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                                                                     EXHIBIT 4.1

                          AMENDMENT TO RIGHTS AGREEMENT
                    BETWEEN COULTER PHARMACEUTICAL, INC. AND
                    CHASEMELLON SHAREHOLDER SERVICES, L.L.C.

     THIS AMENDMENT TO RIGHTS AGREEMENT (the "Amendment") is made as of October
15, 2000, by and between COULTER PHARMACEUTICAL, INC., a Delaware corporation
(the "Company"), and CHASEMELLON SHAREHOLDER SERVICES, L.L.C., a New Jersey
limited liability company, as rights agent (the "Rights Agent").

     WHEREAS, the Company is entering into an Agreement and Plan of Merger (as
the same may be amended from time to time, the "Merger Agreement") among the
Company, Corixa Corporation, a Delaware corporation ("Parent"), and Clearwater
Acquisition Corporation, a Delaware corporation and a wholly owned subsidiary of
Parent (the "Merger Sub"), pursuant to which Merger Sub will merge with and into
the Company, and the Company will survive as a wholly owned subsidiary of
Parent, and the former stockholders of the Company will receive shares of common
stock of Parent;

     WHEREAS, the Company and the Rights Agent are parties to that certain
Rights Agreement, dated as of July 30, 1997 (the "Rights Agreement");

     WHEREAS, the Company desires to amend the Rights Agreement in connection
with the execution and delivery of the Merger Agreement; and

     WHEREAS, the Board of Directors of the Company has approved this Amendment
and authorized its appropriate officers to execute and deliver the same to the
Rights Agent.

     NOW, THEREFORE, in accordance with the procedures for amendment of the
Rights Agreement set forth in Section 27 thereof, and in consideration of the
foregoing and the mutual agreements herein set forth, the parties hereby agree
as follows:

     1    Capitalized terms that are not otherwise defined herein shall have the
meanings ascribed to them in the Rights Agreement.

     2    The definition of "Acquiring Person" set forth in Section 1(a) of the
Rights Agreement is amended by adding the following sentence to the end of that
section:

          Notwithstanding the foregoing, no Person shall be or become an
     Acquiring Person by reason of (i) the execution and delivery of the
     Agreement and Plan of Merger, dated as of October 15, 2000, among Corixa
     Corporation, a Delaware corporation ("Parent"), Clearwater Acquisition
     Corporation, a Delaware corporation and a wholly owned subsidiary of Parent
     ( "Merger Sub") and the Company (the "Merger Agreement") or the execution
     of any amendment thereto, (ii) the execution and delivery of the Coulter
     Voting Agreements (as such term is defined in the Merger Agreement) or the
     execution of any amendment thereto, (iii) the merger of Merger Sub with and
     into the Company, or (iv) the consummation of any other transaction
     contemplated by the Merger Agreement, as it may be amended from time to
     time.

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     3    The definition of "Shares Acquisition Date" in Section l(n) of the
Rights Agreement is hereby amended by adding the following sentence to the end
of that section:

          Notwithstanding anything else set forth in this Agreement, a Shares
     Acquisition Date shall not be deemed to have occurred by reason of (i) the
     execution and delivery or amendment of the Merger Agreement or the Coulter
     Voting Agreements, (ii) the merger of Merger Sub with and into the Company,
     or (iii) the consummation of any other transaction contemplated by the
     Merger Agreement.

     4    Section 3(a) of the Rights Agreement is hereby amended by adding the
following sentence to the end of that section:

          Notwithstanding anything else set forth in this Agreement, no
     Distribution Date shall be deemed to have occurred by reason of (i) the
     execution and delivery or amendment of the Merger Agreement or the Coulter
     Voting Agreements, (ii) the merger of Merger Sub with and into the Company,
     or (iii) the consummation of any other transaction contemplated by the
     Merger Agreement.

