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                                                                   EXHIBIT 10.20

                                  BIOGEN, INC.

                        1983 EMPLOYEE STOCK PURCHASE PLAN
       (As amended and restated through December 14, 2001, effective as of
                               December 31, 2001)

I.   PURPOSE AND DEFINITIONS

          A.   Purpose of the Plan: The Plan is intended to encourage ownership
of Shares by all employees of the Company and its Affiliates.

          B.   Definitions: Unless otherwise specified or unless the context
otherwise requires, the following terms, as used in this Employee Stock Purchase
Plan, have the following meanings:

          1.   "Affiliate" means (a) a corporation in respect of which the
          Company owns directly or indirectly fifty percent (50%) or more of the
          voting securities thereof or which is otherwise controlled by the
          Company; or (b) to the extent not inconsistent with Section 423(a) of
          the Code, an unincorporated trade or business controlled by the
          Company which has elected, for federal income tax purposes, to be
          either (i) classified as an association taxable as a corporation or
          (ii) disregarded as an entity separate from its owner (as provided in
          Section 301.7701-3 of the federal income tax regulations). For
          purposes of this definition, the Company shall be deemed to control
          another entity if the Company possesses, directly or indirectly, the
          power to direct or cause the direction of the management and policies
          of such entity, whether through ownership of voting securities, by
          contract or otherwise.

          2.   "Board" means the Board of Directors of the Company.

          3.   "Code" means the United States Internal Revenue Code of 1986, as
          amended from time to time.

          4.   "Committee" means the Stock and Option Plan Administration
          Committee of the Board or, if such committee ceases to exist, the
          Board or a committee thereof to which responsibility for administering
          the Plan shall have been delegated.

          5.   "Company" means Biogen, Inc., a Massachusetts Corporation.

          6.   "Compensation" means salary and wages, including overtime pay,
          received by an Employee before any salary reduction by the employee
          under Code Sections 401(k) or 125, but excluding bonus, incentive and
          similar payments and all other forms of non-cash remuneration.

          7.   "Custodian" shall have the meaning set forth in Section IV.C.


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          8.   "Employee" means an individual employed by the Company or an
          Affiliate as a common law employee (determined under the regular
          personnel policies, practices and classifications of the Company or
          the Affiliate, as applicable) whose customary employment is 20 hours
          or more per week and who resides or performs his principal duties
          within the United States, except as is otherwise determined by the
          Committee (ii) employees of non-U.S. Affiliates, except as is
          otherwise determined by the Committee. An individual is not considered
          an Employee for purposes of the Plan if the individual is classified
          as a consultant or contractor under the Company or an Affiliate's
          regular personnel classifications and practices, or if the individual
          is a party to an agreement to provide services to the Company or an
          Affiliate without participating in the Plan, notwithstanding that such
          individual may be treated as a common law employee for payroll tax,
          coverage requirements under Section 410(b) of the Code,
          nondiscrimination requirements under Section 401(a)(4) of the Code or
          other legal purposes.

          9.   "Employee Share Price" shall have the meaning set forth in
          Section V.A.

          10.  "Enrollment Dates" are the earliest date participation is
          permitted hereunder by the Committee when the Plan is first made
          operative and each successive January 1 and July 1 thereafter.

          11.  "Exchange Act" means the United States Securities Exchange Act of
          1934, as amended from time to time.

          12.  "Fair Market Value"

               (i)  If Shares are purchased by the Plan on a U.S. securities
               exchange or the Nasdaq National Market or the Nasdaq SmallCap
               Market, the actual purchase price of such Shares shall be such
               Shares' Fair Market Value.

