Document:

<PAGE>
                                                                   Exhibit 10.3

*Certain confidential information contained in this document, marked by
brackets, has been omitted and filed with the Securities and Exchange Commission
pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as amended.

                                                               February 12, 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:

        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. 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 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. ("SB"), as amended to date, is hereby
further amended as follows. This Letter Agreement (the "Letter") is a binding
agreement of Corixa and SB.

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

        Section 6.2 of the Agreement is hereby amended and restated to read in
full as follows:

        6.2 MILESTONE PAYMENTS ON BEXXAR. In consideration for the rights
granted hereunder with respect to BEXXAR, SB shall pay to Corixa the following
amounts in cash within twenty (20) days after the following events occur with
respect to BEXXAR (in its unconjugated and radiolabeled form) in the applicable
territories:

                                                                          Page 1
<PAGE>

                          TERRITORY A MILESTONE EVENTS

<TABLE>
<CAPTION>
          MILESTONE EVENT                                               AMOUNT
          ---------------                                               ------
          <S>                                                           <C>
          Acceptance of all BLAs necessary for commercialization of     $[*] in equity as
          BEXXAR [*] in TERRITORY A.                                    detailed in
                                                                        Section 6.2.1(a)

          REGULATORY APPROVAL of BEXXAR in TERRITORY A for [*],                       [*]
          whether or not a [*].

          REGULATORY APPROVAL of BEXXAR in TERRITORY A for [*]                        [*]

          First calendar year in which NET SALES in TERRITORY A exceed U.S.       [*]
          $[*]

          First calendar year in which NET SALES in TERRITORY A exceed U.S.       [*]
          $[*]

          REGULATORY APPROVAL of BEXXAR in TERRITORY A for [*].                       [*]

          REGULATORY APPROVAL of BEXXAR in TERRITORY A for [*].                       [*]

          Sub-Total for TERRITORY A                                                   [*]
</TABLE>

<TABLE>
<CAPTION>
                                 OTHER MILESTONE EVENTS
          ------------------------------------------------------------------
          MILESTONE EVENT                                             AMOUNT
          ---------------                                             ------
          <S>                                                         <C>
          Notification to SB by Corixa that Institutional Review      [*]
          Board Approval has been received by Corixa for at least
          [*] Study

          Initiation of the [*]                                       [*]

          Notification to SB by Corixa that Institutional Review      [*]
          Board Approval has been received by Corixa [*]

          Subtotal for DEVELOPMENT Events                             [*]

          TOTAL BEXXAR MILESTONE PAYMENTS                             [*]
</TABLE>

No payment shall be owed for a milestone which is not achieved and in no event
shall the milestone payments made by SB to Corixa upon the occurrence of the
milestone events set forth in the tables above in this Section 6.2 exceed the
amounts set forth opposite the milestone events in such tables. In the event
that REGULATORY APPROVAL of BEXXAR is granted concurrently for the [*] in the
FIELD in TERRITORY A, then the applicable milestone payments set forth in this
Section 6.2 for REGULATORY APPROVAL with regard to such [*] shall both be due
from SB to Corixa, and shall both be paid within the time frame provided in this
Section 6.2 (and the next [*] for which REGULATORY APPROVAL is granted shall be,
and shall trigger the milestone payment for, the [*]).

* Confidential Treatment Requested

                                                                          Page 2
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               6.2.1  DEFINITIONS RELATING TO MILESTONE PAYMENTS.

                      (a) The payment owed for Acceptance of all BLAs necessary
for commercialization of BEXXAR for the [*] in TERRITORY A shall be in the form
of the purchase by SB of unregistered shares of Corixa common stock at a [*]%
premium to Share Price. The "Share Price" shall be equal to the average closing
price for Corixa common stock on the NASDAQ National Market System as reported
in the Wall Street Journal for the ten (10) trading days prior to the date of
Filing of a BLA necessary for commercialization of BEXXAR for the [*] in
TERRITORY A. Such purchase of stock shall occur pursuant to the form of Stock
Purchase Agreement attached hereto as Exhibit B, which shall be executed by the
Parties concurrent with the execution of this Letter. SB confirms that once the
milestone of Acceptance of all BLAs necessary for commercialization of BEXXAR
for [*] in TERRITORY A has been achieved, SB's purchase obligation shall not be
affected, and any stock purchased by SB as a result of such milestone shall not
become redeemable and its investment shall not otherwise be refunded, [*], or is
approved for an indication which is [*].

                      (b) "Filing of a BLA necessary for commercialization of
BEXXAR for the [*] in TERRITORY A" shall be deemed to have occurred upon the
filing of the first constituent portion of all BLAs filed by or on behalf of
Corixa or SB under this Agreement necessary for commercialization of BEXXAR for
the [*] in TERRITORY A.

                      (c) "Acceptance of all BLAs necessary for
commercialization of BEXXAR for the [*] in TERRITORY A" of a given BLA shall be
deemed to have occurred upon the earlier of (i) written notice of acceptance
from the FDA of all BLAs filed by or on behalf of Corixa or SB under this
Agreement necessary for commercialization of BEXXAR for the [*] in TERRITORY A
or (ii) sixty (60) days following filing of such BLA with the FDA, assuming the
filing Party has not received a "Notice of Refusal to File" from the FDA with
respect to such BLA.

