<PAGE>
                                                                   Exhibit 10.43

*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
<PAGE>

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:
   -------------------------------


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