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                                                                     EXHIBIT 4.1

               SECOND AGREEMENT AND AMENDMENT TO RIGHTS AGREEMENT

                  This SECOND AGREEMENT AND AMENDMENT TO RIGHTS AGREEMENT (this
"Amendment") is being entered into as of June 20, 2003, between Biogen, Inc., a
Massachusetts corporation (the "Company"), and EquiServe Trust Company, N.A., as
rights agent (the "Rights Agent").

                  WHEREAS, the Company and the Rights Agent are parties to a
Rights Agreement, dated as of May 8, 1999, between the Company and the Rights
Agent, as previously amended by an Agreement and Amendment to Rights Agreement,
dated as of May 31, 2002, between State Street Bank and Trust Company, the
Company and EquiServe Trust Company, N.A. (as so amended, the "Rights
Agreement"); and

                  WHEREAS, it is proposed that the Company enter into an
Agreement and Plan of Merger (as it may be amended from time to time, the
"Merger Agreement") by and among IDEC Pharmaceuticals Corporation, a Delaware
corporation ("IDEC"), Bridges Merger Corporation, a Massachusetts corporation
and wholly owned subsidiary of IDEC ("Merger Sub"), and the Company, pursuant to
which Merger Sub will merge with and into the Company; and

                  WHEREAS, pursuant to Section 27 of the Rights Agreement, the
Company and the Rights Agent may supplement or amend the Rights Agreement in
accordance with the provisions of such Section 27. The Company now desires to
amend the Rights Agreement as set forth in this Amendment and deems such
amendments to be necessary and desirable. Capitalized terms used but not defined
herein shall have the meanings ascribed to such terms in the Rights Agreement.

                  NOW THEREFORE, in consideration of the premises and the mutual
agreements herein set forth, the parties hereby agree as follows:

                  1. AMENDMENT OF SECTION 1(a). Section 1(a) of the Rights
Agreement is hereby replaced in its entirety to read as follows:

                           "(a) 'ACQUIRING PERSON' shall mean any Person (as
                  such term is hereinafter defined) who or which, together with
                  all Affiliates and Associates (as such terms are hereinafter
                  defined) of such Person, shall be the Beneficial Owner (as
                  such term is hereinafter defined) of 10% or more of the Common
                  Shares of the Company then outstanding. Notwithstanding the
                  foregoing, (A) the term Acquiring Person shall not include (i)
                  the Company, (ii) any Subsidiary (as such term is hereinafter
                  defined) of the Company or (iii) any employee



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                  benefit plan of the Company or any Subsidiary of the Company
                  or any entity holding Common Shares of the Company for or
                  pursuant to the terms of any such employee benefit plan and
                  (B) no Person shall become an 'Acquiring Person' (x) as the
                  result of an acquisition of Common Shares of the Company by
                  the Company which, by reducing the number of shares
                  outstanding, increases the proportionate number of shares
                  beneficially owned by such Person to 10% or more of the Common
                  Shares of the Company then outstanding; provided, however,
                  that, if a Person shall become the Beneficial Owner of 10% or
                  more of the Common Shares of the Company then outstanding by
                  reason of share acquisitions by the Company and shall, after
                  such share acquisitions by the Company, become the Beneficial
                  Owner of any additional Common Shares of the Company, then
                  such Person shall be deemed to be an 'Acquiring Person,' (y)
                  if such Person, prior to June 20, 2003 became the Beneficial
                  Owner of 10% or more of the Common Shares of the Company then
                  outstanding, unless such Person becomes the Beneficial Owner
                  of (1) 1% (or any percentage greater than 1%) of the Common
                  Shares of the Company then outstanding in excess of (2) the
                  percentage of Common Shares of the Company then outstanding
                  Beneficially Owned by such Person on (aa) June 20, 2003 or
                  (bb) thereafter, whichever such percentage is less; provided,
                  however, that the sum of the percentage referred to in clause
                  (y)(2) immediately above, plus 1%, shall not be less than 10%
                  or (z) if such Person is the Beneficial Owner of less than 15%
                  of the Common Shares of the Company then outstanding and has
                  reported or is required to report such Person's Beneficial
                  Ownership on Schedule 13G under the Exchange Act (or any
                  comparable or successor report), or on Schedule 13D under the
                  Exchange Act (or any comparable or successor report) which
                  Schedule 13D does not state any intention to or reserve the
                  right to control or influence the management or policies of
                  the Company or engage in any of the actions specified in Item
                  4 (or any comparable or successor item) of such schedule
                  (other than the disposition of the Common Shares of the
                  Company) and, within 10 Business Days of being requested by
                  the Company to advise it regarding the same, certifies to the
                  Company that such Person acquired Common Shares of the Company
                  in excess of 9.99% inadvertently or without knowledge of the
                  terms of the Rights and who, together with all of such
                  Person's Affiliates and Associates, thereafter does not
                  acquire additional Common Shares of the Company while the
                  Beneficial Owner of 10% or more of the Common Shares of the
                  Company then outstanding; provided,



