<PAGE>

                                                                   EXHIBIT 10.13

                                BIOGEN IDEC INC.

                  VOLUNTARY EXECUTIVE SUPPLEMENTAL SAVINGS PLAN
              (As Amended and Restated; Effective January 1, 2004)

<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<CAPTION>

                                                                                                             Page
<S>                                                                                                          <C>
ARTICLE 1         INTRODUCTION.............................................................................    1
      1.1         Purpose and Effective Date...............................................................    1

ARTICLE 2         DEFINITIONS..............................................................................    1
      2.1         401(k) restoration.......................................................................    1
      2.2         Base salary..............................................................................    1
      2.3         Biogen Idec..............................................................................    1
      2.4         Biogen SERP..............................................................................    1
      2.5         Board....................................................................................    1
      2.6         Bonus....................................................................................    1
      2.7         Change in Control........................................................................    1
      2.8         Code.....................................................................................    1
      2.9         Committee................................................................................    1
      2.10        Compensation Committee...................................................................    1
      2.11        Disability...............................................................................    2
      2.12        Employee.................................................................................    2
      2.13        Employer.................................................................................    2
      2.14        ERISA....................................................................................    2
      2.15        Participant..............................................................................    2
      2.16        Plan.....................................................................................    2
      2.17        Plan year................................................................................    2
      2.18        Prior plan...............................................................................    2
      2.19        Savings Plan.............................................................................    2
      2.20        Service..................................................................................    2
      2.21        Transition Contribution..................................................................    2
      2.22        Voluntary Deferred Compensation..........................................................    2
      2.23        Years of service.........................................................................    2

ARTICLE 3         PARTICIPATION............................................................................    2
      3.1         Eligibility and Participation............................................................    2
      3.2         End of Participation.....................................................................    3

ARTICLE 4         SAVINGS DEPOSITS AND DEFERRALS BY PARTICIPANTS; EMPLOYER CREDITS.........................    3
      4.1         401(k) Restoration.......................................................................    3
      4.2         Voluntary Deferrals......................................................................    4
      4.3         Transition Contribution..................................................................    4
      4.4         Sign-Up Procedure for 401(k) Restoration and Voluntary Deferral Compensation.............    4

ARTICLE 5         PARTICIPANTS' ACCOUNTS...................................................................    5
      5.1         Participant Accounts.....................................................................    5
      5.2         Participant's Account Value..............................................................    6
      5.3         Vesting..................................................................................    7

ARTICLE 6         DISTRIBUTIONS TO PARTICIPANT.............................................................    8
      6.1         Distributions for Financial Hardship.....................................................    8
      6.2         In-Service Distribution(s) at a Time Specified by Participant............................    8
      6.3         Distribution upon Death of a Participant.................................................    9
      6.4         Distribution upon Participant's Termination of Employment................................    9
      6.5         Installment Distributions in Certain Cases...............................................    9

ARTICLE 7         MISCELLANEOUS............................................................................   10
      7.1         Amendment or Termination of Plan.........................................................   10
      7.2         Benefits Not Currently Funded............................................................   10
</TABLE>

<PAGE>

<TABLE>
<S>                                                                                                           <C>
      7.3         No Assignment............................................................................   11
      7.4         Responsibilities and Authority of Committee..............................................   11
      7.5         Limitation on Rights Created by Plan.....................................................   11
      7.6         Tax Withholding..........................................................................   12
      7.7         Text Controls............................................................................   12
      7.8         Applicable State Law.....................................................................   12
</TABLE>

<PAGE>

                                    ARTICLE 1
                                  INTRODUCTION

1.1      PURPOSE AND EFFECTIVE DATE. The purpose of this plan is to provide
certain key executives and managers of Biogen Idec (or its subsidiaries) with
additional tax-deferred savings opportunities supplementing those available
under the Savings Plan. This plan allows participants whose compensation exceeds
the amount of compensation that may be taken into account by the Savings Plan
for any plan year (the Code Section 401(a)(17) limits) to make savings deposits
hereunder from such excess compensation with matching Biogen Idec contributions
on the same basis as is provided in the Savings Plan, and allows participants to
make additional, unmatched savings deposits from base salary or bonus if elected
by a participant.

         This plan also contains account balances previously maintained under
the amended and restated IDEC Pharmaceuticals Corporation Deferred Compensation
Plan, the Biogen, Inc. Voluntary Executive Supplemental Savings Plan, and the
Biogen, Inc. Supplemental Executive Retirement Plan.

         The plan is effective January 1, 2004.

                                    ARTICLE 2
                                   DEFINITIONS

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

2.1      401(k) RESTORATION means that component of the plan which permits a
participant to make savings deposits from applicable compensation in excess of
the limit imposed by Section 401(a)(17) of the Code.

2.2      BASE SALARY means the base salary established for any participant by
his employer as in effect from time to time; the entire amount of a
participant's base salary will be taken into account in accordance with the
terms of this plan without regard to any dollar limitation on applicable
compensation that may be imposed under the Savings Plan; base salary includes
all components of a participant's applicable compensation other than bonus.