     5    Clause (i) of Section 7(a) of the Rights Agreement is hereby amended
to delete the phrase "(the "Final Expiration Date")" so that it shall read as
follows:

          (i)  the Close of Business on July 30, 2007,

     6    Section 7(a) of the Rights Agreement is further amended by deleting
the word "or" at the end of clause (ii) of Section 7(a) and by adding the
following clause at the end of Section 7(a):

          , or (iv) the moment in time immediately prior to the Effective Time
     (as such term is defined in the Merger Agreement) (the earliest to occur of
     the events described in clauses (i) and (iv) of this section shall be
     referred to as the "Final Expiration Date").

     7    Section 11(a)(ii) of the Rights Agreement is hereby amended by adding
the following sentence to the end of that section:

          Notwithstanding anything else set forth in this Agreement, no event
     requiring an adjustment under this Section 11(a)(ii) shall be deemed to
     have occurred by reason of (i) the execution and delivery or amendment of
     the Merger Agreement or the Coulter Voting Agreements, (ii) the merger of
     Merger Sub with and into the Company, or (iii) the consummation of any
     other transaction contemplated by the Merger Agreement.

     8    The language of Section 13(a) of the Rights Agreement prior to clause
(z) thereof is hereby amended to read as follows:

          (a)  In the event that, following the Shares Acquisition Date or, if a
     Transaction is proposed, the Distribution Date, directly or indirectly (x)
     the Company shall consolidate with, or merge with and into, any Interested
     Stockholder, or if in such merger or consolidation all holders of Common
     Stock are not treated alike, any other Person, (y) any Interested
     Stockholder, or if in such merger or consolidation all holders of Common
     Stock are not treated alike, any other Person shall consolidate with the
     Company, or merge with and into the Company, and the Company shall be the
     continuing or surviving corporation of such merger (other than, in the case
     of either

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     transaction described in (x) or (y), (i) a merger or consolidation which
     would result in all of the voting power represented by the securities of
     the Company outstanding immediately prior thereto continuing to represent
     (either by remaining outstanding or by being converted into securities of
     the surviving entity) all of the voting power represented by the securities
     of the Company or such surviving entity outstanding immediately after such
     merger or consolidation and the holders of such securities not having
     changes as a result of such merger or consolidation or (ii) the merger of
     Merger Sub with and into the Company), or

The remaining portion of Section 13(c) of the Rights Agreement shall be
unchanged and shall remain in full force and effect.

     9    The first phrase of Section 13(c) of the Rights Agreement is hereby
amended to read as follows:

          The Company shall not consummate any such consolidation, merger (other
     than the merger of Merger Sub with and into the Company), sale or transfer
     unless the Principal Party shall have a sufficient number of authorized
     shares of its Common Shares that have not been issued or reserved for
     issuance to permit the exercise in full of the Rights in accordance with
     this Section 13, and unless prior thereto the Company and each Principal
     Party and each other Person who may become a Principal Party as a result of
     such consolidation, merger, sale or transfer shall have (i) executed and
     delivered to the Rights Agent a supplemental agreement provided for the
     terms set forth in paragraphs (a) and (b) of this Section 13 and (ii)
     prepared, filed and had declared and remain effective a registration
     statement under the Act on the appropriate form with respect to the Rights
     and the securities exercisable upon the exercise of the Rights and further
     providing that, as soon as practicable after the date of any consolidation,
     merger, sale or transfer of assets mentioned in paragraph (a) of this
     Section 13, the Principal Party at its own expense will:

The remaining portion of Section 13(c) of the Rights Agreement shall be
unchanged and shall remain in full force and effect.

     10   Clause (ii) of Section 13(d) of the Rights Agreement is hereby amended
to read as follows:

          (ii) merge with or into (other than by the merger of Merger Sub with
     and into the Company),

     11   Clause (iv) of Section 25(a) of the Rights Agreement is hereby amended
to read as follows:

          (iv) to effect any consolidation or merger into or with (other than
     the merger of Merger Sub with and into the Company), or to effect any sale
     or other transfer (or to permit one or more of its Subsidiary to effect any
     sale or other transfer), in one or more transactions, of 50% or more of the
     assets or earning power of the Company and its Subsidiary (taken as a
     whole), to any other person,

     12   Agreement, as amended by this Amendment, shall remain in full force
and effect in accordance with its terms.

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     13   All the covenants and provisions of this Amendment by or for the
benefit of the Company or the Rights Agent shall bind and inure to the benefit
of their respective successors and assigns hereunder.