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                  however, that if the Person requested to so certify fails to
                  do so within 10 Business Days, then such Person shall become
                  an Acquiring Person immediately after such 10-Business-Day
                  period. Notwithstanding anything in this Agreement to the
                  contrary, neither IDEC nor any of its existing or future
                  Affiliates or Associates shall be deemed to be an Acquiring
                  Person solely by virtue of (i) the execution or delivery of
                  the Merger Agreement, (ii) the acquisition of Common Shares of
                  the Company (including the conversion of shares of common
                  stock of Merger Sub into Common Shares or other common stock
                  of the Company) pursuant to the Merger Agreement or (iii) the
                  consummation of the other transactions contemplated by the
                  Merger Agreement."

                  2. AMENDMENT OF SECTION 1(o). Section 1(o) of the Rights
Agreement is hereby replaced in its entirety to read as follows:

                           "(s) 'SHARES ACQUISITION DATE' shall mean the earlier
                  of the first date of (i) the public announcement by the
                  Company or an Acquiring Person that an Acquiring Person has
                  become such or (ii) the public disclosure of facts by the
                  Company or an Acquiring Person indicating that an Acquiring
                  Person has become such."

                  3. AMENDMENT OF SECTION 1. Section 1 of the Rights Agreement
is hereby amended to add the following subparagraphs at the end thereof:

                           (r) "'AMENDMENT NO. 2' shall mean the Second
                  Agreement and Amendment to Rights Agreement, dated as of June
                  20, 2003, by and between the Company and the Rights Agent."

                           (s) "'AMENDMENT NO. 2 EFFECTIVE TIME' shall mean the
                  date and time the Amendment No. 2 is fully executed and
                  delivered."

                           (t) "'IDEC' shall mean IDEC Pharmaceuticals
                  Corporation, a Delaware corporation."

                           (u) "'MERGER' shall have the meaning set forth in the
                  Merger Agreement."

                           (v) "'MERGER AGREEMENT' shall have the meaning set
                  forth in Section 35 hereof."



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                           (w) "'MERGER SUB' shall have the meaning set forth in
                  Section 35 hereof."

                  4. AMENDMENT OF SECTION 3(a). Section 3(a) of the Rights
Agreement is hereby amended to add the following sentence at the end thereof:

                  "Notwithstanding anything in this Agreement to the contrary, a
                  Distribution Date shall not be deemed to have occurred solely
                  by virtue of (i) the execution or delivery of the Merger
                  Agreement, (ii) the acquisition of Common Shares of the
                  Company (including the conversion of shares of common stock of
                  Merger Sub into Common Shares or other common stock of the
                  Company) pursuant to the Merger Agreement or (iii) the
                  consummation of the other transactions contemplated by the
                  Merger Agreement."

                  5. AMENDMENT OF SECTION 7(a). Section 7(a) of the Rights
Agreement is hereby amended to add the following sentence at the end thereof:

                  "Notwithstanding anything in this Agreement to the contrary,
                  none of (i) the execution or delivery of the Merger Agreement,
                  (ii) the acquisition of Common Shares of the Company
                  (including the conversion of shares of common stock of Merger
                  Sub into Common Shares or other common stock of the Company)
                  pursuant to the Merger Agreement or (iii) the consummation of
                  the other transactions contemplated by the Merger Agreement
                  shall be deemed to be events that cause the Rights to become
                  exercisable pursuant to the provisions of this Section 7 or
                  otherwise."

                  6. AMENDMENT OF SECTION 11. Section 11 of the Rights Agreement
is hereby amended to add the following sentence after the first sentence of said
Section:

                  "Notwithstanding anything in this Agreement to the contrary,
                  none of (i) the execution or delivery of the Merger Agreement,
                  (ii) the acquisition of Common Shares of the Company
                  (including the conversion of shares of common stock of Merger
                  Sub into Common Shares or other common stock of the Company)
                  pursuant to the Merger Agreement or (iii) the consummation of
                  the other transactions



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                  contemplated by the Merger Agreement shall be deemed to be
                  events of the type described in this Section 11 or to cause
                  the Rights to be adjusted or to become exercisable in
                  accordance with this Section 11."

                  7. AMENDMENT OF SECTION 13. Section 13 of the Rights Agreement
is hereby amended to add the following subparagraph at the end thereof:

                  "(e) Notwithstanding anything in this Agreement to the
                  contrary, none of (i) the execution or delivery of the Merger
                  Agreement, (ii) the acquisition of Common Shares of the
                  Company (including the conversion of shares of common stock of
                  Merger Sub into Common Shares or other common stock of the
                  Company) pursuant to the Merger Agreement or (iii) the
                  consummation of the other transactions contemplated by the
                  Merger Agreement shall be deemed to be events of the type
                  described in this Section 13 or to cause the Rights to be
                  adjusted or to become exercisable in accordance with this
                  Section 13 or otherwise to be subject to any restrictions
                  contained in this Section 13."

