all of the tendered shares, and the Trustee shall
 withdraw the directed number of shares from the tender offer prior to the
 tender offer withdrawal deadline. A participant shall not be limited as to the
 number of directions to tender or withdraw that the participant may give to the
 Trustee.

                     (4) A direction by a participant to the Trustee to tender
 shares of Sponsor Stock 

                     (5) credited to the participant's accounts shall not be
 considered a written election under the Plan by the participant to withdraw, or
 have distributed, any or all of his withdrawable shares. The Trustee shall
 credit to each account of the participant from which the tendered shares were
 taken the proceeds received by the Trustee in exchange for the shares of
 Sponsor Stock tendered from that account. Pending receipt of directions
 (through the administrator) from the participant or the Named Fiduciary, as
 provided in the Plan, as to which of the remaining investment options the
 proceeds should be invested in, the Trustee shall invest the proceeds in the
 Mutual Fund described in Schedule "C". 

                     (vii) SHARES CREDITED. For all purposes of this Section,
 the number of shares of Sponsor Stock deemed "credited" to a participant's
 accounts shall be determined as of the last preceding valuation date.






                                       12

<PAGE>   13

                     (viii) GENERAL. With respect to all rights other than the
 right to vote, the right to tender, and the right to withdraw shares previously
 tendered, in the case of Sponsor Stock credited to a participant's accounts,
 the Trustee shall follow the directions of the participant and if no such
 directions are received, the directions of the Named Fiduciary. The Trustee
 shall have no duty to solicit directions from participants.

                     (ix) CONVERSION. All provisions in this Section 4(e) shall
 also apply to any securities received as a result of a conversion of Sponsor
 Stock.

         (f) NOTES. The Administrator shall act as the Trustee's agent for the
 purpose of holding all trust investments in participant loan notes and related
 documentation and as such shall (i) hold physical custody of and keep safe the
 notes and other loan documents, (ii) collect and remit all principal and
 interest payments to the Trustee, (iii) keep the proceeds of such loans
 separate from the other assets of the Administrator and clearly identify such
 assets as Plan assets, and (iv) cancel and surrender the notes and other loan
 documentation when a loan has been paid in full. To originate a participant
 loan, the Plan participant shall direct the Trustee as to the type of loan to
 be made from the participant's individual account. Such directions shall be
 made by Plan participants by use of the telephone exchange system maintained
 for such purpose by the Trustee or its agent. The Trustee shall determine,
 based on the current value of the participant's account, the amount available
 for the loan. Based on the monthly interest rate supplied by the Sponsor in
 accordance with the terms of the Plan, the Trustee shall advise the participant
 of such interest rate, as well as the installment payment amounts. The Trustee
 shall forward the loan document to the participant for execution and submission
 for approval to the Administrator. The Administrator shall have the
 responsibility for approving the loan and instructing the Trustee to send the
 loan proceeds to the Administrator or to the participant if so directed by the
 Administrator. In all cases, such instruction by the Administrator shall be
 made within thirty (30) days of the participant's initial request (the
 origination date).

         (g) RELIANCE OF TRUSTEE ON DIRECTIONS.




                                       13


<PAGE>   14

             (i) The Trustee shall not be liable for any loss, or by reason of
 any breach, which arises from any participant's exercise or non-exercise of
 rights under this Section 4 over the assets in the participant's accounts.

             (ii) The Trustee shall not be liable for any loss, or by reason of
 any breach, which arises from the Named Fiduciary's exercise or non-exercise of
 rights under this Section 4, unless it was clear on their face that the actions
 to be taken under the Named Fiduciary's directions were prohibited by the
 fiduciary duty rules of section 404(a) of ERISA or were contrary to the terms
 of the Plan or this Agreement.

         (h) TRUSTEE POWERS. The Trustee shall have the following
 powers and authority:

             (i) Subject to paragraphs (b), (c), (d) and (e) of this Section 4,
 to sell, exchange, convey, transfer, or otherwise dispose of any property held
 in the Trust, by private contract or at public auction. No person dealing with
 the Trustee shall be bound to see to the application of the purchase money or
 other property delivered to the Trustee or to inquire into the validity,
 expediency, or propriety of any such sale or other disposition.

             (ii) Subject to paragraphs (b) and (c) of this Section 4, to invest
 in guaranteed investment contracts and short term investments (including
 interest bearing accounts with the Trustee or money market mutual funds advised
 by aff~liates of the Trustee) and in collective investment funds maintained by
 the Trustee for qualified plans, in which case the provisions of each
 collective investment fund in which the Trust is invested shall be deemed
 adopted by the Sponsor and the provisions thereof incorporated as a part of
 this Trust as long as the fund remains exempt &om taxation under Sections
 401(a) and 501(a) of the Internal Revenue Code of 1986, as amended.

             (iii) To cause any securities or other property held as part of the
 Trust to be registered in the Trustee's own name, in the name of one or more of
 its nominees, or in the Trustee's account with the Depository Trust Company of
 New York and to hold any investments in bearer form, but the books and records
 of the Trustee shall at all times show that all such investments are part of
 the Trust.



                                       14


<PAGE>   15

             (iv) To keep that portion of the Trust in cash or cash balances as
 the Named Fiduciary or Administrator may, from time to time, specifically
 instruct the Trustee.

             (v) To make, execute, acknowledge, and deliver any and all
 documents of transfer or conveyance and to carry out the powas herein granted.

             (vi) To settle, compromise, or submit to arbitration any claims,
 debts, or damages due to or arising from the Trust; to commence or defend suits
 or legal or administrative proceedings; to represent the Trust in all suits and
 legal and administrative hearings; and to pay all reasonable expenses arising
 from any such action, from the Trust if not paid by the Sponsor.

             (vii) With prior written approval from the Sponsor to employ legal,
 accounting, clerical, and other assistance as may be required in carrying out
 the provisions of this Agreement and to pay their reasonable expenses and
 compensation from the Trust if not paid by the Sponsor.

             (viii) To do all other acts although not specifically mentioned
 herein, as the Trustee may deem necessary to carry out any of the foregoing
 powers and the purposes of the Trust.

SECTION 5. RECORDKEEPING AND ADMINISTRATIVE SERVICES TO BE PERFORMED.

          (a) GENERAL. The Trustee shall perform those recordkeeping and
 administrative functions described in Schedule "A" attached hereto. These
 recordkeeping and administrative functions shall be performed within the
 framework of the Administrator's written directions regarding the Plan's
 provisions, guidelines and interpretations.

          (b) ACCOUNTS. The Trustee shall keep accurate accounts of all
 investments, receipts, disbursements, and other transactions hereunder, and
 shall report the value of the assets held in the Trust as of the last day of
 each fiscal quarter of the Plan and, if not on the last day of a fiscal
 quarter, the date on which the Trustee resigns or is removed as provided in
 Section 8 of this Agreement or is terminated as provided in Section 10 (a
 "Reporting Date"). Within thirty (30) days following each Reporting Date or
 within sixty (60) days in the case of a Reporting Date caused by the
 resignation or removal of the Trustee, or the termination of this Agreement,
 the 




                                       15


<PAGE>   16

 Trustee shall file with the Administrator a written account setting forth all
 investments, receipts, disbursements, and other transactions effected by the
 Trustee between the Reporting Date and the prior Reporting Date, and setting
 forth the value of the Trust as of the Reporting Date. Except as otherwise
 required under ERISA, upon the expiration of six (6) months from the date of
 filing such account with the Administrator, the Trustee shall have no liability
 or further accountability to anyone with respect to the propriety of its acts
 or transactions shown in such account, except with respect to such acts or
 transactions as to which the Sponsor shall within such six (6) month period
 file with the Trustee written objections.