2.3      BIOGEN IDEC means Biogen Idec Inc., a Delaware corporation, or any
successor to it or to all or the major portion of its assets or business which
assumes the obligations of Biogen Idec Inc. under this plan.

2.4      BIOGEN SERP means the Biogen, Inc. Supplemental Executive Retirement
Plan, as in effect immediately prior to January 1, 2004 (or other date of
transfer referred to in Section 3.1(d)).

2.5      BOARD means the Board of Directors of Biogen Idec.

2.6      BONUS means the amount of compensation paid to a participant in
addition to his base salary and designated as such participant's bonus by his
employer; the entire amount of any such bonus will be taken into account in
accordance with the terms of this plan without regard to any dollar limitation
on applicable compensation that may be imposed under the Savings Plan.

2.7      CHANGE IN CONTROL means a "Corporate Change in Control" as defined in
the IDEC Pharmaceuticals Corporation 2003 Omnibus Equity Plan, as in effect from
time to time (or the corresponding provisions of any successor instrument).

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

2.9      COMMITTEE means the committee designated by the Board to administer
this Plan.

2.10     COMPENSATION COMMITTEE means the Compensation Committee of the Board.

                                        1
<PAGE>

2.11     DISABILITY means "disability" as defined under the long-term disability
program of Biogen Idec or another employer covering a participant, or, if no
such program is in effect with respect to such participant, then "disability"
means "total and permanent disability" as defined in Section 22(e)(3) of the
Code.

2.12     EMPLOYEE means a person who is classified as a regular, common law
employee of Biogen Idec (or other employer) under the regular personnel
classifications and practices of his employer. An individual will not be
considered an employee for purposes of this plan if the individual is classified
as a consultant or contractor under Biogen Idec's (or other employer's) regular
personnel classifications and practices or he is a party to an agreement to
provide services to Biogen Idec (or other 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.

2.13     EMPLOYER means Biogen Idec and each direct or indirect subsidiary or
other affiliate of Biogen Idec that employs persons who are or may be eligible
to participate in this plan.

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

2.15     PARTICIPANT means an employee of Biogen Idec (or other employer) who is
eligible to participate in this plan in accordance with Section 3.1 hereof and
who has an account described in Section 5.1 hereunder or for whom an amount has
been transferred to this plan from a prior plan.

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

2.17     PLAN YEAR means the 12-month periods commencing on January 1, 2004 and
on each subsequent January 1 while the plan remains in effect.

2.18     PRIOR PLAN means the amended and restated IDEC Pharmaceuticals
Corporation Deferred Compensation Plan and the Biogen, Inc. Voluntary Executive
Supplemental Savings Plan, each as in effect immediately prior to January 1,
2004 (or other date of transfer referred to in Section 3.1(d)).

2.19     SAVINGS PLAN means the Biogen Idec 401(k) Savings Plan, as amended from
time to time. Any term defined in the Savings Plan will have the same meaning
when used in this plan unless otherwise defined herein.

2.20     SERVICE means the sum of a participant's employment (a) with Biogen
Idec since November 12, 2003 and (b) with either Biogen, Inc. or IDEC
Pharmaceuticals Corporation prior to November 12, 2003 (including in each case
service with any subsidiary or other affiliate of such entity).

2.21     TRANSITION CONTRIBUTION means an amount that would be contributed by
Biogen Idec to a participant's account under the Savings Plan, as determined
under Appendix C of the Savings Plan but (a) without regard to the
nondiscrimination limits or the Code Section 415 limits imposed on the Savings
Plan and (b) reduced by the amount of the actual contribution on the
participant's behalf to the Savings Plan in accordance with Appendix C thereof.

2.22     VOLUNTARY DEFERRED COMPENSATION means that component of the plan which
permits a participant to defer from 1% to 80% of his base salary and from 1% to
100% of his bonus.

2.23     YEARS OF SERVICE means full years of completed continuous service as a
regular employee, determined in accordance with the personnel policies and
practices of a participant's employer.

                                    ARTICLE 3
                                  PARTICIPATION

3.1      ELIGIBILITY AND PARTICIPATION.

         (a)      401(k) Restoration. A person (i) who is an employee of Biogen
Idec (or another employer), (ii) who is eligible to participate in the Savings
Plan, and (iii) whose base salary and bonus for a plan year exceed the

                                        2
<PAGE>

limit under Code Section 401(a)(17) applicable to such year will be eligible to
be a participant in the 401(k) restoration component of the plan.

         (b)      Voluntary Deferred Compensation. An employee (i) who has the
job title of Senior Director or Vice President or more senior officer of Biogen
Idec (or another employer) or (ii) who is designated as eligible by the
Compensation Committee will be eligible to be a participant in the voluntary
deferred compensation component of the plan. Participation in this component of
the plan is voluntary and no eligible employee will be required to participate.