                  8. AMENDMENT OF SECTION 21. Section 21 of the Rights Agreement
is hereby amended to add the following sentence after the first sentence of said
Section:

                  "In the event the transfer agency relationship in effect
                  between the Company and the Rights Agent terminates, the
                  Rights Agent will be deemed to resign automatically on the
                  effective date of such termination; and any required notice
                  will be sent by the Company."

                  9. AMENDMENT OF SECTION 23(b). Section 23(b) of the Rights
Agreement is hereby replaced in its entirety to read as follows:

                  "(b) The Board of Directors of the Company may, at its option,
                  at any time prior to the earlier of (i) the Close of Business
                  on the tenth Business Day following the Shares Acquisition
                  Date; or (ii) the Final Expiration Date, redeem all but not
                  less than all of the then outstanding Rights at a redemption
                  price of $.001 per Right, appropriately adjusted to reflect
                  any stock split, stock dividend or similar transaction
                  occurring after the date hereof (such redemption price being
                  hereinafter referred to as the "Redemption Price"). The
                  redemption of the Rights by the Board of Directors may be made



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                  effective at such time, on such basis and subject to such
                  conditions as the Board of Directors in its sole discretion
                  may establish."

                  10. ADDITION OF SECTION 35. The Rights Agreement is hereby
amended to add the following new Section 35:

                  "SECTION 35. MERGER WITH BRIDGES MERGER CORPORATION

                  The Company, IDEC and Bridges Merger Corporation, a
                  Massachusetts corporation and wholly owned subsidiary of IDEC
                  ("Merger Sub"), intend to enter into an Agreement and Plan of
                  Merger, of even date of the Amendment No. 2, to be entered
                  into following the Amendment No. 2 Effective Time (as it may
                  be amended from time to time, the "Merger Agreement"),
                  pursuant to which Merger Sub shall merge with and into the
                  Company, with the Company continuing as the surviving
                  corporation. Notwithstanding anything in this Agreement to the
                  contrary, if the Merger Agreement shall be terminated for any
                  reason, then, effective as of the time of such termination,
                  the following provisions which were added to this Agreement by
                  the Amendment No. 2 shall be deemed repealed and deleted
                  without any further action on the part of the Company or the
                  Rights Agent: (1) the last sentence of Section 1(a) hereof,
                  (2) subsections (r) through (w) of Section 1 hereof, (3) the
                  last sentence of Section 3(a) hereof, (4) the last sentence of
                  Section 7(a) hereof, (5) the second sentence of Section 11
                  hereof and (6) Section 13(e) hereof."

                  11. ADDITION OF SECTION 36. The Rights Agreement is hereby
amended to add the following new Section 36:

                  "SECTION 36. FORCE MAJEURE

                  Notwithstanding anything to the contrary contained herein, the
                  Rights Agent shall not be liable for any delays or failures in
                  performance resulting from acts beyond its reasonable control
                  including, without limitation, acts of God, terrorist acts,
                  shortage of supply, breakdowns or malfunctions, interruptions
                  or malfunction of computer facilities, or loss of data due to
                  power failures or mechanical difficulties with information
                  storage or retrieval systems, labor difficulties, war, or



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                  civil unrest, to the extent such acts or events are actually
                  beyond the Rights Agent's reasonable control."

                  12. EFFECTIVENESS. This Amendment shall be deemed effective as
of the date first set forth above, as if executed and delivered on such date.
Except as amended hereby, the Rights Agreement shall remain in full force and
effect and shall be otherwise unaffected hereby.

                  13. MISCELLANEOUS. This Amendment shall be deemed to be a
contract made under the laws of the Commonwealth of Massachusetts and for all
purposes shall be governed by and construed in accordance with the laws of the
Commonwealth of Massachusetts applicable to contracts to be made and performed
entirely within the Commonwealth of Massachusetts without giving effect to the
principles of conflict of laws thereof that would require the application of any
other law. This Amendment may be executed in separate counterparts, each of such
counterparts shall for all purposes be deemed to be an original, and all such
counterparts shall together constitute but one and the same instrument. If any
provision, covenant or restriction of this Amendment is held by a court of
competent jurisdiction or other authority to be invalid, illegal or
unenforceable, the remainder of the terms, provisions, covenants and
restrictions of this Amendment shall remain in full force and effect and shall
in no way be affected, impaired or invalidated.


                           [Signature Page To Follow]



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                  IN WITNESS WHEREOF, this Second Agreement and Amendment to
Rights Agreement is executed under seal as of the date first set forth above.


                                  BIOGEN, INC.


                                  By:   /s/  Anne Marie Cook
                                      ------------------------------------------
                                      Name:  Anne Marie Cook
                                      Title: Vice President, Chief Corporate
                                             Counsel



                                  EQUISERVE TRUST COMPANY, N.A., as Rights Agent


                                  By:   /s/  Dennis V. Moccia
                                      ------------------------------------------
                                      Name:  Dennis V. Moccia
                                      Title: Managing Director