          (c) INSPECTION AND AUDIT. All records generated by the Trustee in
 accordance with paragraphs (a) and (b) shall be open to inspection and audit,
 during the Trustee's regular business hours prior to the termination of this
 Agreement, by the Administrator or any person designated by the Administrator.
 Upon the resignation or removal of the Trustee or the termination of this
 Agreement, the Trustee shall provide to the Administrator, at no expense to the
 Sponsor, in the format regularly provided to the Administrator, a statement of
 each participant's accounts including trial balance and/or normal
 administrative reports as of the resignation, removal, or termination, and the
 Trustee shall provide to the Administrator or the Plan's new recordkeeper such
 further records as are reasonable, at the Sponsor's expense.

          (d) EFFECT OF PLAN AMENDMENT. A confirmation of the current qualified
 status of the Plan is attached hereto as Schedule FN. The Trustee's provision
 of the recordkeeping and administrative services set forth in this Section 5
 shall be conditioned on the Sponsor delivering to the Trustee a copy of any
 amendment to the Plan as soon as administratively feasible following the
 amendment's adoption, with, if requested, an IRS determination letter or an
 opinion of counsel substantially in the form of Schedule NF covering such
 amendment, and on the Administrator providing the Trustee on a timely basis
 with all the information the Administrator deems necessary for the Trustee to
 perform the recordkeeping and administrative services and such other
 information as the Trustee may reasonably request.

          (e) RETURNS, REPORTS AND INFORMATION. The Administrator shall be
 responsible for the preparation and filing of all returns, reports, and
 information required of the Trust or Plan by law. The Trustee shall provide the
 Administrator with such information as the Administrator may reasonably request
 to make these filings. Unless otherwise agreed upon by the Sponsor and 




                                       16


<PAGE>   17

 the Trustee, the Administrator shall also be responsible for making any
 disclosures to Participants required by law except, such disclosures as may be
 required under federal or state truth-in-lending laws with regard to
 Participant loans which shall be provided by the Trustee.

 SECTION 6. COMPENSATION AND EXPENSES. Within thirty (30) days of receipt of the
 Trustee's bill, which shall be computed and billed in accordance with Schedule
 "B" attached hereto and made a part hereof, as amended from time to time, the
 Sponsor shall send to the Trustee a payment in such amount or the Sponsor may
 direct the Trustee to deduct such amount from participants' accounts. All
 expenses of the Trustee relating directly to the acquisition and disposition of
 investments constituting part of the Trust, and all taxes of any kind
 whatsoever that may be levied or assessed under existing or future laws upon or
 in respect of the Trust or the income thereof, shall be a charge against and
 paid from the appropriate Plan participants' accounts.

 SECTION 7. DIRECTIONS AND INDEMNIFICATION.

          (a) IDENTITY OF ADMINISTRATOR AND NAMED FIDUCIARY. The Trustee shall
 be fully protected in relying on the fact that the Named Fiduciary and the
 Administrator under the Plan are the individuals or persons named as such above
 or such other individuals or persons as the Sponsor may notify the Trustee in
 writing.

          (b) DIRECTIONS FROM ADMINISTRATOR. Whenever the Administrator provides
 a direction to the Trustee, the Trustee shall not be liable for any loss, or by
 reason of any breach, arising from the direction if the direction is contained
 in a writing (or is oral and immediately confirmed in a writing) signed by any
 individual whose name and signature have been submitted (and not withdrawn) in
 writing to the Trustee by the Administrator in the form attached hereto as
 Schedule "D", provided the Trustee reasonably believes the signature of the
 individual to be genuine, unless it is clear on the direction's face that the
 actions to be taken under the direction would be prohibited by the fiduciary
 duty rules of ERISA or would be contrary to the terms of the plan or this
 Agreement or would otherwise be improper. Such direction may also be made via
 EDT in accordance with procedures agreed to by the Administrator and the
 Trustee; provided, however, that the Trustee shall be fully protected in
 relying on such direction as if it were a direction made in writing by the
 Administrator. The Trustee shall have no responsibility to ascertain any
 direction's (i) accuracy, (ii) compliance with the terms of the Plan or any
 applicable law, or (iii) effect for tax purposes or otherwise.




                                       17


<PAGE>   18

          (c) DIRECTIONS FROM NAMED FIDUCIARY. Whenever the Named Fiduciary or
 Sponsor provides a direction to the Trustee, the Trustee shall not be liable
 for any loss, or by reason of any breach, arising from the direction (i) if the
 direction is contained in a writing (or is oral and immediately confirmed in a
 writing) signed by any individual whose name and signature have been submitted
 (and not withdrawn) in writing to the Trustee by the Named Fiduciary in the
 form attached hereto as Schedule "E" and (ii) if the Trustee reasonably
 believes the signature of the individual to be genuine, unless it is clear on
 the direction's face that the actions to be taken under the direction would be
 prohibited by the fiduciary duty rules of ERISA or would be contrary to the
 terms of the Plan or this Agreement or would otherwise be improper.

          (d) CO-FIDUCIARY LIABILITY. In any other case, the Trustee shall not
 be liable for any loss, or by reason of any breach, arising from any act or
 omission of another fiduciary under the Plan except as provided in section
 405(a) of ERISA.

          (e) INDEMNIFICATION. The Sponsor shall indemnify the Trustee against,
 and hold the Trustee harmless from, any and all loss, damage, penalty,
 liability, cost, and expense, including without limitation, reasonable
 attorneys' fees and disbursements, that may be incurred by or, imposed upon the
 Trustee by reason of any claim, regulatory proceeding, or litigation arising
 from any act done or omitted to be done by any individual or person with
 respect to the Plan or Trust, excepting only any and all loss, etc., arising
 from the mistake, negligence or bad faith of the Trustee (or any agent
 affiliate or contractor of the Trustee performing the Trustee's duties
 hereunder) or breach by the Trustee of the terms of this Agreement.

          (f) SURVIVAL. The provisions of this Section 7 shall survive the
 termination of this Agreement.

SECTION 8. RESIGNATION OR REMOVAL OF TRUSTEE.

          (a) RESIGNATION. The Trustee may resign at any time upon sixty (60)
 days' notice in writing to the Sponsor, unless a shorter period of notice is
 agreed upon by the Sponsor.




                                       18


<PAGE>   19

          (b) REMOVAL. The Sponsor may remove the Trustee at any time upon sixty
 (60) days' notice in writing to the Trustee, unless a shorter period of notice
 is agreed upon by the Trustee.

 SECTION 9. SUCCESSOR TRUSTEE.

          (a) APPOINTMENT. If the office of Trustee becomes vacant for any
 reason, the Sponsor may in writing appoint a successor trustee under this
 Agreement. The successor trustee shall have all of the rights, powers,
 privileges, obligations, duties, liabilities, and immunities granted to the
 Trustee under this Agreement. The successor trustee and predecessor trustee
 shall not be liable for the acts or omissions of the other with respect to the
 Trust.

          (b) ACCEPTANCE. When the successor trustee accepts its appointment
 under this Agreement in writing, title to and possession of the Trust assets
 shall immediately vest in the successor trustee without any further action on
 the part of the predecessor trustee. The predecessor trustee shall execute all
 instruments and do all acts that reasonably may be necessary or reasonably may
 be requested in writing by the Sponsor or the successor trustee to vest title
 to all Trust assets in the successor trustee or to deliver all Trust assets to
 the successor trustee.