         (c)      Transition Contribution. An employee (i) whose "additional
employer contribution" as determined under Appendix C of the Savings Plan is
limited because of limits on compensation, limits on contributions or
nondiscrimination requirements and (ii) who is designated by the committee will
be eligible to be a participant in the transition contribution component of the
plan.

         (d)      Prior Plan and Biogen SERP. Each employee who is not eligible
to be a participant under subsection (a), (b), or (c) above, or who is eligible
but declines to participate under subsection (b) above, but who was a prior plan
and/or Biogen SERP participant and whose prior plan and/or Biogen SERP account
balance was transferred to this plan effective as of January 1, 2004 (or such
later date as the committee specifies) will be a participant solely with respect
to such transferred prior plan and/or Biogen SERP account balance. This will not
include a person who is a vested participant under the Biogen SERP but not an
employee (i.e., a person who terminated employment from Biogen, Inc. or from
Biogen Idec on or before the date of transfer referred to in the preceding
sentence); such a person's benefits under the Biogen SERP are governed by the
provisions of Section 5.1(g).

         An eligible employee under subsection (a), (b) or (c) above will become
a participant hereunder when he makes a savings deposit or a voluntary deferral
under this plan or when Biogen Idec makes a contribution on his behalf.

3.2      END OF PARTICIPATION. A participant's participation in this plan will
end upon the termination of his service as an employee of Biogen Idec (or other
employer) because of death or any other reason.

         In addition, in the case of a participant who was designated as
eligible by the Compensation Committee, his participation will end upon the
Compensation Committee's specifying that he is no longer eligible to
participate. In such event, his participation will end effective as of the later
of the date of the Compensation Committee's action or the date specified by the
Compensation Committee; provided that no such action will retroactively deprive
a participant of any amount credited to his account or any benefit he was
entitled to under this plan calculated as of the effective date of his
termination of participation.

         Upon the termination of a participant's participation in this plan in
accordance with this section, the participant may make no further savings
deposits or voluntary deferrals hereunder and there will be no additional
employer matching credits to such participant's account. However, the
participant will be entitled to receive any vested amounts in his accounts in
accordance with this plan.

                                    ARTICLE 4
        SAVINGS DEPOSITS AND DEFERRALS BY PARTICIPANTS; EMPLOYER CREDITS

4.1      401(k) RESTORATION.

         (a)      Savings Deposits. Each eligible employee (under Section
3.1(a)) who has elected to participate in the Savings Plan will have savings
deposits to this plan taken from his base salary and from his bonus in the same
percentages applicable to such amounts as in effect under the Savings Plan as of
January 1 of a year, provided that savings deposits to this plan will be taken
only from the portion of his applicable compensation that exceeds the limit
under Code Section 401(a)(17) for such year (his "excess applicable
compensation"). The percentages so determined shall remain in effect under this
plan for the balance of the plan year, subject to the fourth paragraph of
Section 4.4(a).

                                        3
<PAGE>

         All amounts by which a participant reduces his bonus or the remainder
of his applicable compensation under the preceding paragraph are referred to
herein as the participant's savings deposits. The amount by which a
participant's savings deposits for a plan year hereunder do not exceed 3% of his
excess applicable compensation are referred to herein as his matchable savings
deposits.

         (b)      Employer Matching Credits.

                  (i)      Amount of Matching Employer Credits. For each
                           calendar quarter (or a shorter period of time
                           specified by the committee) during a plan year, each
                           employer will credit a matching contribution amount
                           to the account of each participant employed by such
                           employer who makes matchable savings deposits under
                           subsection (a) above during such calendar quarter (or
                           such shorter period of time). The employer's matching
                           contribution credits will be equal to 200% of the
                           participant's matchable savings deposits during the
                           calendar quarter (or such shorter period of time).

                  (ii)     Time for Making Employer Matching Credits. The
                           employer's matching amounts under subsection (i) will
                           be credited to participants' accounts as soon as
                           practicable after each calendar quarter (or such
                           shorter period of time specified by the committee).

4.2      VOLUNTARY DEFERRALS. Each eligible employee (under Section 3.1(b)) may
make voluntary deferrals under the plan from his base salary in any whole
percentage of his base salary from a minimum of 1% to a maximum of 80% by
agreeing to reduce his base salary by such amount. In addition, each eligible
employee may make voluntary deferrals under the plan from his bonus in any whole
percentage of his bonus from a minimum of 1% to a maximum of 100% by agreeing to
reduce his bonus by such amount.

4.3      TRANSITION CONTRIBUTION. Each participant (under Section 3.1(c)) who is
eligible to receive an additional employer contribution as described in Appendix
C of the Savings Plan will be eligible to receive a transition contribution
under this plan if his contribution under Appendix C was limited by
nondiscrimination requirements or by limits imposed by Code Section 401(a)(17)
or 415. The amount of such contribution will be the amount the participant would
have received under the terms and conditions of Appendix C of the Savings Plan
if such nondiscrimination requirements and such Code Section 401(a)(17) or 415
limits did not apply, reduced by any amount contributed to the Savings Plan on
his behalf under Appendix C of the Savings Plan. Any such transition
contribution hereunder will be made at the same time as the additional employer
contribution under the Savings Plan.

