Document:

<PAGE>
                                                                   EXHIBIT 10.42

*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.

                                                                  April 20, 2000

Coulter Pharmaceutical, Inc
600 Gateway Boulevard South
San Francisco, California 94080-7014, U.S.A.
Attention:  Chief Financial Officer

     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.
("Coulter), 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 Coulter and SB.

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

A.      MANUFACTURE DEVELOPMENT COSTS

        Within five (5) days after the date of mutual execution of this Letter
(the "Letter Effective Date"), SB shall pay Coulter U.S. $[*] for out-of-pocket
expenses incurred by Coulter during the third and fourth quarters of 1999 as
well as U.S. $[*] for FTE expenses Coulter incurred during the fourth quarter of
1998 and for the entire 1999 calendar year. Coulter agrees that this payment
completely satisfies all of SB's obligations for payment of out-of-pocket
expenses, FTE expenses, and MANUFACTURE DEVELOPMENT COSTS under the Agreement
for the fourth quarter of 1998 as well as for the entire 1999 calendar year.

        SB's share of MANUFACTURE DEVELOPMENT COSTS under the Agreement shall be
capped at U.S. $[*]. To the extent that aggregate MANUFACTURE DEVELOPMENT COSTS
would otherwise cause SB's share of such costs to exceed U.S. $[*], Coulter
shall bear the full amount of such excess, provided that Coulter shall be
repaid, from the SB share of the Joint P&L profits under Section 7.1.14, the
amount of such excess MANUFACTURE DEVELOPMENT COSTS that would have been borne
by SB in the absence of the cap set forth in the first sentence of this
paragraph. Such repayment to Coulter shall occur prior to any payment of profits
to SB.

* Confidential Treatment Requested
<PAGE>

B.      DETERMINATION AND PAYMENT OF MANUFACTURE DEVELOPMENT COSTS AND
        DEVELOPMENT COSTS

        Within sixty (60) days following the Letter Effective Date, the JDC
shall determine a mechanism under which the parties will develop timelines to
enable the JOINT MANUFACTURE DEVELOPMENT PLAN and JOINT DEVELOPMENT PLAN and the
associated MANUFACTURE DEVELOPMENT COSTS and DEVELOPMENT COSTS to be approved by
the JDC in accordance with SB's budgeting process for such costs.

        Subject to the MANUFACTURE DEVELOPMENT COSTS cap outlined above, SB
agrees that it shall pay Coulter [*] of SB's share of all invoices received from
Coulter which are related to approved MANUFACTURE DEVELOPMENT COSTS and
DEVELOPMENT COSTS within the time period provided in the Agreement, even if SB
disputes all or some of the amounts on such invoices, provided that such
invoices are sent to SB with reasonable supporting documentation in a form
agreed upon in advance and acceptable to the JDC, and provided further that once
the parties have resolved such dispute, (a) Coulter shall promptly refund any
overpayment to SB, together with interest at a rate equal to the prime rate as
reported by the Chase Manhattan Bank, New York calculated on the number of days
between the time SB made such payment and the date that SB receives such refund,
and (b) if SB still owes Coulter money, all such late payments shall be made
together with interest at a rate equal to the prime rate as reported by the
Chase Manhattan Bank, New York calculated on the number of days such payment
amount is delinquent.

C.      TERRITORIES B AND C

        Coulter and SB agree that SB's rights in TERRITORIES B and C will
terminate effective as of June 30, 2000 (the "B/C Termination Effective Date"),
without any further notice obligation by SB.

        Coulter agrees that SB will not be obligated to make Coulter any payment
under Section 9.10.5 of the Agreement or any other payment related to a EUROPEAN
FACILITY. Coulter further agrees that SB has no further obligation with respect
to a EUROPEAN FACILITY.

D.      ALLOCATION OF SUPPLY.

        Article 9.5 of the Agreement is hereby amended to read in full as
follows

        9.5 ALLOCATION IN THE EVENT OF PRODUCT SHORTAGES.

        9.5.1 ALLOCATION OF RADIOLABELED ANTIBODY. In the event of shortages of
RADIOLABELED ANTIBODY, any RADIOLABELED ANTIBODY material produced by Nordion,
Kanata, from the facility in existence as of the Letter Effective Date, shall be
allocated as follows: [*]% of the manufactured vials in each batch shall be
allocated to TERRITORY A and [*]% of the manufactured vials in each batch shall
be allocated for use by Coulter in the rest of the world. If after the Letter
Effective Date, SB and Coulter jointly fund further expansion at Nordion,
Kanata, or build a second site for TERRITORY A purposes, this expanded capacity
shall not be available to Coulter for

* Confidential Treatment Requested

                                      -2-
<PAGE>

use by Coulter or its other licensees in the rest of the world unless Coulter or
its other licensees fund an appropriate part of the costs of the additional
capacity. If such funding is provided by Coulter or its other licensees,
apportionment of available RADIOLABELED ANTIBODY for use outside of TERRITORY A
shall be limited to the equivalent percentage of the funding contribution made
by Coulter and its other licensees. Shortages of RADIOLABELED ANTIBODY which
arise from shortages of UNCONJUGATED ANTIBODY rather than a capacity constraint
in connection with radiolabelling shall not be allocated in accordance with this
Section 9.5.1, but rather the available UNCONJUGATED ANTIBODY shall be
radiolabeled for use in TERRITORY A or in the rest of the world in accordance
with the allocation by territory set forth in Section 9.5.2, unless the Parties
then agree differently.