          (c) RECORDKEEPING. If as a result of the appointment of a successor
 trustee hereunder, the Trustee is no longer performing recordlceeping and
 administrative services under Section S hereof, the Trustee will furnish copies
 of plan records to the Sponsor or to a successor recordkeeper designated by the
 Sponsor, and will cooperate as reasonably requested by the Sponsor in effecting
 the transition of recordkeeping functions to the successor recordkeeper.

          (d) CORPORATE ACTION. Any successor of the Trustee or successor
 trustee, through sale or transfer of the business or trust department of the
 Trustee or successor trustee, or through reorganization, consolidation, or
 merger, or any similar transaction, shall, upon consummation of the
 transaction, become the successor trustee under this Agreement.

 SECTION 10. TERMINATION. This Agreement may be terminated at any time by the
 Sponsor upon sixty (60) days' notice in writing to the Trustee. On the date of
 the termination of this 




                                       19


<PAGE>   20

 Agreement, the Trustee shall forthwith transfer and deliver to such individuals
 or entities as the Sponsor shall designate, all cash and assets then
 constituting the Trust and copies of all records and data relating to the
 Trustee's recordkeeping and administrative services under Section S hereof. If,
 by the termination date, the Sponsor has not notified the Trustee in writing as
 to whom the assets and cash are to be transferred and delivered, the Trustee
 may bring an appropriate action or proceeding for leave to deposit the assets
 and cash in a court of competent jurisdiction. The Trustee shall be reimbursed
 by the Sponsor for all costs and expenses of the action or proceeding
 including, without limitation, reasonable attorneys' fees and disbursements.

 SECTION 11. RESIGNATION, REMOVAL, AND TERMINATION NOTICES. All notices of
 resignation, removal, or termination under this Agreement must be in writing
 and mailed to the party to which the notice is being given by certified or
 registered mail, return receipt requested, or delivered via a receipted
 delivery method, to the Sponsor c/o Vice President-General Counsel, Biogen,
 Inc., Fourteen Cambridge Center, Cambridge, MA 02142, and to the Trustee c/o
 John M. Kimpel, Fidelity Investments, 82 Devonshire Street, Boston,
 Massachusetts 02109, or to such other individuals addresses as the parties have
 notified each other of in the foregoing manner.

 SECTION 12. DURATION. This Trust shall continue in effect without limit
 as to time, subject, however, to the provisions of this Agreement
 relating to amendment, modification, and termination thereof.

 SECTION 13. AMENDMENT OR MODIFICATION. This Agreement may be amended or
 modified at any time and from time to time only by an instrument executed by
 both the Sponsor and the Trustee. Notwithstanding the foregoing, to reflect
 increased operating costs the Trustee may once each calendar year (beginning
 April 1, 1996) amend Schedule "B" without the Sponsor's consent upon
 seventy-five (75) days advance written notice to the Sponsor.

SECTION 14. GENERAL.

          (a) PERFORMANCE BY TRUSTEE. ITS AGENTS OR AFFILIATES. The Sponsor
 acknowledges and authorizes that the services to be provided under this
 Agreement shall be provided by the Trustee, its agents or afffiliates,
 including Fidelity Investments Institutional Operations 



                                       20


<PAGE>   21

 Company or its successor, and that certain of such services may be provided
 pursuant to one or more other contractual agreements or relationships.

          (b) ENTIRE AGREEMENT. This Agreement contains all of the terms
 agreed upon between the parties with respect to the subject matter hereof.

          (c) WAIVER. No waiver by either party of any failure or refusal to
 comply with an obligation hereunder shall be deemed a waiver of any other or
 subsequent failure or refusal to so comply.

          (d) SUCCESSORS AND ASSIGNS. The stipulations in this Agreement shall
 inure to the benefit of, and shall bind, the successors and assigns of the
 respective parties.

          (e) PARTIAL INVALIDITY. If any term or provision of this Agreement or
 the application thereof to any person or circumstances shall, to any extent, be
 invalid or unenforceable, the remainder of this Agreement, or the application
 of such term or provision to persons or circumstances other than those as to
 which it is held invalid or unenforceable, shall not be affected thereby, and
 each term and provision of this Agreement shall be valid and enforceable to the
 fullest extent permitted by law.

          (f) SECTION HEADINGS. The headings of the various sections and
 subsections of this Agreement have been inserted only for the purposes of
 convenience and are not part of this Agreement and shall not be deemed in any
 mannerto modify, explain, expand or restrict any of the provisions of this
 Agreement.

 SECTION 15. GOVERNING LAW.

          (a) MASSACHUSETTS LAW CONTROLS. This Agreement is being made in the
 Commonwealth of Massachusetts, and the Trust shall be administered as a
 Massachusetts trust. The validity, construction, effect, and administration of
 this Agreement shall be governed by and interpreted in accordance with the laws
 of the Commonwealth of Massachusetts, except to the extent those laws are
 superseded under section 514 of ERISA.



                                       21



<PAGE>   22

          (b) TRUST AGREEMENT CONTROLS. The Trustee is not a party to the
 Plan, and in the  event of any conflict between the provisions of the
 Plan and the provisions of this Agreement, the provisions of this
 Agreement shall control.

     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
 executed by their duly authorized officers as of the day and year first above
 written.



                                             BIOGEN, INC.

 Attest: /s/ Michael J. Astrue           By: /s/ Frank A. Burke, Jr.
         ----------------------------        -----------------------------------
         Secretary                           Frank A. Burke, Jr.
                                             Vice President-Human Resources



                                             FIDELITY MANAGEMENT TRUST COMPANY


 Attest: /s/                             By: /s/
         ----------------------------        -----------------------------------
         Clerk                               Senior Vice President





                                       22

<PAGE>   23


                                SCHEDULE "A"
                  RECORDKEEPING & ADMINISTRATIVE SERVICES

ADMINISTRATION

*   Establishment and maintenance of participant account and election 
    percentages.

*   Maintenance of six (6) plan investment options:
        -Fidelity Retirement Government Money Market Portfolio
        -Fidelity Intermediate Bond Portfolio
        -Fidelity Puritan Fund
        -Fidelity Magellan Fund
        -Fidelity Growth Company Fund
        -Biogen Stock - (Employer matching contributions/accounts only)

*   All five Fidelity Mutual Funds shall be eligible for the following money
    classifications other than Employer Match.

*   Maintenance of four (4) money classifications:
        -Basic Account
        -Supplemental Account
        -Rollover
        -Employer Match

*   Processing of mutual fund trades.

*   The Trustee will provide only the recordkeeping and administrative services
    set forth on this Schedule "A" and no others.

PROCESSING
*   Bi-weekly processing of cnntribution data.
*   Daily processing of transfers and changes of future allocations.
*   Bi-weekly processing of loans, withdrawals, and distributions.

OTHER SERVICES AS MORE FULLY DETERMINED IN THE OCTOBER 6, 1993 LETTER OF
UNDERSTANDING
*   Processing of corrective distributions for 401(k)/401(m) compliance,
    including calculation of earnings.
*   Monthly trial balance
*   Quarterly administrative reports
*   Quarterly participant statements
*   1099-Rs
*   Participant Loans
*   Performance of section 401 (k)/401 (m) limitation testing upon request. In
    order to obtain this service, the client shall be required to provide the
    information identified in the Fidelity Discrimination Testing Package 
    Guidelines.
*   Employee communications describing available investment options, including
    multimedia informational materials and group presentations.
*   In-service and hardship withdrawals as approved and directed by the Sponsor.







