<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 406 OF THE SECURITIES ACT OF 1933, 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 [*] days after the following events occur with respect to BEXXAR
(in its unconjugated and radiolabeled form) in the applicable territories:


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<TABLE>
<CAPTION>

                          TERRITORY A MILESTONE EVENTS

MILESTONE EVENT                                               AMOUNT
------------------------------------------------------------  -----------------
<S>                                                           <C>
Acceptance of all BLAs necessary for commercialization of                  $[*]
BEXXAR [*] in TERRITORY A.

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

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>


                             OTHER MILESTONE EVENTS

<TABLE>
<CAPTION>
MILESTONE EVENT                                             AMOUNT
----------------------------------------------------------  -------------------
<S>                                                         <C>
Notification to SB by Corixa that Institutional Review      $[*]
Board Approval has been received by Corixa [*]

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 [*] for the [*], then the
applicable milestone payments set forth in this Section 6.2 for REGULATORY
APPROVAL with regard to such [*] shall [*] be due from SB to Corixa, and shall
[*] 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 [*]).

          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 [*] to
Share Price. The "Share Price" shall


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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 [*]. 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 the [*] 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, [*].

                    (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 [*] 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) [*] 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 [*] shall 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 [*] has been
received by Corixa for [*] shall mean the date that Corixa sends SB written
notification that [*].

                    (f) The term "Notification to SB by Corixa that [*] has been
received by Corixa for [*]" shall mean the date that Corixa sends SB written
notification that [*].

[*]

               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: /s/ Signature Illegible
                                           ------------------------------------




                                        Title:
                                              ----------------------------------

                                        Date: February 12, 2001
                                             -----------------------------------

AGREED TO AND ACCEPTED:

CORIXA CORPORATION

By: /s/ Steven Gillis
   ----------------------------------

Title: Chairman and CEO
      -------------------------------

Date: February 12, 2001
     --------------------------------


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