        9.5.2 ALLOCATION OF UNCONJUGATED ANTIBODY. Coulter shall place orders
for UNCONJUGATED ANTIBODY for use in TERRITORY A in consultation with SB, and
shall place orders for use in the rest of the world in its own discretion. Any
orders placed with BI Pharma or any other vendor of UNCONJUGATED ANTIBODY which
is a supplier for use both inside and outside of TERRITORY A (a "Vendor") shall
be a "Vested Order" for purposes of this Section 9.5.2 if it is a firm order
which has been accepted by the Vendor and is scheduled for delivery within
twenty-four (24) months after the date of such order. To the extent that a
Vendor is able to deliver UNCONJUGATED ANTIBODY material in accordance with
Vested Orders, such material shall be purchased by, and the benefits and risks
of ownership shall belong to, the party which was responsible for such Vested
Order. Thus, Vested Orders which had been placed for the purpose of supply in
TERRITORY A shall be allocated solely to TERRITORY A and Vested Orders which had
been placed for the purposes of supply outside of TERRITORY A shall be allocated
solely to Coulter for use outside of TERRITORY A, in each case except by the
mutual consent of the parties.

        In the event of shortages of UNCONJUGATED ANTIBODY, the following rules
shall apply. Attached hereto as Exhibit A is a set of examples demonstrating the
application of the following rules.

        (a)     If Coulter seeks to place a Vested Order for UNCONJUGATED
ANTIBODY and is advised by a Vendor that such Vendor will not accept the full
amount of such order, whether such order is for purposes of supply in TERRITORY
A or elsewhere in the world, then Coulter shall consult with SB regarding the
allocation of available supply. Unless the Parties otherwise agree, all future
Vested Orders shall be allocated [*]% to TERRITORY A and [*]% for use outside of
TERRITORY A so long as such conditions of supply shortage prevail, with each lot
being allocated in such pro rata manner

        (b)     If a Vendor is unable to deliver the full amount of a Vested
Order, either because the amount delivered is less than the amount ordered or
because some or all of the delivery is not accepted for any reason, including,
without limitation, non-compliance with Specifications, then the available
UNCONJUGATED ANTIBODY in each calendar year in which a shortage of supply
prevails shall be allocated as between TERRITORY A and the rest of the world pro
rata on the basis of the aggregate Vested Orders for delivery in that calendar
year that had been placed for use in TERRITORY A and in the rest of the world.
If the shortfall or failure occurs in a batch which is part of a manufacturing
campaign, then the acceptable material manufactured

* Confidential Treatment Requested

                                      -3-
<PAGE>

as part of that campaign, and all future deliveries in that calendar year, shall
be allocated between TERRITORY A and the rest of the world so as to make the
aggregate allocation of material on a year-to-date basis equal, as nearly as
practicable, to such pro rata allocation. (Such allocation on a campaign basis
could require that material already received as part of the same campaign for
use in one territory be redesignated for use in a different territory, if such
redesignation is necessary in order to achieve the specified pro rata
allocation. However, material received as part of previous campaigns shall not
be redesignated except by mutual consent.) As used herein, a manufacturing
campaign shall refer to two or more batches manufactured in the same
manufacturing plant in temporal proximity to each other. The allocation rule set
forth in this Section 9.5.2(b) shall restart for each calendar year, without any
carryover of shortfalls realized for a particular territory in the prior
calendar year.

In the event either Party desires to reduce the size of a Vested Order
previously placed, the percentage by which such Vested Order may be reduced
under the contract with the Vendor shall be applied equally to TERRITORY A and
to the rest of the world unless the Parties otherwise agree. By way of example,
if the supply agreement permits a reduction in Vested Orders of up to 25% for
orders with delivery dates at a specified time period in the future, and the
Parties desire to reduce the Vested Order for that time period in TERRITORY A by
35% and the rest of the world by 10%, then the reduction for Territory A shall
be limited to 25% while the order for the rest of the world is reduced by the
full 10%. In that case, by mutual agreement, any further reduction which would
have been available for the order for the rest of the world may be reallocated
to permit a further reduction in the Vested Order for TERRITORY A.