4.4      SIGN-UP PROCEDURE FOR 401(k) RESTORATION AND VOLUNTARY DEFERRAL
         COMPENSATION.

         (a)      Sign-Up Procedure. An eligible employee who wishes to reduce
his base salary and/or bonus with respect to a particular plan year in order to
make voluntary deferrals under Section 4.2 must complete an enrollment form
specifying the amount of his voluntary deferrals (with separate percentages for
his base salary and bonus if desired), agreeing to reduce his base salary and/or
bonus by the amount(s) desired, and providing such other information as the
committee may require.

         A participant's initial enrollment form (or another form specified by
the committee) will also specify the time for payment (or the commencement of
installment payments) under Section 6.4 and the form of payment (lump sum or
installments in accordance with Section 6.5(a) below) of his accounts hereunder.
The time specified for payment may be anytime the participant indicates, but not
later than the participant's termination of employment. In addition, a
participant's initial enrollment form may (but is not required to) specify one
or more in-service distributions to the participant in accordance with Section
6.2 if desired by the participant.

         A participant's enrollment form electing savings deposits or voluntary
deferrals for any plan year must be filed with the committee by such deadline as
the committee specifies, but in any event before the start of such plan year. A
participant may change the amount of his voluntary deferrals (but not the time
for payment or the form of payment of his accounts except as provided below)
with respect to any subsequent plan year by filing a new enrollment form before
the start of such subsequent plan year, and the change will become effective as
of the first day of such subsequent plan year. Once a participant has elected to
defer base salary and/or bonus, his enrollment

                                        4
<PAGE>

form will remain in effect for future plan years unless the participant changes
or terminates his prior elections by filing a new enrollment form in accordance
with the preceding sentence.

         After a plan year has begun, a participant may not change the amount of
savings deposits established for such plan year or the amount of voluntary
deferrals (if any) he had elected for such plan year. However, if a participant
has an unforeseeable financial hardship (as defined in Section 6.1) or other
significant financial difficulty during a year, with the consent of the
committee the participant may reduce or cancel his savings deposits and/or
voluntary deferrals election for the balance of that year.

         (b)      Change of Election. Notwithstanding the second paragraph of
subsection (a) above, at anytime prior to the date for payment originally
elected by the participant, if the participant is still an employee of Biogen
Idec (or another employer or other subsidiary or affiliate) at such time, the
participant may elect to defer the time when his account(s) would otherwise be
payable (or installment payments would otherwise begin) to a subsequent date
specified by him (not later than the latest time permitted under subsection (a))
or may elect installments (or a greater number of installments, subject to the
limitations of the plan). If such election becomes effective as provided below,
then the participant's account(s) will be payable at the time specified in his
subsequent election. The participant's election under this subsection (b) will
become effective if any of the following criteria is satisfied: (i) the
participant remains an employee of Biogen Idec (or another employer or other
subsidiary or affiliate) for at least one year after making such election, (ii)
the participant's service as an employee of Biogen Idec (or another employer or
other subsidiary or affiliate) ends due to disability, or (iii) the
participant's employment as an employee of Biogen Idec (or another employer or
other subsidiary or affiliate) is involuntarily terminated without cause.

         A participant may make only one election under this subsection (b) to
further defer payment.

                                    ARTICLE 5
                             PARTICIPANTS' ACCOUNTS

5.1      PARTICIPANT ACCOUNTS.

         (a)      401(k) Restoration Accounts. Savings deposits by a participant
under Section 4.1(a) will be credited to an account in the name of such
participant. Such account will be called his 401(k) restoration account.

         (b)      Employer Matching Credits Accounts. Employer credits on a
participant's behalf under Section 4.1(b) will be credited to an account in the
name of such participant. Such account will be called his employer matching
credits account.

         (c)      Voluntary Deferred Compensation Accounts. Voluntary
deferrals by a participant under Section 4.2 will be credited to an account in
the name of such participant. Such account will be called his voluntary deferred
compensation account.

         (d)      Transition Contribution Accounts. Transition contributions
made on a participant's behalf under Section 4.3 will be credited to an account
in the name of such participant. Such account will be called his transition
contribution account.

         (e)      Prior Plan Account. Account balances as of December 31, 2003
(or such later date as the committee specifies) for a participant in a prior
plan will be transferred to this plan from such prior plan and the transferred
amount will be credited to an account in the name of such participant. Such
account will be called his prior plan account.

         (f)      Biogen SERP Account. Amounts transferred to this plan from
the Biogen SERP on behalf of a participant will be credited to an account in the
name of such participant. Such account will be called his Biogen SERP account.
The amount so transferred on behalf of a participant in the excess benefit
formulas in Section 4.2 of the SERP will be the amount credited to such
participant's SERP cash balance account as of December 31, 2003 (or such later
date as

                                        5
<PAGE>

the committee specifies) (calculated in accordance with the terms of the SERP in
effect on such date) of the accrued supplemental pension as of such date.