               (ii) In all other circumstances including if Shares are purchased
               by the Plan from the Company, in determining such Shares' Fair
               Market Value, if the Shares are then listed on any U.S.
               securities exchange or traded on the Nasdaq National Market or
               the Nasdaq SmallCap Market AND sale prices are regularly reported
               for the Shares, then the Fair Market Value shall be the
               arithmetic mean between the "high" and "low" sale prices for the
               Shares reported on the applicable composite tape or other
               comparable reporting system on the applicable date, or if the
               applicable date is not a trading day, on the most recent trading
               day immediately prior to the applicable date. If sale prices are
               not regularly reported for the Shares as described in the
               immediately preceding sentence but bid and asked prices for the
               Shares are regularly reported, then the Fair Market Value shall
               be the mean between the closing or last bid and asked prices for
               the Shares on the applicable date or, if the applicable date is
               not a trading day, on the most recent trading day immediately
               prior to the applicable date. If sale prices are not regularly
               reported for the Shares as described in two



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               immediately preceding sentences, then the Fair Market Value shall
               be such value as the Committee in good faith determines.

          13.  "Monthly Share Purchase Date" shall have the meaning set forth in
          Section IV.C.

          14.  "Option" means a right or option granted under the Plan.

          15.  "Participant" means an Employee who is enrolled in the Plan,
          provided however that no Employee may be granted an Option under the
          Plan if, immediately after the Option is granted, such Employee owns
          stock possessing five percent (5%) or more of the total combined
          voting power (or in the case of non-voting stock, value) of all
          classes of issued and outstanding stock of the Company or Affiliate(s)
          (other than wholly owned subsidiaries of the Company.) For purposes of
          determining stock ownership the applicable rules of the Code
          (including attribution) shall control.

          16.  "Participant's Survivors" means a deceased Participant's legal
          representatives and/or any person or persons who acquired the
          Participant's rights to an Option by will or by the laws of descent
          and distribution including where appropriate his/her estate.

          17.  "Plan" means this Employee Stock Purchase Plan, as amended and
          restated from time to time.

          18.  "Shares" means the Common Stock of the Company, par value $0.01
          per share, as to which Options have been or may be granted under the
          Plan or any shares of capital stock into which the Shares are changed
          or for which they are exchanged within the provisions of Article VI of
          the Plan.

II.  SHARES SUBJECT TO THE PLAN

          A.   Subject to the terms of Article VI, the maximum aggregate number
of Shares which may be optioned and purchased from time to time shall be One
Million (1,000,000) Shares.

          If an Option ceases to be "outstanding", in whole or in part, the
Shares which were subject to such Option but not purchased shall be available
for the granting of the other Options. An Option shall be treated as
"outstanding" until such Option is exercised in full, or terminates or expires
under the provisions of the Plan.

          B.   No options shall be granted after December 31, 2007.

III. ADMINISTRATION OF THE PLAN

          The Plan shall be administered by the Committee. Subject to the
provisions of the Plan, the Committee is authorized to interpret the provisions
of the Plan and each Option, and to make any



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rules and determinations which it deems necessary or advisable for the
administration of the Plan provided, however, that all such interpretations,
rules, determinations, terms and conditions shall be made and prescribed in the
context of preserving the tax status of the Options under the Plan granted to
Employees subject to United States federal income taxation and the Plan within
the meaning of Section 423 of the Code. In addition, the Plan is intended to
comply in all respects with Rule 16b-3 or its successors promulgated under the
Exchange Act, with respect to Participants who are subject to Section 16 of the
Exchange Act. The Plan will be interpreted in a manner which comports with this
intention.

IV.  ELIGIBILITY FOR PARTICIPATION

          A.   Subject to the limits in Article II, on the January 1 Enrollment
Date of each year in the case of an Employee who is a Participant on such
January 1 Enrollment Date and on the July 1 Enrollment Date of each year in the
case of an Employee who is a Participant on such July 1 Enrollment Date, the
Company will be deemed to have granted to each such Participant an Option to
purchase, during a six-month period commencing on the Enrollment Date on which
such Option is granted and in the manner provided hereunder, such number of
Shares as have an aggregate Employee Share Price (as determined under Section
V.A) equal to $2,500.00. If, on any Enrollment Date, an insufficient number of
Shares remains available under the Plan to grant to each Participant an Option
to purchase such number of Shares, then the number of Shares subject to each
Option to be granted on such Enrollment Date shall be reduced equally so that
the aggregate number of Share subject to all Options granted on such Enrollment
Date shall not exceed the number of Shares then available under the Plan.