                                       23
<PAGE>   24


BIOGEN, INC.                            FIDELITY MANAGEMENT TRUST COMPANY

By: /s/ Frank A. Burke, Jr.    4/1/94   By: /s/                          4/14/97
    ---------------------------------       ------------------------------------
                                Date                                      Date





















                                       24
<PAGE>   25



                                  SCHEDULE "B"
                                  FEE SCHEDULE

Annual Recordkeeping Fee           $80,000 per year*, billed and payable 
                                    quarterly.

Loan Fee                            Establishment fee of S35.00 per loan 
                                    account; annual fee of $15.00 per loan 
                                    account.

Return of Excess Fee                $25.00 per participant, a one-time charge 
                                    per calculation and check generation.

Remote Access Fee                   $1,000 per year, plus a monthly charge for
                                    TYMNET usage. A one-time installation fee of
                                    $1,500 will also be charged to the sponsor 
                                    in the first year.

Other Fees: ADP testing, extraordinary expenses resulting from large numbers of
simultaneous manual transactions or from errors not caused by Fidelity, or for
reports not contemplated in this Agreement. The Administrator may withdraw
reasonable administrative fees from the Trust by written direction to the
Trustee.

* This fee will be imposed pro rata for each CALENDAR quarter during the year.
If the Sponsor terminates this Agreement before the end of a calendar yeadr then
the recordkeeping fee will be pro-rated only for the period of time prior to
termination. There will be no recordkeeping fee charged after termination of the
Agreement.

TRUSTEE FEES:

To the extent that assets are invested in Sponsor Stock, .25% of such assets in
the Trust payable pro rata quarterly on the basis of such assets as of the
calendar quarter's last valuation date, subject to a $10,000 annual minimum.

Note: These fees have been negotiated and accepeed based on current plan assets
of $6.0 million, current participation of 300 participants and projected net
cash flows of $.9 million per year. Fees will be subject to revision if these
Plan tharacteristics change significantly by either falling below or exceeding
current projected levels. Fees also have been based on the use of up to six (6)
Fidelity managed investment options, and such fees will be subject to revision
if additional investment options are added.

Increases in participant levels have been contemplated and would not impact fees
until after the 2 year fee guarantee.

    BIOGEN, INC.                             FIDELITY  MANAGEMENT TRUST COMPANY

BY: /s/ Frank A. Burke, Jr.    4/1/94    BY: /s/                         4/14/94
    ---------------------------------        -----------------------------------
                                Date         Senior Vice President        Date





                                       25



<PAGE>   26

                                SCHEDULE "C"


                             INVESTMENT OPTIONS

      In accordance with Section 4(b), the Named Fiduciary hereby directs the
 Trustee that participants' individual accounts may be invested in the following
 investment options:

 -Fidelity Retirement Government Money Market Portfolio
 -Fidelity Intermediate Bond Portfolio
 -Fidelity Puritan Fund
 -Fidelity Magellan Fund
 -Fidelity Growth Company Fund
 -Biogen Stock - (Employer matching contributions/accounts only)

      The mutual fund advised by Fidelity Management & Research Company referred
 to in Section 4(c) shall be Fidelity Retirement Government Money Market
 Portfolio.



        BIOGEN, INC.


BY: /s/ Frank A. Burke, Jr.    4/1/94
    ---------------------------------
                                DATE










                                       26

<PAGE>   27



                               SCHEDULE "G"

                       TELEPHONE EXCHANGE PROCEDURES


 The following telephone exchange procedures are currently employed by Fidelity
 Institutional Retirement Services Company (FIRSCO).

 Telephone exchange hours are 8:30 a.m. (EST) to 8:00 p.m. (EST) on each
 business day. A "business day" is any day on which the New York Stock Exchange
 is open.

 FIRSCO reserves the right to change these telephone exchange procedures at its
 discretion and agrees to communicate them immediately to Sponsor.

                                MUTUAL FUNDS

        EXCHANGES BETWEEN MUTUAL FUNDS

        Participants may call on any business day to exchange between the
        mutual funds. If the request is received before 4:00 p.m. (EST), it
        will receive that day's trade date. Calls received after 4:00 p.m.
        (EST) will be processed on a next business day basis.



        BIOGEN, INC.


By: /s/ FRANK A. BURKE, JR.        4/1/94
    -------------------------------------
                                    Date




                                       27


<PAGE>   28



                   FIRST AMENDMENT TO TRUST AGREEMENT BETWEEN
                      FIDELITY MANAGEMENT TRUST COMPANY AND
                                   BIOGEN, INC

       THIS FIRST AMENDMENT, dated as of the first day of January, 1995, by and
between Fidelity Management Trust Company (the "Trustee") and Biogen, Inc. (the
"Sponsor"); and

                                   WITNESSETH:

       WHEREAS, the Trustee and the Sponsor heretofore entered into a trust
agreement dated April 1, 1994 with regard to the Biogen Savings Plan (the
"Plan"); and

       WHEREAS, the Trustee and the Sponsor now desire to amend said trust
agreement as provided for in Section 13 thereof;

       NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the trust agreement by:

(1)    Amending and adding the following mutual funds to the "investment option"
section to Schedules "A" and "C", as follows:

             Fidelity Asset Manager:  Growth
             Fidelity International Growth & Income Fund

       IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this First
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.

BIOGEN, INC                             FIDELITY MANAGEMENT TRUST COMPANY

By /s/ Frank A. Burke, Jr.   11/30/94   By /s/                          12/12/94
   ----------------------------------      -------------------------------------
                               Date        Vice President                  Date



<PAGE>   29



                      SECOND AMENDMENT TO TRUST AGREEMENT BETWEEN
                      FIDELITY MANAGEMENT TRUST COMPANY AND
                                   BIOGEN, INC

       THIS SECOND AMENDMENT, dated as of the fifteenth day of October, 1995, by
and between Fidelity Management Trust Company (the "Trustee") and Biogen, Inc.
(the "Sponsor"); and

                                   WITNESSETH:

       WHEREAS, the Trustee and the Sponsor heretofore entered into a trust
agreement dated April 1, 1994 with regard to the Biogen Savings Plan (the
"Plan"); and

       WHEREAS, the Trustee and the Sponsor now desire to amend said trust
agreement as provided for in Section 13 thereof;

       NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the trust agreement by:

(1) Amending and adding the following mutual funds to the "investment option"
section to Schedules "A" and "C", as follows:

       -  Fidelity U.S. Equity Index Portfolio
       -  Fidelity U.S. Bond Index Portfolio


       IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Second
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.

BIOGEN, INC                             FIDELITY MANAGEMENT TRUST COMPANY

By /s/ Frank A. Burke, Jr.    9/20/95   By /s/                           10/9/95
   ----------------------------------      -------------------------------------
                               Date        Vice President                  Date





<PAGE>   30






                   THIRD AMENDMENT TO TRUST AGREEMENT BETWEEN
                      FIDELITY MANAGEMENT TRUST COMPANY AND
                                  BIOGEN, INC.

       THIS THIRD AMENDMENT, dated as of the third day of October, 1996, by and
between Fidelity Management Trust Company (the "Trustee") and Biogen, Inc. (the
"Sponsor");

                                   WITNESSETH:

       WHEREAS, the Trustee and the Sponsor heretofore entered into a Trust
Agreement dated April 1, 1994, with regard to the Biogen Savings Plan (the
"Plan"); and

       WHEREAS, the Trustee and the Sponsor now desire to amend said trust
agreement as provided for in Section 13 thereof;

       NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the trust agreement by:

       (1)    Amending the "investment options" portion of Schedules "A" and "C"
              by adding the following:

              Fidelity Value Fund

       (2)    Amending the Annual Participant Fee on Schedule "B" as follows:

              EFFECTIVE OCTOBER 1, 1996:
              Annual Recordkeeping Fee:          $20.00 per participant, billed
                                                 and payable quarterly.

       IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Third
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.


BIOGEN, INC                             FIDELITY MANAGEMENT TRUST COMPANY


By /s/ Frank A. Burke, Jr.    10/3/96   By /s/                          10/23/96
   ----------------------------------      -------------------------------------
                               Date        Vice President                  Date





<PAGE>   31





                   FOURTH AMENDMENT TO TRUST AGREEMENT BETWEEN
                      FIDELITY MANAGEMENT TRUST COMPANY AND
                                  BIOGEN, INC.

      THIS FOURTH AMENDMENT, dated as of the fifteenth day of September, 1997,
by and between Fidelity Management Trust Company (the "Trustee") and Biogen,
Inc. (the "Sponsor");

                                   WITNESSETH:

      WHEREAS, the Trustee and the Sponsor heretofore entered into a Trust
Agreement dated April 1, 1994, with regard to the Biogen Savings Plan (the
"Plan"); and

      WHEREAS, the Sponsor now desires, and hereby directs the Trustee, in
accordance with Section 7(c), to liquidate all participant balances held in the
Fidelity International Growth & Income Fund on February 27, 1998, and to invest
the proceeds in the USAA International Fund. The parties hereto agree that the
Trustee shall have no discretionary authority with respect to this sale and
transfer directed by the Sponsor. Any variation from the procedure described
herein may be instituted only at the express written directions of the Sponsor;
and

      WHEREAS, the Trustee and the Sponsor now desire to amend said Trust
Agreement as provided for in Section 13 thereof;

      NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the Trust Agreement by:

      (1)  Amending Section 4(b)(i) AVAILABLE INVESTMENT OPTIONS by redefining
           "Mutual Fund" as follows:

           (i) securities issued by investment companies advised by Fidelity
           Management & Research Company ("Fidelity Mutual Funds") and certain
           securities issued by registered investment companies not advised by
           Fidelity Management & Research Company ("Non-Fidelity Mutual Funds")
           (collectively referred to as "Mutual Funds").

      (2)  Inserting the following sentence before all others in 
           subsection 4(d):

           All transactions involving Non-Fidelity Mutual Funds shall be done in
           accordance with the OPERATIONAL GUIDELINES FOR NON-FIDELITY MUTUAL
           FUNDS attached hereto as Schedule "H".

      (3)  Adding a Schedule "H" OPERATIONAL GUIDELINES FOR
           NON-FIDELITY MUTUAL FUNDS as attached.

      (4)  Amending the "investment options" section of Schedules "A" and "C" by
           restating as follows:

                 - Fidelity Low-Priced Stock Fund
                 - Janus Worldwide Fund
<PAGE>   32

                 - USAA International Fund
                 - Fidelity International Growth & Income Fund (frozen to new 
                   investments as of February 27, 1998)
                 - Fidelity Value Fund
                 - Spartan U.S. Equity Index Fund
                 - Fidelity U.S. Bond Index Portfolio
                 - Fidelity Asset Manager: Growth
                 - Fidelity Retirement Government Money Market Portfolio
                 - Fidelity Intermediate Bond Fund
                 - Fidelity Puritan Fund
                 - Fidelity Magellan Fund
                 - Fidelity Growth Company Fund
                 - Biogen Stock

      (5)  Amending Schedule "B" by adding "Non-Fidelity Mutual Funds" as 
           follows:

           Non-Fidelity Mutual Funds:      .25% annual administration fee on all
                                           Non-Fidelity Mutual Fund assets (to 
                                           be paid by the Non-Fidelity Mutual 
                                           Fund vendor).

      IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Fourth
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.

BIOGEN, INC.                               FIDELITY MANAGEMENT TRUST COMPANY



By /s/ Frank A. Burke, Jr.    9/12/97   By /s/ Cheryl L. Gladstone       9/23/97
   ----------------------------------      -------------------------------------
                                Date       Vice President                  Date






                                       2
<PAGE>   33


                                SCHEDULE "H"

            OPERATIONAL GUIDELINES FOR NON-FIDELITY MUTUAL FUNDS

   PRICING

   By 7:00 p.m. Eastern Time ("ET") each Business Day, the Non-Fidelity Mutual
   Fund Vendor (Fund Vendor) will input the following information ("Price
   Information") into the Fidelity Participant Recordkeeping System ("FPRS") via
   the remote access price screen that Fidelity Investments Institutional
   Operations Company, Inc. ("FIIOC"), an affiliate of the Trustee, has provided
   to the Fund Vendor: (1) the net asset value for each Fund at the Close of
   Trading, (2) the change in each Fund's net asset value from the Close of
   Trading on the prior Business Day, and (3) in the case of an income fund or
   funds, the daily accrual for interest rate factor ("mil rate"). FIIOC must
   receive Price Information each Business Day (a "Business Day" is any day the
   New York Stock Exchange is open). If on any Business Day the Fund Vendor does
   not provide such Price Information to FIIOC, FIIOC shall pend all associated
   transaction activity in the Fidelity Participant Recordkeeping System
   ("FPRS") until the relevant Price Information is made available by Fund
   Vendor.

   TRADE ACTIVITY AND WIRE TRANSFERS

   By 7:00 a.m. ET each Business Day following Trade Date ("Trade Date PLUS
   One"), FIIOC will provide, via facsimile, to the Fund Vendor a consolidated
   report of net purchase or net redemption activity that occurred in each of
   the Funds up to 4:00 p.m. ET on the prior Business Day. The report will
   reflect the dollar amount of assets and shares to be invested or withdrawn
   for each Fund. FIIOC will transmit this report to the Fund Vendor each
   Business Day, regardless of processing activity. In the event that data
   contained in the 7:00 a.m. ET facsimile transmission represents estimated
   trade activity, FIIOC shall provide a final facsimile to the Fund Vendor by
   no later than 9:00 a.m. ET. Any resulting adjustments shall be processed by
   the Fund Vendor at the net asset value for the prior Business Day.

   The Fund Vendor shall send via regular mail to FIIOC transaction confirms for
   all daily activity in each of the Funds. The Fund Vendor shall also send via
   regular mail to FIIOC, by no later than the fifth Business Day following
   calendar month close, a monthly statement for each Fund. FIIOC agrees to
   notify the Fund Vendor of any balance discrepancies within twenty (20)
   Business Days of receipt of the monthly statement.




                                       3


<PAGE>   34

   For purposes of wire transfers, FIIOC shall transmit a daily wire for
   aggregate purchase activity and the Fund Vendor shall transmit a daily wire
   for aggregate redemption activity, in each case including all activity across
   all Funds occurring on the same day.

   PROSPECTUS DELIVERY

   FIIOC shall be responsible for the timely delivery of Fund prospectuses and
   periodic Fund reports ("Required Materials") to Plan participants, and shall
   retain the services of a third-party vendor to handle such mailings. The Fund
   Vendor shall be responsible for all materials and production costs, and
   hereby agrees to provide the Required Materials to the third-party vendor
   selected by FIIOC. The Fund Vendor shall bear the costs of mailing annual
   Fund reports to Plan participants. FIIOC shall bear the costs of mailing
   prospectuses to Plan participants.

   PROXIES

   The Fund Vendor shall be responsible for all costs associated with the
   production of proxy materials. FIIOC shall retain the services of a
   third-party vendor to handle proxy solicitation mailings and vote tabulation.
   Expenses associated with such services shall be billed directly to the Fund
   Vendor by the third-party vendor.