         (g)      Certain Special Provisions. Participants' prior plan accounts
and Biogen SERP accounts will be governed by the applicable provisions of this
plan as in affect from time to time.

         For persons who were participants in the Biogen SERP before the
transfer date referred to in subsection (f) above and are entitled to a vested
benefit under subsection (f) above, but who are not active participants under
this plan and therefore do not have a Biogen SERP account hereunder, the Biogen
SERP benefit (including the amount, time and form of payment) will be determined
under the terms of the Biogen SERP. The applicable terms will be those in effect
as of December 31, 2003 (plus subsequent amendments, if any). For this purpose,
the Biogen SERP, as in effect on December 31, 2003 (or other date of transfer so
referred to) and as subsequently amended, is deemed to be an appendix to this
plan and is incorporated as such by this reference.

5.2      PARTICIPANT'S ACCOUNT VALUE.

         (a)      Investments. A participant's accounts will be credited with
deemed investment results as if the amounts were invested in one or more
designated investment funds and all dividends and distributions on shares or
other interests of a particular investment fund were reinvested in such fund.
The investment funds available for this purpose will be those from time to time
available as investment options for participants' accounts under the Savings
Plan, plus the investment funds specified in subsections (b) and (c) below.
Investment funds hereunder are for the sole purpose of providing the basis for
crediting deemed investment results to participants' accounts, and do not
represent any actual funds or assets held hereunder for the benefit of
participants.

         Each participant will indicate with his initial enrollment form (or
another form specified by the committee) the investment fund or funds (and the
proportion in each fund when the participant designates more than one) he wishes
to designate for this purpose. Thereafter, a participant may change his
designation either with respect to the deemed investment of future contributions
or the deemed transfer of amounts from a previously designated investment fund
to another fund. The committee shall establish the frequency with which such a
change may be made, the method of making such a change, and the effective date
of such a change, and shall prescribe such other rules and procedures as it
deems appropriate. Such designation will remain in effect until subsequently
changed by the participant in accordance with this paragraph.

         Notwithstanding the preceding paragraph, the committee may establish
one or more default investment funds that will be used to determine deemed
investment results in the case of any participant or group of participants who
have not made a designation under the preceding paragraph. Such default
investment fund(s) will be used to determine deemed investment results
applicable to the account of such participant or participants until any such
participant makes a designation of investment fund(s) in accordance with the
plan.

         Deemed investment results under this subsection will be credited to a
participant's accounts effective as of the last day in each calendar quarter (or
such shorter time specified by the committee).

         The value of a participant's accounts at any point in time will be his
savings deposits, voluntary deferrals, employer matching credits, transition
contributions on his behalf, and prior plan and/or Biogen SERP transfer amounts,
increased or decreased by deemed investment results as provided in this
subsection (a) through the most recent calendar quarter (or such shorter time
specified by the committee), and reduced by any distributions from the
participant's accounts.

         (b)      Fixed Income Option. In addition to the investment funds
offered under the Savings Plan as described in (a) above, a participant may
elect to have his accounts credited with the deemed investment results as if
such amounts were invested in a fixed income option earning a rate of return
specified by the committee. The rate of return under the fixed income option
will be 8% for the 2004 plan year. The rate of return of future plan years will
be determined each year by the committee.

         (c)      Exception for Certain Prior Plan Accounts. Former participants
in the IDEC Pharmaceuticals Corporation Deferred Compensation Plan whose
accounts were credited with interest under the fixed income option available
under that plan immediately prior to the date such account was transferred to
this plan may continue to

                                       6
<PAGE>

have such transferred amount credited with interest under that fixed income
option. Any additional contributions made under this plan will be credited with
deemed investment results as described in subsection (a) or (b) above. Amounts
being credited with interest under this subsection (c) may be transferred to an
option described in subsection (a) or (b) above, but no amounts credited to a
participant's accounts may be transferred into the fixed income option under
this subsection (c) (even if such amounts had previously been invested in such
investment fund and then transferred to another investment fund).

         (d)      Bookkeeping Accounts. Participants' accounts and subaccounts
will be maintained on the books of the participant's employer for bookkeeping
purposes only; such accounts will not represent any interest in any trust or in
any segregated asset.

         In order to facilitate the administration of the plan, the committee
may arrange for a participant's account to be divided for record keeping
purposes into two or more subaccounts, in accordance with procedures established
by the committee.

5.3      VESTING.

         (a)      401(k) Restoration Account. A participant will have a fully
vested interest in his 401(k) restoration account at all times.

         (b)      Employer Matching Credits Account. A participant who is an
active employee will have a fully vested interest in his employer matching
credits account at all times on and after his 55th birthday. Before that date,
such a participant will have a vested interest in that percentage of his
employer matching credits account specified in the following table based upon
his number of years of service under the plan:

<TABLE>
<CAPTION>
Years of Service           Vested Percentage
----------------           -----------------
<S>                        <C>
Less than 1                        0%
1                                 25%
2                                 50%
3                                 75%
4 or more                        100%
</TABLE>

         (c)      Voluntary Deferred Compensation Account. A participant will
have a fully vested interest in his voluntary deferred compensation account at
all times.