                      (d) The term "Initiation of [*] mean the date that [*],
which study is being performed in accordance with the FD&C ACT and applicable
regulations promulgated thereunder (including without limitation 21 CFR Part
312).

                      (e) The term "Notification to SB by Corixa that
Institutional Review Board Approval has been received by Corixa for at least [*]
sites for the Fission Comparability Study" shall mean the date that Corixa sends
SB written notification that at least [*] of the sites selected by the JDC have
received full approval by each of the appropriate Institutional Review Boards
for the Fission Comparability Study. The term "Fission Comparability Study"
means the Fission Comparability Study that is Study # 9 under the Clinical Plan
agreed by Corixa and SB on January 16, 2001.

                      (f) The term "Notification to SB by Corixa that
Institutional Review Board Approval has been received by Corixa for the "[*]"
shall mean the date that Corixa sends SB written notification that at least [*]
of the sites selected by the JDC have received full approval by the appropriate
Institutional Review Board for the [*]. The term "[*]" means the [*], Protocol #
CP [*], that is Study # [*] under the Clinical Plan agreed by Corixa and SB on
January 16, 2001.

In the event that one or more of the clinical trials defined in Sections
6.2.1(d), 6.2.1(e) or 6.2.1(f) are canceled, substituted or delayed, the Parties
will substitute appropriate alternative clinical

* Confidential Treatment Requested

                                                                          Page 3
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trial(s) therefor as approved by the JDC. Such alternative clinical trial(s)
must be Territory A Trial(s) (each as defined in Section 3.2.3).

               6.2.2 NON-REFUNDABLE AND NON-CREDITABLE MILESTONE PAYMENTS. All
amounts paid under this Section 6.2 shall be non-refundable and non-creditable,
provided, however, in the event of termination of this Agreement by SB due to
material breach by Corixa, the foregoing shall not preclude SB from seeking
whatever damages are available at law.

        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 duplicate copies
of this Letter below and return one such fully executed copy to SB.

                                     Very truly yours,
                                     SMITHKLINE BEECHAM CORPORATION

                                     By:
                                        -------------------------------
                                     Title:
                                        -------------------------------
                                     Date:
                                        -------------------------------

AGREED TO AND ACCEPTED:

CORIXA CORPORATION

By:
   -------------------------------
Title:
   -------------------------------
Date:
   -------------------------------

                                                                          Page 4<PAGE>
                                                                   Exhibit 10.4

*Certain confidential information contained in this document, marked by
brackets, has been omitted and filed with the Securities and Exchange Commission
pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, 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. In consideration for (a) the [*] paid by Corixa to MDS Nordion, Inc.
("Nordion") pursuant to Section 3.03 of the 131Iodine Supply Agreement, made as
of September 30, 2000, between Coulter and Nordion and (b) the [*] paid by
Corixa to Lonza Biologics PLC ("Lonza") pursuant to Section 2.2 of the
Settlement Agreement, made as of December 11, 2000, between Coulter and Lonza,
GSK shall pay to Corixa by wire transfer of immediately available funds, [*]
within five (5) days following approval by the US Food and Drug Administration
of BEXXAR for sale in TERRITORY A. Corixa agrees that this payment completely
satisfies all of SB's obligations for payment of out-of-pocket expenses, FTE
expenses, MANUFACTURE DEVELOPMENT COSTS or other expenses under the Agreement
solely for any costs or expenses incurred by Coulter and Corixa as a result of
the Agreement between Lonza Biologics PLC and Coulter Pharmaceutical, Inc.,
dated May 28, 1998 (the "Lonza Agreement"), including, without limitation, the
Settlement Agreement referenced above. Corixa also agrees that this payment
completely satisfies all of SB's obligations for payment of out-of-pocket
expenses, FTE expenses, MANUFACTURE DEVELOPMENT COSTS or other expenses under
the Agreement solely for any costs or expenses

* Confidential Treatment Requested

                                                                          Page 1
<PAGE>

incurred by Coutler and Corixa as a result of Coulter Pharmaceutical, Inc.'s
approval of the construction of a hot cell by Nordion under the aforementioned
Iodine Supply Agreement. The parties further agree that the foregoing payment by
GSK to Corixa will not count against the [*] Manufacturing Development Costs cap
set forth in the second paragraph of Part A of the Letter Amendment to the
Agreement, dated April 20, 2000, between GSK and Coulter (the "April 2000 Letter
Amendment").

        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 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:
                                          -------------------------------
                                       Title:
                                             ----------------------------
                                       Date:
                                            -----------------------------
AGREED TO AND ACCEPTED:

CORIXA CORPORATION

By:
   -----------------------------

* Confidential Treatment Requested

                                                                          Page 2
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Title:
      -------------------------
Date:
     --------------------------

                                                                          Page 3

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