   PARTICIPANT COMMUNICATIONS

   The Fund Vendor shall provide internally-prepared fund descriptive
   information approved by the Funds' legal counsel for use by FIIOC in its
   written participant communication materials. FIIOC shall utilize historical
   performance data obtained from third-party vendors (currently Morningstar,
   Inc., FACTSET Research Systems and Lipper Analytical Services) in telephone
   conversations with plan participants and in quarterly participant statements.
   The Sponsor hereby consents to FIIOC's use of such materials and acknowledges
   that FIIOC is not responsible for the accuracy of such third-party
   information. FIIOC shall seek the approval of the Fund Vendor prior to
   retaining any other third-party vendor to render such data or materials under
   this Agreement.

   COMPENSATION

   FIIOC shall be entitled to fees as set forth in a separate agreement with the
   Fund Vendor.



                                       4
<PAGE>   35



                   FIFTH AMENDMENT TO TRUST AGREEMENT BETWEEN
                      FIDELITY MANAGEMENT TRUST COMPANY AND
                                  BIOGEN, INC.

      THIS FIFTH AMENDMENT, dated as of the fifteenth day of December, 1997, by
and between Fidelity Management Trust Company (the "Trustee") and Biogen, Inc.
(the "Sponsor");

                                   WITNESSETH:

      WHEREAS, the Trustee and the Sponsor heretofore entered into a Trust
Agreement dated April 1, 1994, with regard to the Biogen Savings Plan (the
"Plan"); and

      WHEREAS, the Trustee and the Sponsor now desire to amend said Trust
Agreement as provided for in Section 13 thereof;

      NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the Trust Agreement by:

      (1)  Amending Section 4(e) by restating as follows:

           (e) SPONSOR STOCK. Trust investments in Sponsor Stock shall be made
via the Biogen Stock Fund (the "Stock Fund"). Investments in the Stock Fund
shall consist primarily of shares of Sponsor Stock. In order to satisfy daily
participant exchange or withdrawal requests for transfers and payments, the
Stock Fund shall also include cash or short-term liquid investments in
accordance with this paragraph. Such holdings will include Fidelity
Institutional Cash Portfolios: Money Market Portfolio: Class I or such other
Mutual Fund or commingled money market pool as agreed to by the Sponsor and
Trustee. The Named Fiduciary shall, after consultation with the Trustee,
establish and communicate to the Trustee in writing a target percentage and
drift allowance for such short-term liquid investments. The Trustee shall be
responsible for ensuring that the actual cash held in the Stock Fund falls
within the agreed upon range over time. Each participant's proportional interest
in the Stock Fund shall be measured in units of participation, rather than
shares of Sponsor Stock. Such units shall represent a proportionate interest in
all of the assets of the Stock Fund, which includes shares of Sponsor Stock,
short-term investments and at times, receivables for dividends and/or Sponsor
Stock sold and payables for Sponsor Stock purchased. The Trustee shall determine
a daily net asset value ("NAV") for each unit outstanding of the Stock Fund.
Valuation of the Stock Fund shall be based upon the 4:00 p.m. NASDQ closing
price of the stock, or if unavailable, the latest available price as reported by
the principal national securities exchange on which the Sponsor Stock is traded.
The NAV shall be adjusted by dividends paid on the shares of Sponsor Stock held
by the Stock Fund, gains or losses realized on sales of Sponsor Stock,
appreciation or depreciation in the market price of those shares owned, and
interest on the short-term investments held by the Stock Fund, expenses 





<PAGE>   36

that, pursuant to Sponsor direction, the Trustee accrues from the Stock Fund,
and commissions on purchases and sales of Sponsor Stock. Investments in the
Stock Fund shall be subject to the following limitations:

                 (i)   ACQUISITION LIMIT.  Pursuant to the Plan, the Trust
may be invested in Sponsor Stock to the extent necessary to comply with
investment directions in accordance with this Agreement.

                 (ii)  FIDUCIARY DUTY OF NAMED FIDUCIARY. The Named Fiduciary
shall continually monitor the suitability under the fiduciary duty rules of
section 404(a)(1) of ERISA (as modified by section 404(a)(2) of ERISA) of
acquiring and holding Sponsor Stock. The Trustee shall not be liable for any
loss, or by reason of any breach, which arises from the directions of the Named
Fiduciary with respect to the acquisition and holding of Sponsor Stock, unless
it is clear on their face that the actions to be taken under those directions
would be prohibited by the foregoing fiduciary duty rules or would be contrary
to the terms of this Agreement.

                 (iii) Purchase and sales of Sponsor Stock shall be made on the
open market as necessary to maintain the target cash percentage and drift
allowance for the Stock Fund, provided that:

                       (A)  If the Trustee is unable to purchase or sell
the total number of shares required to be purchased or sold on such day as
a result of market conditions; or

                       (B)  If the Trustee is prohibited by the Securities
and Exchange Commission, the NASDQ, or any other regulatory body from purchasing
or selling any or all of the shares required to be purchased or sold on such
day, then the Trustee shall purchase or sell such shares as soon as possible
thereafter. The Trustee may follow directions from the Administrator or Named
Fiduciary to deviate from the above purchase and sale procedures provided that
such direction is made in writing by the Administrator or Named Fiduciary.

                 (iv) EXECUTION OF PURCHASES AND SALES.

                 (A) Purchases and sales of units in the Stock Fund (other than
for exchanges) shall be made on the date on which the Trustee receives from the
Administrator in good order all information, documentation, and wire transfers
of funds (if applicable), necessary to accurately effect such transactions.
Exchanges of units in the Stock Fund shall be made in accordance with the
Telephone Exchange Guidelines attached hereto as Schedule "G". The Trustee may
follow directions from the Administrator or Named Fiduciary 



                                       2


<PAGE>   37

to deviate from the above purchase and sale procedures provided that such
direction is made in writing by the Administrator or Named Fiduciary.

                      (B) PURCHASES AND SALES FROM OR TO SPONSOR. If directed by
the Sponsor in writing prior to the trading date, the Trustee may purchase or
sell Sponsor Stock from or to the Sponsor if the purchase or sale is for
adequate consideration (within the meaning of section 3(18) of ERISA) and no
commission is charged. If Sponsor contributions (employer) or contributions made
by the Sponsor on behalf of the participants (employee) under the Plan are to be
invested in Sponsor Stock, the Sponsor may transfer Sponsor Stock in lieu of
cash to the Trust. In either case, the number of shares to be transferred will
be determined by dividing the total amount of Sponsor Stock to be purchased or
sold by the 4:00 p.m. NASDQ closing price of the Sponsor Stock on the trading
date.

                      (C) USE OF AN AFFILIATED BROKER. The Sponsor hereby
directs the Trustee to use Fidelity Brokerage Services, Inc. ("FBSI") to provide
brokerage services in connection with any purchase or sale of Sponsor Stock
which results from directions received from Plan participants. FBSI shall
execute such directions directly or through its affiliate, National Financial
Services Company ("NFSC"). The provision of brokerage services shall be subject
to the following:

                           (1) As consideration for such brokerage services, the
Sponsor agrees that FBSI shall be entitled to remuneration under this
authorization provision in the amount of three and one-half cents ($.035)
commission on each share of Sponsor Stock. Any change in such remuneration may
be made only by a signed agreement between Sponsor and Trustee.