         (d)      Transition Contribution Account. A participant who is an
active employee will have a fully vested interest in his transition contribution
account at all times on and after his 65th birthday. Before that date, such a
participant will have a vested interest in that percentage of his transition
contribution account specified in the following table based upon his number of
years of service under the plan:

<TABLE>
<CAPTION>
Years of Service    Vested Percentage
----------------    -----------------
<S>                 <C>
Less than 2                0%
2                         20%
3                         50%
4                         60%
5                         70%
6                         80%
7 or more                100%
</TABLE>

         (e)      Prior Plan Account. A participant will have a fully vested
interest in his prior plan account at all times.

         (f)      Biogen SERP Account. A participant who is an active employee
will have a fully vested interest in his Biogen SERP account at all times on and
after his 65th birthday. Before that date, such a participant will have a

                                       7
<PAGE>

vested interest in that percentage of his Biogen SERP account specified in the
following table based upon his number of years of service under the plan:

<TABLE>
<CAPTION>
Years of Service    Vested Percentage
----------------    -----------------
<S>                 <C>
Less than 2                0%
2                         20%
3                         50%
4                         60%
5                         70%
6                         80%
7 or more                100%
</TABLE>

         (g)      Full Vesting upon Death, Disability or Change in Control.
Notwithstanding subsections (b), (d) and (f) above:

                  (i)      If a participant's employment by his employer (or
                           another Biogen Idec subsidiary or affiliate) is
                           terminated because of the participant's death or
                           disability, all his accounts hereunder will be fully
                           vested regardless of his number of years of service.

                  (ii)     In the event of a change in control of Biogen Idec,
                           all accounts of all participants will be fully vested
                           regardless of a participant's number of years of
                           service.

         (h)      Meaning of "Fully Vested." Reference to any account of a
participant as "fully vested" means that such account is not subject to
forfeiture; however, all participant accounts, including fully vested accounts,
are subject to fluctuation as a result of the crediting of deemed investment
results (including losses) to such accounts as provided in the plan.

                                    ARTICLE 6
                          DISTRIBUTIONS TO PARTICIPANT

6.1      DISTRIBUTIONS FOR FINANCIAL HARDSHIP. If a participant has a serious
financial hardship, he may apply to the committee for a distribution from the
plan prior to his termination of employment with his employer or other
designated time for payment. If such application for a hardship distribution is
approved by the committee, the distribution will be made as soon as practicable
after the later of the date specified in the participant's application or the
date of approval by the committee. The amount of the distribution will be the
amount needed to alleviate the participant's financial hardship, as determined
by the committee, up to a maximum of the participant's vested account balances.
Such a distribution will be made from the participant's accounts in a single
lump-sum payment. If such a participant's account has two or more subaccounts,
the committee will determine which subaccounts will be debited to reflect the
financial hardship distribution.

         Financial hardship will be limited to the following: bankruptcy or
impending bankruptcy, unexpected and unreimbursed major expenses resulting from
illness to person or accident to person or property, and to other types of
unforeseeable and unreimbursed expenses of a major nature that normally would
not be budgetable. Financial hardship shall not include foreseeable expenses
such as down payments on a home, purchase of an auto, or college or other
educational expenses.

6.2      IN-SERVICE DISTRIBUTION(S) AT A TIME SPECIFIED BY PARTICIPANT. If, in
his initial enrollment or other election form, a participant designated payment
of his vested account(s) (or a specified portion thereof) at a specified time(s)
and he is still an employee of Biogen Idec (or another employer or other
subsidiary or affiliate) at such time(s), the participant will receive payment
of the amount to be distributed in accordance with such election, payable on or
as soon as practicable after the designated date(s). A participant's election
for in-service distributions under this Section 6.2 may be for a single payment
or up to five annual payments, in each case in an amount or portion specified by
the participant in his enrollment or other election form. Each payment will be
the amount specified (or the entire vested balance remaining in the
participant's accounts, if less).

         Any amount in a participant's accounts hereunder not distributed to the
participant under this Section 6.2 will be distributed under Section 6.3 or 6.4,
whichever may be applicable, and Section 6.5, if applicable. If a

                                       8
<PAGE>


participant is receiving multiple payments under this Section 6.2 and dies or
otherwise terminates employment, payments under this subsection will cease and
subsequent payments will be governed by Section 6.3 or 6.4, as the case may be.

6.3      DISTRIBUTION UPON DEATH OF A PARTICIPANT.

         (a)      In general. If a participant dies while still an employee of
Biogen Idec (or another employer or other subsidiary or affiliate) or after
termination of such employment, but before the complete distribution of his
vested accounts hereunder, his beneficiary will receive the total amount
remaining in his vested accounts. Except as otherwise provided in Section 6.5,
distribution will be made in a single sum payment on a date determined by the
committee, but not later than one year after the committee receives such
evidence of the participant's death and of the right of any beneficiary to
receive payment as it deems necessary.

