<PAGE>
                                                                   EXHIBIT 10.44


*CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
BRACKETS, HAS BEEN OMITTED AND FILED WITH THE SECURITIES AND EXCHANGE COMMISSION
PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.


                                                                October 18, 2001


Corixa Corporation
1124 Columbia Street
Suite 200
Seattle, Washington 98104, U.S.A.
Attention:  Chairman

     RE: Collaboration Agreement Dated October 23, 1998 between
         Coulter Pharmaceutical, Inc. and SmithKline Beecham Corporation

Dear Sir:

     This Letter Agreement sets forth our agreement that as to the matters set
forth herein in connection with the Collaboration Agreement (the "Agreement"),
executed as of October 23, 1998 by and between Coulter Pharmaceutical, Inc., a
company incorporated under the laws of the State of Delaware, with its principal
place of business at 600 Gateway Boulevard, South San Francisco, California
94080-7014, U.S.A. ("Coulter") which has been acquired by Corixa Corporation, a
company incorporated under the laws of the State of Delaware, with its principal
place of business at 1124 Columbia Street, Suite 200, Seattle, Washington 98104,
U.S.A. ("Corixa"), and SmithKline Beecham Corporation (a GlaxoSmithKline
Company), a company incorporated under the laws of the Commonwealth of
Pennsylvania, with its principal place of business at One Franklin Plaza,
Philadelphia, Pennsylvania 19101, U.S.A. ("GSK"), as amended to date, the
parties hereto agree as follows. This Letter Agreement (the "Letter") is a
binding agreement of Corixa and GSK.

     All terms used, but not defined, in this Letter shall have the respective
meanings set forth in the Agreement.

     1.   [*].

     2.   The parties hereby replace the second paragraph of Part B of the April
2000 Letter Amendment in its entirety with the following:

          "Subject to the MANUFACTURING DEVELOPMENT COSTS cap outlined above,
          GSK agrees that it shall pay Corixa one hundred percent (100%) of
          GSK's share of all invoices received from Corixa that are related to
          approved MANUFACTURING DEVELOPMENT COSTS and DEVELOPMENT COSTS, in
          each case within thirty (30) days following receipt of such invoice,
          provided that such invoices are sent to GSK with supporting
          documentation as described in Exhibit A to the Letter Agreement, dated
          October 18, 2001 between GSK and Corixa, which Exhibit A shall be
          incorporated in this Letter Agreement by this reference. In the event
          of any disagreement with respect to the amounts invoiced in accordance
          with the foregoing for either clinical or development services, the
          parties shall resolve


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* Confidential Treatment Requested.

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          such disagreement within the ninety (90) day period that follows GSK's
          receipt of the invoice that is the basis of such disagreement. If the
          parties determine that an amount is due GSK as a result of such
          resolution, such amount shall at Corixa's option either be credited
          against the amount payable by GSK to Corixa pursuant to the next
          fiscal quarter's invoice or shall be paid to GSK by Corixa within
          thirty (30) days following such resolution."

     All other terms and conditions of the Agreement shall remain in full force
and effect.

     If Corixa agrees with the terms and conditions of this Letter, please have
an appropriate representative of Corixa sign and date the enclosed duplicate of
this Letter as indicated below and return it to GSK.

                                        Very truly yours,
                                        SMITHKLINE BEECHAM CORPORATION
                                        (a GlaxoSmithKline Company)

                                        By: /s/ Paul A. Holcombe, Jr.
                                           -------------------------------------
                                        Title: Senior VP and General Counsel-US
                                              ----------------------------------
                                        Date:  October 18, 2001
                                              ----------------------------------

AGREED TO AND ACCEPTED:

CORIXA CORPORATION

By: /s/ Steven Gillis
   ----------------------------------
Title: Chairman & CEO
      -------------------------------
Date:  October 19, 2001
      -------------------------------


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