          B.   Employee Contributions: Each eligible Employee may, on an
enrollment application and payroll withholding form approved by the Committee
and filed with his/her employer's payroll department no later than thirty (30)
days prior to a January or July Enrollment Date, elect to participate and make
contributions by payroll deduction of any whole percentage form 1% to 10% of
such Employee's Compensation payable on each payroll period.

          In the event that the amount contributed by a Participant during an
Option exercise period (i.e., the six-month periods commencing on an Enrollment
Date) is in excess of the maximum amount which may be applied to purchase shares
for such Participant, such excess shall be returned to the Participant. No
interest shall accrue or be payable to any Participant with respect to any
amount contributed by the Participant, whether such sums are applied to purchase
Shares or are returned to the Participant.

          Payroll deductions may only be increased by a Participant effective as
of an Enrollment Date, but may be decreased effective with respect to any
payroll period, provided written election on a change authorization and payroll
withholding form approved by the Committee is received by the Participant's
employer's payroll department no later than thirty (30) days prior thereto.

          C.   Application of Payroll Contributions:


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               1.   The employer will remit to a bank, stock brokerage firm or
          other custodian (the "Custodian") selected by the Company, the
          accumulated withheld funds of all electing Participants together with
          employer contributions pursuant to Section IV.G, if any, as soon as
          reasonably possible following the end of the month in which the
          deductions were made. Prior to such remittance, Participant
          contributions may be commingled with other Company funds. Not less
          frequently than monthly, the Custodian shall buy from the Company or,
          if the Committee prior thereto approves, give an order to the stock
          broker selected by the Company to purchase (or if the Custodian shall
          be such stock broker, shall itself purchase) in the open market, the
          total number of Shares purchasable with the monies available from such
          remittance. The date on which Shares are so acquired shall be referred
          to as the "Monthly Share Purchase Date." The Committee shall instruct
          the Custodian whether to purchase Shares from the Company or on the
          open market, after giving due consideration to any applicable
          securities laws and the advice of the chief financial officer of the
          Company. A Participant shall be deemed to have exercised his/her
          Options on the Monthly Share Purchase Date to the extent of such
          purchase unless prior thereto the Participant shall have effectively
          withdrawn pursuant to the terms hereof.

               The Company may, in its sole and absolute discretion, refuse to
          sell Shares to the Custodian under the Plan if to do so would be
          violative of any commitment or restriction (whether legally binding or
          otherwise) not to issue or sell its own shares, as from time to time
          exists, and whether such commitment or restriction existed prior to or
          subsequent to the adoption of the Plan or for any other reason the
          Company deems appropriate. The refusal of the Company to sell Shares
          to the Custodian under the Plan shall not adversely affect the Plan's
          right and power to acquire such Shares from any other source the
          Committee deems appropriate.

               2.   The certificates representing the Shares so purchased shall
          be issued in the name of and delivered to the Custodian and the
          account of each electing Participant shall be credited with the number
          of Shares to which he/she shall be entitled on the basis of his/her
          proportion of the aggregate remittance.

               3.   Any cash dividends paid on Shares shall automatically be
          used to purchase additional shares of the Common Stock of the Company,
          unless a Participant in writing instructs the Custodian to the
          contrary. The purchases described in the preceding sentence, whether
          purchased by the Custodian from the Company or in the open market,
          shall be in addition to the number of Shares purchasable pursuant to
          Section II.A and Section IV.A and shall not be of Shares optioned
          under the Plan. Section IV.G with respect to employer's contribution
          shall be inapplicable with respect to shares of the Common Stock of
          the Company acquired under this Paragraph IV.C.3.