         (b)      Beneficiary. The beneficiary to receive the payment described
in subsection (a) above will be the same person or persons who are to receive
benefits payable upon the participant's death under the Savings Plan. If more
than one person is a beneficiary, death benefits hereunder will be paid to them
in the same proportions as under the Savings Plan. In the event that a
participant does not participate in the Savings Plan, the participant may
designate one or more beneficiaries to receive a 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 such designation of beneficiary will be in
writing on such form as the committee may prescribe or deem acceptable, and will
be effective upon filing with the committee.

         Any portion of a distribution payable upon the death of a participant
that is not disposed of by a designation of beneficiary under the preceding
paragraph, for any reason whatsoever, will be paid to the participant's 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.

6.4      DISTRIBUTION UPON PARTICIPANT'S TERMINATION OF EMPLOYMENT.
Following a participant's termination of employment for any reason other than
death, except as otherwise provided in Section 6.5, the participant will receive
a single sum payment equal to his vested account balance, payable on a date
determined by the committee but not later than one year after the committee's
receipt of satisfactory evidence of the participant's termination of employment.
If a participant terminates employment because of a disability, payment of his
account balance may be accelerated with the consent of the committee.

6.5      INSTALLMENT DISTRIBUTIONS IN CERTAIN CASES.

         (a)      Participant. Notwithstanding the provisions of Section 6.4, a
participant may, at the time of filing his initial enrollment form under Section
4.4 (or, if applicable, in a subsequent election), designate that the amount
payable to him hereunder will be paid in a number (minimum of two and maximum of
fifteen) of annual installment payments, as specified by the participant.

         (b)      Beneficiary. Notwithstanding Section 6.3, a participant may
designate that, if the participant dies before receiving the entire amount
payable to him hereunder, the beneficiary will receive either:

                  (i)      A number of annual installment payments equal to:

                           (A)      the number the participant elected for
                                    himself under subsection (a) above (if the
                                    participant dies before receiving any
                                    installment payments), or

                           (B)      the number of remaining installment payments
                                    due to the participant under subsection (a)
                                    above (if the participant dies after
                                    receiving one or more installment payments);
                                    or

                  (ii)     A single payment.

         Payment to the beneficiary will be made or begin as provided in Section
6.3(a).

                                       9
<PAGE>

         If the participant fails to designate the form of payment to the
beneficiary, the default form will be installments under (i) above. If
installment payments are payable to the beneficiary, a participant may
subsequently change the form of payment to his beneficiary (but not the form of
payment to himself) to a single payment by filing a written instrument so
specifying with the committee. Notwithstanding the foregoing, a beneficiary may
request a form of payment other than that which the participant designated on
his election form, subject to the approval of the committee.

         (c)      Installment Payments. Where installment payments are due, the
first annual installment payment will be paid out on the date specified in
Section 6.3 or 6.4 (whichever is applicable) and subsequent annual installments
will be paid approximately on succeeding anniversaries of the first payment
date. The amount of each annual installment payment will be determined by
multiplying the then amount of the participant's vested account balances by a
fraction whose numerator is one and whose denominator is the number of remaining
annual installment payments.

         (d)      Death of Beneficiary. If a participant's designated
beneficiary is receiving installment payments and dies before receiving payment
of all the annual installments, the designated beneficiary's estate will receive
a lump-sum payment of the amount remaining to be distributed to such deceased
beneficiary. Such payment will be made as soon as practicable after the
committee's receipt of satisfactory evidence of the death of the designated
beneficiary.

                                    ARTICLE 7
                                  MISCELLANEOUS

         7.1      AMENDMENT OR TERMINATION OF PLAN. Biogen Idec, by action of
the Board or of the Compensation Committee (or such other committee thereof or
officer or officers of Biogen Idec to whom the Board or Compensation Committee
has delegated this authority), at any time and from time to time, may amend or
modify any or all of the provisions of this plan or may terminate this plan
without the consent of any participant (or beneficiary or other person claiming
through a participant). No termination or amendment of the plan may reduce the
amounts credited to the accounts of any participant under the plan (including a
participant whose employment with the employer was terminated before such
termination or amendment) or the vested percentages of such accounts. However,
Biogen Idec may change the deemed investment options under Section 5.2, and
Biogen Idec may upon termination of this plan pay participants' account balances
to the participants regardless of the times elected for payment (or the start of
installment payments) elected by the participants and may pay such amounts in
single sum payments regardless of whether installment distributions would
otherwise be payable under Section 6.5. In addition, Biogen Idec may, from time
to time, make any amendment that it deems necessary or desirable to satisfy the
applicable requirements of the tax laws and ruling and regulations thereunder in
order to preserve, if possible, the tax deferral features of this plan for
participants. No diminution or restriction on a participant's opportunity to
make elections or withdrawals, or exercise other privileges or rights hereunder,
pursuant to the preceding sentence will be deemed to violate the rights of any
participant or beneficiary hereunder so long as such change does not effect a
forfeiture of any of a participant's account balances hereunder or render an
account balance (or portion thereof) which previously was nonforfeitable
forfeitable.