                           (2) Following the procedures set forth in Department
of Labor Prohibited Transaction Class Exemption 86-128 (PTCE 86-128), the
Trustee will provide the Sponsor with the following documents: (1) a description
of FBSI's brokerage placement practices; (2) a copy of PTCE 86-128; and (3) a
form by which the Sponsor may terminate this direction to use a broker
affiliated with the Trustee. The Trustee will provide the Sponsor with this
termination form annually, as well as quarterly and annual reports which
summarize all securities transaction-related charges incurred by the Plan.

                           (3) Any successor organization of FBSI, through
reorganization, consolidation, merger or similar transactions, shall, upon
consummation of such transaction, become the successor broker in accordance with
the terms of this authorization provision.





                                       3


<PAGE>   38

                           (4) The Trustee and FBSI shall continue to rely on
this authorization provision until notified to the contrary. The Sponsor
reserves the right to terminate this authorization upon written notice to FBSI
(or its successor) and the Trustee, in accordance with Section 11 of this
Agreement.

                 (iv) SECURITIES LAW REPORTS. The Named Fiduciary shall be
responsible for filing all reports required under Federal or state securities
laws with respect to the Trust's ownership of Sponsor Stock, including, without
limitation, any reports required under section 13 or 16 of the Securities
Exchange Act of 1934, and shall immediately notify the Trustee in writing of any
requirement to stop purchases or sales of Sponsor Stock pending the filing of
any report. The Trustee shall provide to the Named Fiduciary such information on
the Trust's ownership of Sponsor Stock as the Named Fiduciary may reasonably
request in order to comply with Federal or state securities laws.

                 (v)  VOTING AND TENDER OFFERS. Notwithstanding any other
provision of this Agreement the provisions of this Section shall govern the
voting and tendering of Sponsor Stock. The Sponsor, after consultation with the
Trustee, shall provide and pay for all printing, mailing, tabulation and other
costs associated with the voting and tendering of Sponsor Stock.

                      (A)  VOTING.

                           (1) When the issuer of Sponsor Stock prepares for any
annual or special meeting, the Sponsor shall notify the Trustee at least thirty
(30) days in advance of the intended record date and shall cause a copy of all
proxy solicitation materials to be sent to the Trustee. If requested by the
Trustee, the Sponsor shall certify to the Trustee that the aforementioned
materials represents the same information that is distributed to shareholders of
Sponsor Stock. Based on these materials the Trustee shall prepare a voting
instruction form and shall provide a copy of all proxy solicitation materials to
be sent to each Plan participant with an interest in Sponsor Stock held in the
Trust, together with the foregoing voting instruction form to be returned to the
Trustee or its designee. The form shall show the proportional interest in the
number of full and fractional shares of Sponsor Stock credited to the
participant's accounts held in the Stock Fund.

                           (2) Each participant with an interest in the Stock
Fund shall have the right to direct the Trustee as to the manner in which the
Trustee is to vote (including not to vote) that number of shares of Sponsor
Stock reflecting such participant's proportional interest in the Stock Fund
(both vested and






                                       4

<PAGE>   39

unvested). Directions from a participant to the Trustee concerning the voting of
Sponsor Stock shall be communicated in writing, or by mailgram or similar means
as is agreed upon by the Trustee and the Sponsor. These directions shall be held
in confidence by the Trustee and shall not be divulged to the Sponsor, or any
officer or employee thereof, or any other person except to the extent that the
consequences of such directions are reflected in reports regularly communicated
to any such persons in the ordinary course of the performance of the Trustee's
services hereunder. Upon its receipt of the directions, the Trustee shall vote
the shares of Sponsor Stock reflecting the participant's proportional interest
in the Stock Fund as directed by the participant. Except as otherwise required
by law, the Trustee shall not vote shares of Sponsor Stock reflecting a
participant's proportional interest in the Stock Fund for which it has received
no direction from the participant.

                           (3) The Trustee shall vote that number of shares of
Sponsor Stock not credited to participants' accounts in the same proportion on
each issue as it votes those shares credited to participants' accounts for which
it received voting directions from participants.

                      (B)  TENDER OFFERS.

                           (1) Upon commencement of a tender offer for any
securities held in the Trust that are Sponsor Stock, the Sponsor shall timely
notify the Trustee in advance of the intended tender date and shall cause a copy
of all materials to be sent to the Trustee. The Sponsor shall certify to the
Trustee that the aforementioned materials represent the same information
distributed to shareholders of Sponsor Stock. Based on these materials and after
consultation with the Sponsor the Trustee shall prepare a tender instruction
form and shall provide a copy of all tender materials to be sent to each plan
participant, together with the foregoing tender instruction form, to be returned
to the Trustee or its designee. The tender instruction form shall show the
number of full and fractional shares of Sponsor Stock that reflect the
participants proportional interest in the Stock Fund (both vested and unvested).

                           (2) Each participant shall have the right to direct
the Trustee to tender or not to tender some or all of the shares of Sponsor
Stock reflecting such participant's proportional interest in the Stock Fund
(both vested and unvested). Directions from a participant to the Trustee
concerning the tender of Sponsor Stock shall be communicated in writing, or by
mailgram or such similar means as is agreed upon by the Trustee and the Sponsor.
These directions shall be held in confidence by the Trustee and shall not be
divulged to the Sponsor, or any officer or employee thereof, or any other person
except to the extent that the consequences of such directions are reflected in
reports regularly communicated to any such persons in the ordinary course of the




                                       5


<PAGE>   40

performance of the Trustee's services hereunder. The Trustee shall tender or not
tender shares of Sponsor Stock as directed by the participant. Except as
otherwise required by law, the Trustee shall not tender shares of Sponsor Stock
reflecting a participant's proportional interest in the Stock Fund for which it
has received no direction from the participant.

                           (3) Except as otherwise required by law, the Trustee
shall tender that number of shares of Sponsor Stock not credited to
participants' accounts in the same proportion as the total number of shares of
Sponsor Stock credited to participants' accounts for which it has received
instructions from Participants.

                           (4) A participant who has directed the Trustee to
tender some or all of the shares of Sponsor Stock reflecting the participant's
proportional interest in the Stock Fund may, at any time prior to the tender
offer withdrawal date, direct the Trustee to withdraw some or all of the
tendered shares reflecting the participant's proportional interest, and the
Trustee shall withdraw the directed number of shares from the tender offer prior
to the tender offer withdrawal deadline. Prior to the withdrawal deadline, if
any shares of Sponsor Stock not credited to participants' accounts have been
tendered, the Trustee shall redetermine the number of shares of Sponsor Stock
that would be tendered under Section 4(e)(v)(B)(3) if the date of the foregoing
withdrawal were the date of determination, and withdraw from the tender offer
the number of shares of Sponsor Stock not credited to participants' accounts
necessary to reduce the amount of tendered Sponsor Stock not credited to
participants' accounts to the amount so redetermined. A participant shall not be
limited as to the number of directions to tender or withdraw that the
participant may give to the Trustee.

                           (5) A direction by a participant to the Trustee to
tender shares of Sponsor Stock reflecting the participant's proportional
interest in the Stock Fund shall not be considered a written election under the
Plan by the participant to withdraw, or have distributed, any or all of his
withdrawable shares. The Trustee shall credit to each proportional interest of
the participant from which the tendered shares were taken the proceeds received
by the Trustee in exchange for the shares of Sponsor Stock tendered from that
interest. Pending receipt of directions (through the Administrator) from the
participant or the Named Fiduciary, as provided in the Plan, as to which of the
remaining investment options the proceeds should be invested in, the Trustee
shall invest the proceeds in the investment option described in Schedule "C".