               4.   By enrolling in the Plan, each Participant is deemed to have
          authorized the establishment of a brokerage account in his/her name at
          a securities brokerage firm selected




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          by or approved of by the Committee.

          D.   Transfer of Certificates to Electing Participants:

               1.   Upon request by a Participant and receipt by the Custodian
          of written notice to such effect from the Company, all or any portion
          of the Shares in the Participant's account shall be transferred by the
          Custodian out of its name into the name of the Participant and a
          certificate evidencing them shall be issued in the name of and
          delivered to the Participant.

               2.   In order to preserve the intended purposes of the Plan as
          set forth in Section 1.A, Employees who become Participants in the
          Plan agree not to transfer or otherwise dispose of Shares acquired on
          their behalf under the Plan (other than in the case of an Employee's
          death or total and permanent disability as determined by the
          Committee) prior to one year from the date of acquisition of such
          Shares on their behalf.

          E.   Shares Retained by the Custodian: Accumulation of Shares not
transferred to Participants under Section IV.D shall be held by the Custodian
for the account of the Participant entitled thereto, but all rights accruing to
an owner of record on such Shares shall belong to and be vested in the
Participant for whose account it is being held, including the right to receive
any and all dividend payable in respect of such Shares whether in cash, shares
of the Company's Common Stock or otherwise, and the right to receive all notices
of stockholders' meetings (which shall be forwarded to the Participant by the
Custodian without delay) and to direct the Custodian how to vote thereat to the
same extent as if such Shares were held in street name by a brokerage firm or
otherwise.

          F.   Withdrawal:

               1.   An electing Participant may discontinue his/her election and
          withdraw from the Plan as of the payroll period next following 30 days
          from the date written notice on a change authorization and payroll
          withholding form approved by the Committee is received by his/her
          employer's payroll department; provided, however, that an electing
          Participant who shall have discontinued his/her election and withdrawn
          from the Plan may not resubscribe to the Plan prior to the Enrollment
          Date coincident with or next following twelve (12) months from the
          effective date of such discontinuance.

               2.   A Participant shall be deemed to have discontinued his/her
          election and withdrawn from the Plan immediately upon the occurrence
          of any of the following:

               a.   The termination for any reason of the Participant's
               employment with the Company or with an Affiliate. A Participant's
               employment shall not be deemed terminated by reason of a transfer
               to another employer which is the Company or an Affiliate. A
               Participant who has elected participation under the Plan who is
               absent from work with the Company or with an Affiliate because of
               temporary disability (any disability other than a permanent or
               total disability) or who is on leave of absence for any



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               purpose authorized by his/her employer and permitted by an
               authoritative interpretation (e.g., regulation, ruling, case law,
               etc.) of Section 423 of the Code, shall not, during the period of
               any such absence, be deemed, by virtue of such absence alone, to
               have terminated his/her employment with the Company or with an
               Affiliate, except as the Committee may otherwise expressly
               provide or determine.

               b.   Death of the Participant.

               c.   The filing with or levying upon the Company or the Custodian
               of any judgment, attachment, garnishment, or other court order
               affecting either the Participant's earnings or his/her account
               under the Plan.

               d.   Termination of the Plan prior to its expiration.

               e.   Expiration of the Plan.

               3.   Upon the discontinuance of an election and withdrawal from
          the Plan by a Participant, all Shares in the account of the
          Participant shall be transferred out of the Custodian's name and into
          the name of the Participant and a certificate evidencing such Shares
          shall be issued in the name of and delivered to the Participant; and
          all dividends and remaining cash if any credited to his/her account
          shall be paid to the Participant.

          G.   Employer Contribution: Each Participant's employer will, as
frequently as is necessary contribute an amount equal to the difference between
the Employee Share Price as determined at Section V.A and the cost per share to
the Custodian if the Shares are not acquired from the Company. If the Shares are
acquired from the Company, the Company shall sell such Shares to the Custodian
at a price equal to the Employee Share Price.