         In addition, any amendment provided for under the preceding paragraph
may be made by the committee, or by the Chairman, Chief Executive Officer or
Executive Vice President - Human Resources of Biogen Idec except for an
amendment that would materially increase or reduce the benefits of the plan to
participants or materially increase the cost of maintaining the plan to the
employers; such committee or officers may not terminate the plan.

7.2      BENEFITS NOT CURRENTLY FUNDED.

         (a)      Nothing in this plan will be construed to create a trust or to
obligate Biogen Idec to segregate a fund, purchase an insurance contract or
other investment, or in any other way currently to fund the future payment of
any benefits hereunder, nor will anything herein be construed to give any
participant or any other person rights to any specific assets of Biogen Idec or
any other entity. However, in order to make provision for its obligations
hereunder, Biogen Idec may in its discretion purchase an insurance contract or
other investment; any such contract or investment will be a general asset
belonging to Biogen Idec, and no participant or beneficiary will have any rights

                                       10
<PAGE>

to any such asset. The rights of a participant or beneficiary hereunder will be
solely those of a general, unsecured creditor of his employer.

         (b)      Notwithstanding subsection (a) above, Biogen Idec in its sole
discretion may establish a grantor trust of which it is treated as the owner
under Code Section 671 to provide for the payment of benefits hereunder, subject
to such terms and conditions as Biogen Idec may deem necessary or advisable to
ensure that benefits are not includable, by reason of the trust, in the taxable
income of trust beneficiaries before actual distribution and that the existence
of the trust does not cause the plan or any other arrangement to be considered
funded for purposes of Title I of ERISA.

7.3      NO ASSIGNMENT.

         (a)      No participant or beneficiary will have any power or right to
transfer, assign, anticipate or otherwise encumber any benefit or amount payable
under this plan, nor shall any such benefit or amount payable be subject to
seizure or attachment by any creditor of a participant or a beneficiary, or to
any other legal, equitable or other process, or be liable for, or subject to,
the debts, liabilities or other obligations of a participant or beneficiary
except as otherwise required by law.

         (b)      Notwithstanding subsection (a) above, all or a portion of a
participant's account balances may be assigned to the participant's spouse,
former spouse, or other dependent (for purposes of this section, an "alternate
recipient") in connection with a court order or property settlement agreement
awarding such portion(s) to the alternate recipient. If any portion of an
account so assigned is not fully vested at such time, such portion will vest
only in accordance with the applicable provisions of this plan based upon the
participant's years of service. Upon receipt of a copy of the relevant
provisions of any such order or property settlement agreement, certified to be
accurate and in effect by the participant, and an acknowledgment by the
alternate recipient that such alternate recipient will be responsible for income
taxes on such amounts when distributed or made available to such alternate
recipient and that such amounts are subject to income tax withholding as
provided in this plan, and such other information (including the alternate
recipient's social security number) as the committee may reasonably request, the
committee will assign such amount to a separate account hereunder and will
distribute such account to the alternate recipient as soon as practicable
thereafter (except for any unvested amounts). Notwithstanding the preceding
sentence, in the sole discretion of the committee, the amount credited to the
alternate recipient's account may be retained in the plan and paid to the
alternate recipient as such time or times as the committee determines, but not
later than the time or times that amounts hereunder are distributed to the
participant. Pending payment of an alternate recipient's account to him or her,
such account will be credited with deemed investment results under Section 5.2
based upon the alternate recipient's designation of one or more investment
funds.

7.4      RESPONSIBILITIES AND AUTHORITY OF COMMITTEE. The committee will control
and manage the operation and administration of the plan except to the extent
that such responsibilities are specifically assigned hereunder to Biogen Idec,
the Board or the Compensation Committee.

         The committee will have all powers and authority necessary or
appropriate to carry out its responsibilities for the operation and
administration of the plan. It will have discretionary authority to 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 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 committee will be
conclusive and binding upon all persons, except as otherwise provided herein or
by law, and except that the committee may revoke or modify a determination or
action previously made in error. It is intended that any action or inaction by
the committee will be given the maximum possible deference by any reviewing body
(whether a court or other reviewing body), and will be reversed by such
reviewing court or other body only if found to be arbitrary and capricious.

         Biogen Idec will be the "plan administrator" and the "named fiduciary"
for purposes of ERISA.

7.5      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.

                                       11
<PAGE>

7.6      TAX WITHHOLDING. Any payment hereunder to a participant, beneficiary or
alternate recipient will be subject to withholding of income and other taxes to
the extent required by law.

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

7.8      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.

                                                       BIOGEN IDEC INC.

                                                     By: /s/ James C. Mullen
                                                        -----------------------
                                                        James C. Mullen
                                                        Chief Executive Officer

                                       7