     14   Nothing in this Amendment shall be construed to give to any Person
other than the Company, the Rights Agent and the registered holders of the Right
Certificates (and, prior to the Distribution Date, the Common Shares) any legal
or equitable right, remedy or claim under this Amendment; but this Amendment
shall be for the sole and exclusive benefit of the Company, the Rights Agent and
the registered holders of the Right Certificates (and, prior to the Distribution
Date, the Common Shares).

     15   If any term, provision, covenant or restriction of this Amendment is
held by a court of competent jurisdiction or other authority to be invalid, void
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.

     16   This Amendment shall be deemed to be a contract made under the laws of
the State of Delaware and for all purposes shall be governed by and construed in
accordance with the laws of such State applicable to contracts to be made and
performed entirely within such State.

     17   This Amendment may be executed in any number of counterparts and 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.

     18   The Company hereby certifies to the Rights Agent that this Amendment
is in compliance with Section 27 of the Rights Agreement and that it will notify
the Rights Agent in writing of the occurrence of the Effective Time.

     IN WITNESS WHEREOF, the parties herein have caused this Amendment to be
duly executed and attested, all as of the date and year first above written.

                                    COULTER PHARMACEUTICAL, INC.

                                    By: /s/ Michael F. Bigham
                                       -----------------------------------------
                                    Name:  Michael F. Bigham
                                         ---------------------------------------
                                    Title: President and Chief Executive Officer
                                          --------------------------------------

Attest: /s/ William G. Harris
       -------------------------------------------------
Name:  William G. Harris
     ---------------------------------------------------
Title: Senior Vice President and Chief Financial Officer
      --------------------------------------------------

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                                    CHASEMELLON SHAREHOLDER SERVICES, L.L.C.

                                    By: /s/ Sharon Magidson
                                       -----------------------------------------
                                    Name:  Sharon Magidson
                                         ---------------------------------------
                                    Title: Vice President
                                          --------------------------------------

Attest: /s/ Joseph W. Thatcher
       --------------------------------
Name:  Joseph W. Thatcher
     ----------------------------------
Title: Assistant Vice President
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                                                                  EXHIBIT 10.3.2

                             FIRST AMENDMENT TO THE
                        VNUS MEDICAL TECHNOLOGIES, INC.
                                1995 STOCK PLAN

                    This First Amendment to the VNUS Medical Technologies, Inc.
1995 Stock Plan (the "First Amendment") is adopted as of February 6, 1997 by the
Board of Directors (the "Board") of VNUS Medical Technologies, Inc., a Delaware
corporation (the "Company").

                                    RECITALS

             WHEREAS, the Company wishes to increase the maximum aggregate
number of shares of common stock which may be issued upon exercise of options
under the VNUS Medical Technologies, Inc. 1995 Stock Plan (the "Plan"); and

             WHEREAS, pursuant to Section 14 of the Plan, the Board may amend
the Plan from time to time;

             NOW THEREFORE, BE IT RESOLVED, that the Plan be amended as follows,
effective as of February 6, 1997:

             1. Stock Subject to the Plan.The first paragraph of Section 3 of
the Plan is hereby amended in its entirety to read as follows:

             "3. Stock Subject to the Plan. Subject to the provisions of Section
        12 of the Plan, the maximum aggregate number of Shares that may be
        optioned and sold under the Plan is 2,500,000 shares of Common Stock.
        The Shares may be authorized, but unissued, or reacquired Common Stock."

             2. This First Amendment shall be and hereby is incorporated in and
forms a part of the Plan.

             3. Except as hereby amended, the Plan remains in full force and
effect.

             4. Capitalized terms used herein and not otherwise defined shall
have the meaning ascribed to them in the Plan.

                                      * * *

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