V.  TERMS AND CONDITIONS OF OPTIONS AND ISSUANCE OF SHARES

          No Option shall be granted to a Participant, and no purported grant of
any Option shall be effective, until an enrollment application and payroll
withholding form approved by the Committee shall have been duly executed on
behalf of the Company and by the Participant. Such enrollment application and
payroll withholding form and the agreement constituted thereby shall be subject
to at least the following terms and conditions:

          A.   Employee Share Price: The "Employee Share Price" as of a Monthly
Share Purchase Date shall be eighty-five percent (85%) of the lower of the:

               1.   Fair Market Value of the Shares at the date of grant of the
          Option (i.e., the applicable January 1 or July 1 Enrollment Date); or

               2.   Fair Market Value of the Shares at the Monthly Share
          Purchase Date.



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          B.   Effect of Death and Participant's Survivors: In the event that a
Participant to whom an Option has been granted ceases to be an employee of the
Company or of an Affiliate by reason of such Participant's death, such Option to
the extent exercisable but not exercised on the date of death shall be deemed
exercised by the Participant's Survivors to the extent of any monies contributed
by the Participant and his/her employer prior to the Participant's death.

          A Participant may determine that a designated person shall become the
Participant's Survivor either by selecting a joint account (with a right of
survivorship running to such designated joint owner), or by so designating in
his/her will or otherwise as controlled under the applicable law with respect to
testamentary dispositions. In the absence of a valid disposition the applicable
laws of descent and distribution shall control. The Custodian may require such
proof and indemnification (documentary or otherwise) as it deems necessary and
appropriate before releasing any Shares and/or funds in a Participant's account
to a person other than the Participant.

          C.   Assignability and Transferability of Options: By its terms, an
Option granted to a Participant shall not be transferable by the Participant
otherwise than by will or by the laws of descent and distribution or pursuant to
a qualified domestic relations order as interpreted under Rule 16b-3 promulgated
under the Exchange Act, and shall be exercisable, during the Participant's
lifetime, only by such Participant. Such Option shall not be assigned, pledged
or hypothecated in any way (whether by operation of law or otherwise). Any
attempted transfer, assignment, pledge, hypothecation or other disposition of
any Option or of any rights granted hereunder contrary to the provisions of this
Section V.C shall be null and void.

          D.   All Participants to Have Equal Rights and Privileges: All
Participants shall have equal rights and privileges under the Plan. The fact
that the maximum number of Shares which may be acquired by Participants bears a
uniform relationship to compensation or is limited by a maximum purchase
restriction shall not be deemed to be violative of the foregoing sentence.

VI.  ADJUSTMENTS UPON CHANGES IN CAPITALIZATION

          In the event that the outstanding shares of the Company's Common Stock
are changed into or exchanged for a different number or kind of shares or other
securities of the Company or of another corporation by reason of any
reorganization, merger, consolidation, recapitalization, reclassification,
change in par value, stock split-up, combination of shares or dividend payable
in capital stock, or the like, appropriate adjustment shall be made in the
number and kind of Shares for the purchase of which Options may be granted under
the Plan and, in addition, appropriate adjustment to prevent dilution or
enlargement of the rights granted to or available for Participants shall be made
in the number and kind of Shares and in the Employee Share Price of outstanding
Options so that each Option holder shall be in a position equivalent to the
position the Option holder would have been in had the Option holder exercised
the Options immediately prior to the applicable event. No such adjustment shall
be made which shall, within the meaning of the applicable provisions of the
Code, constitute such a modification, extension or renewal of an Option as to
cause


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it to be considered as the grant of a new Option.

VII. EFFECT OF DISSOLUTION OR LIQUIDATION OF THE COMPANY

          Upon the dissolution or liquidation of the Company other than in
connection with a transaction to which the preceding Article VI is applicable,
all Options granted hereunder shall terminate and become null and void;
provided, however, that if the rights of a Participant or a Participant's
Survivors hereunder have not otherwise terminated and expired, the Participant
or the Participant's Survivors shall be deemed to have exercised such Options to
the extent of any monies contributed by the Participant or his/her employer as
of the date immediately prior to such dissolution or liquidation.