                 (vi)  GENERAL. With respect to all rights other than the right
to vote, the right to tender, and the right to withdraw shares previously
tendered, in the case of Sponsor Stock credited to a 




                                       6


<PAGE>   41

participant's proportional interest in the Stock Fund, the Trustee shall follow
the directions of the participant and if no such directions are received, the
directions of the Named Fiduciary. The Trustee shall have no duty to solicit
directions from participants. With respect to all rights other than the right to
vote and the right to tender, in the case of Sponsor Stock not credited to
participants' accounts, the Trustee shall follow the directions of the Named
Fiduciary.

                 (vii) CONVERSION. All provisions in this Section 4(e) shall 
also apply to any securities received as a result of a conversion of Sponsor
Stock.

      (2)  Amending Section 4(h), TRUSTEE POWERS, by adding the following:

           (ix) To borrow funds from a bank not affiliated with the Trustee in
           order to provide sufficient liquidity to process Plan transactions in
           a timely fashion; provided that the cost of such borrowing shall be
           allocated in a reasonable fashion to the investment fund(s) in need
           of liquidity.

      (3)  Amending the "investment options" section of Schedules "A" and "C" by
           restating as follows:

           *     Maintenance of the following investment options:

                 - Fidelity Money Market Trust:  Retirement
                 - Money Market Portfolio
                 - Fidelity Intermediate Bond Fund
                 - Fidelity Puritan Fund
                 - Fidelity Magellan Fund
                 - Fidelity Growth Company Fund
                 - Biogen Stock Fund
                 - Fidelity Asset Manager:  Growth
                 - Spartan U.S. Equity Index Fund
                 - Fidelity U.S. Bond Index Fund
                 - Fidelity Value Fund
                 - Fidelity Low-Priced Stock Fund
                 - Janus Worldwide Fund
                 - USAA International Fund

      (4)  Amending the "money classifications" Section of Schedule "A" by 
           restating as follows:

           *     Maintenance of the following money classifications:

                 - Basic Account
                 - Supplemental Account
                 - Rollover
                 - Employer Match

      (5)  Restating Schedule "G" TELEPHONE EXCHANGE PROCEDURES as attached.




                                       7



<PAGE>   42

IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Fifth Amendment
to be executed by their duly authorized officers effective as of the day and
year first above written.

BIOGEN, INC.                              FIDELITY MANAGEMENT TRUST COMPANY


By /s/ Frank A. Burke, Jr.     11/4/97    By /s/                         12/1/97
   -----------------------------------       -----------------------------------
                                 Date        Vice President                Date

















                                       8
<PAGE>   43


                             SCHEDULE "G"

                    TELEPHONE EXCHANGE GUIDELINES

The following telephone exchange guidelines are currently employed by Fidelity
Institutional Retirement Services Company (FIRSCO).

Telephone exchange hours are 8:30 a.m. (ET) to 8:00 p.m. (ET) on each business
day. A "business day" is any day on which the New York Stock Exchange is open.

FIRSCO reserves the right to change these telephone exchange guidelines at its
discretion.

                             MUTUAL FUNDS

EXCHANGES BETWEEN MUTUAL FUNDS

Participants may call on any business day to exchange between the mutual funds.
If the request is received before 4:00 p.m. (ET), it will receive that day's
trade date. Calls received after 4:00 p.m. (ET) will be processed on a next
business day basis.

                            SPONSOR STOCK

I.    EXCHANGES FROM MUTUAL FUNDS INTO SPONSOR STOCK

      Sponsor Stock exchanges are processed on a monthly cycle. Participants who
      wish to exchange out of a mutual fund into Sponsor Stock may call between
      the 1st and the 15th of the month. No calls will be accepted after 4:00
      p.m. (ET) on the 15th (or previous business day if the 15th is not a
      business day).

      Mutual fund shares are sold on the 15th of the month (or the previous
      business day if the 15th is not a business day) and the Sponsor Stock is
      purchased within two (2) business days after the date on which the mutual
      fund shares are sold.

II.   EXCHANGES FROM SPONSOR STOCK INTO MUTUAL FUNDS

      Participants who wish to exchange out of Sponsor Stock into mutual funds
      may call between the 1st and the 15th of the month. No calls will be
      accepted after 4:00 p.m. (ET) on the 15th (or previous business day if the
      15th is not a business day).

      The Sponsor Stock is sold on the 16th (or the next business day if the
      16th is not a business day) and the subsequent purchase into mutual funds
      will take place three (3) business days later. This allows for settlement
      of the stock trade at the custodian and the corresponding transfer to
      Fidelity.

BIOGEN, INC.


By:  __________________________
                           Date









                                       9





<PAGE>   1
                                                                       Exhibit 5

        [MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C. LETTERHEAD]
                              One Financial Center
                          Boston, Massachusetts 02111




701 Pennsylvania Avenue, N.W.                            Telephone: 617/542-6000
Washington, D.C. 20004                                   Fax: 617/542-2241
Telephone: 202/434-7300                                  ww.Mintz.com
Fax: 202/434-7400

                                                         Direct Dial Number




                                December 17, 1997


    Biogen, Inc.
    14 Cambridge Center
    Cambridge, MA 02142

    Gentlemen:

         We have acted as counsel to Biogen, Inc., a Massachusetts corporation
    (the "Company"), in connection with the preparation and filing with the
    Securities and Exchange Commission of a Registration Statement on Form S-8
    (the "Registration Statement"), pursuant to which the Company is registering
    the issuance under the Securities Act of 1933, as amended, of a total of
    200,000 shares (the "Shares") of its common stock, $.01 par value per share
    (the "Common Stock"). This opinion is being rendered in connection with the
    filing of the Registration Statement. All capitalized terms used herein and
    not otherwise defined shall have the respective meanings given to them in
    the Registration Statement.

         In connection with this opinion, we have examined the Company's
    Articles Of Organization, and By-Laws, both as currently in effect; such
    other records of the corporate proceedings of the Company and certificates
    of the Company's officers as we have deemed relevant; and the Registration
    Statement and the exhibits thereto.

         In our examination, we have assumed the genuineness of all signatures,
    the legal capacity of natural persons, the authenticity of all documents
    submitted to us as originals, the conformity to original documents of all
    documents submitted to us as certified or photostatic copies and the
    authenticity of the originals of such copies.

         Based upon the foregoing, we are of the opinion that (i) the Shares
    have been duly and validly authorized by the Company and (ii) the Shares,
    when issued and sold, will have been duly and validly issued, fully paid and
    non-assessable shares of the Common Stock, free of preemptive rights.

         Our opinion is limited to Massachusetts law, and we express no opinion
    with respect to the laws of any other jurisdiction. No opinion is expressed
    herein with respect to the qualification of the Shares under the securities
    or blue sky laws of any state or any foreign jurisdiction.



<PAGE>   2

Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.

    December 17, 1997
    Page 2

         We understand that you wish to file this opinion as an exhibit to the
    Registration Statement, and we hereby consent thereto.

                                             Very truly yours,

                                             /s/ Mintz, Levin, Cohn, Ferris
                                                  Glovsky and Popeo, P.C.

                                             Mintz, Levin, Cohn, Ferris,
                                              Glovsky and Popeo, P.C.















<PAGE>   1


                                                                    Exhibit 23.2







                       CONSENT OF INDEPENDENT ACCOUNTANTS


We hereby consent to the incorporation by reference in the Registration
Statement on Form S-8 of our report dated January 14, 1997, which appears on
page 42 of the 1996 Annual Report to Shareholders of Biogen, Inc., which is
incorporated by reference in Biogen, Inc.'s Annual Report on Form 10-K for the
year ended December 31, 1996.



Price Waterhouse LLP
Boston, Massachusetts
December 17, 1997



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