VIII.  TERMINATION OF THE PLAN

         No Options shall be granted after December 31, 2007. The Plan may be
terminated at an earlier date by vote of either the stockholders of the Company
or the Board. The termination of the Plan shall not affect any Options granted
or Shares acquired prior to the effective date of such termination.

IX.  AMENDMENT OF THE PLAN

          Except as provided in the following sentence, the Plan may be amended
by the stockholders of the Company, the Board, or the Committee, including
amendment of the Plan from time to time to designate corporations whose
employees may be offered Options under the Plan from among a group consisting of
the Company and Affiliates. Amendments effecting: (i) any increase in the
aggregate number of Shares which may be issued under the Plan (other than an
increase merely reflecting a change in capitalization such as stock dividend or
stock split) or (ii) changing the designation of corporations whose employees
may be offered options under the Plan, except designations described in the
preceding sentence, must be approved by the stockholders within twelve (12)
months after such amendment is adopted by the Board or by the Committee or such
amendment is void ab initio. In addition, if the scope of any amendment is such
as to require stockholder approval in order to comply with Rule 16b-3 under the
Exchange Act, such amendment shall also require approval by the stockholders. No
amendment shall affect any Options theretofore granted or any Shares theretofore
acquired by a Participant, unless such amendment shall expressly so provide and
unless any Participant to whom an Option has been granted who would be adversely
affected by such amendment consents in writing thereto.

X.  EMPLOYMENT RELATIONSHIP

          Nothing herein contained shall be deemed to prevent the Company or an
Affiliate from terminating the employment of a Participant, nor to prevent a
Participant from terminating his/her employment with the Company or an
Affiliate.


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XI.   OPTION HOLDERS NOT STOCKHOLDERS

          Neither the granting of an Option nor the deduction from payroll shall
constitute an Employee the owner of Shares covered by an Option until such
Shares have been purchased on his/her behalf pursuant to Article IV.

XII.  WITHHOLDING TAXES

          Any taxes subject to withholding payable with respect to the amounts
to be paid to the Custodian pursuant to the provisions hereof will be deducted
by a Participant's employer from the balance of the Participant's salary, and
not reduce the amounts so to be paid to the Custodian.

XIII. USE OF FUNDS BY THE COMPANY

          The proceeds received by the Company from the sale of Shares pursuant
to Options granted under the Plan will be used for general corporate purposes.

XIV.  STATEMENT OF ACCOUNT

         Following each purchase of Shares on behalf of a Participant, such
Participant will receive from the Custodian a statement of his/her account
showing (i) the respective total amounts of payments (by the Participant and, if
applicable, his/her employer) made to the Custodian on behalf of such
Participant under Paragraph IV.C.1, (ii) the Participant's share of any cash
dividends and other cash distributions and of the amount and proceeds of sale of
any other distributions or rights received by the Custodian, (iii) the total
cost of all Shares purchased by the Custodian for the account of such
Participant, (iv) such Participant's share of any stock dividends on the Shares,
and (v) the number of Shares delivered, or to be delivered, to such Participant
with respect to the period since the last statement.

XI.   BROKERAGE COMMISSIONS AND OTHER COSTS

          Brokerage commissions, if any, payable in connection with the purchase
of Shares hereunder (and shares acquired through dividend reinvestment, if any)
and transfer taxes payable in connection with the delivery to Participants of
Shares acquired hereunder (and shares acquired through dividend reinvestment, if
any) together with the other costs and expenses incurred in administering the
Plan, including the fees and expenses of the Custodian, will be borne by the
Company and Affiliates.

XIV.  EFFECTIVE DATE

          The Plan became effective on October 1, 1983.




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