If under the contract with the Vendor Coulter is obligated to take quantities of
UNCONJUGATED ANTIBODY produced in excess of the amount specified in the original
Vested Order (e.g., as a result of the inherent uncertainty of the quantity of
material produced in each batch or due to minimum batch sizes), then any excess
quantities which Coulter is obligated to purchase shall be allocated as between
TERRITORY A and the rest of the world pro rata on the basis of the aggregate
Vested Orders for delivery to each territory in that calendar year.

        The purpose of the foregoing allocation rules is to permit the Parties
jointly (with respect to TERRITORY A) and Coulter independently (with respect to
the rest of the world) to make their respective long-term purchase decisions for
UNCONJUGATED ANTIBODY, with the benefits and risks of such purchase decisions to
be allocated to the Parties jointly, or Coulter independently, as the case may
be.

        Coulter shall have the right to establish an independent capacity for
the manufacture of UNCONJUGATED ANTIBODY and/or RADIOLABELED ANTIBODY for use
outside of TERRITORY A, at its own expense, in which case any capacity arising
from such independent facility may be used by Coulter in its discretion, free
and clear of the allocation rules set forth in this Section 9.5.2.

E.      ALLOCATION OF FACILITIES CHARGES; DETERMINATION OF MATERIALS COST

        Under the current contractual arrangements with Nordion, Coulter is
obligated to pay a facilities charge of approximately $[*] per year (payable on
a quarterly basis), in addition to the

* Confidential Treatment Requested

                                      -4-
<PAGE>

production fee for individual lots of RADIOLABELED ANTIBODY. The Parties agree
that this facilities charge will be allocated as between TERRITORY A and the
rest of the world pro rata on the basis of the number of vials meeting
Specifications that were produced for each of such territories in the
three-month period immediately preceding the date on which each quarterly
payment is due. Any new facilities charges attributable to the production of
UNCONJUGATED ANTIBODY or RADIOLABELED ANTIBODY for facilities which are used for
the production of material for both TERRITORY A and the rest of the world shall
similarly be allocated between such territories pro rata on the basis of actual
production, unless the Parties otherwise agree.

        The price of each unit of UNCONJUGATED ANTIBODY for TERRITORY A and for
the rest of the world shall be the same for each unit produced in the same
campaign. (In other words, the total costs of production, together with
transportation from the vendor to any common radiolabeling site, shall be
allocated across all units of UNCONJUGATED ANTIBODY that meet Specifications,
and that amount shall be charged to TERRITORY A and to the rest of the world for
each unit of UNCONJUGATED ANTIBODY allocated to the respective territory.)

        The processing fee for each unit of RADIOLABELED ANTIBODY shall be
determined and allocated on a lot-by-lot basis. If units of RADIOLABELED
ANTIBODY are produced in the same lot for use in both TERRITORY A and the rest
of the world, the processing fee for such lot shall be allocated pro rata on the
bass of the number of units produced for each territory.

F.      MILESTONE PAYMENTS ON BEXXAR.

        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 Coulter 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:

                          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                          [*]
       either [*]

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

</TABLE>

* Confidential Treatment Requested

                                      -5-
<PAGE>

<TABLE>
<S>                                                                       <C>

       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>
       Initiation of the [*]                                                     [*]

       Initiation of the [*]                                                     [*]

       Initiation of the [*]                                                     [*]

       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 Coulter 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 Coulter, 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 [*]).

        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 Coulter common stock at a [*]%
premium to Share Price. The "Share Price" shall be equal to the average closing
price for Coulter 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 the [*] in TERRITORY A has been achieved, SB's

* Confidential Treatment Requested

                                      -6-
<PAGE>

purchase obligation shall not be affected, and any stock purchased by SIB 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
Coulter 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 Coulter 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 [*] 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 "Initiation of [*] shall mean the date that
[*], which pivotal study is being performed in accordance with the FD&C ACT and
applicable regulations promulgated thereunder (including without limitation 21
CFR Part 312).

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

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 trial(s) therefor as approved
by the JDC. Such alternative clinical trials) 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 Coulter, the foregoing shall not preclude SB from seeking
whatever damages are available at law.

G.      COST SHARING BETWEEN THE PARTIES WITH RESPECT TO MANUFACTURING
        DEVELOPMENT COSTS

        it is agreed that, as of the effective date of the B/C Termination
Effective Date, MANUFACTURE DEVELOPMENT COSTS incurred by either Party
thereafter shall be shared between the Parties as follows:

* Confidential Treatment Requested

                                      -7-
<PAGE>

        (i)     Any MANUFACTURE DEVELOPMENT COSTS which are incurred by either
Party after the B/C Termination Effective Date with respect to activities
related solely to TERRITORY A shall be borne [*] by Coulter and [*] by SB.

        (ii)    Any costs which are incurred by Coulter after the B/C
Termination Effective Date with respect to manufacture development activities
related solely to outside of TERRITORY A shall be borne [*] by Coulter.

        (iii)   Any MANUFACTURE DEVELOPMENT COSTS which are incurred by either
Party after the B/C Termination Effective Date with respect to activities both
inside and outside of TERRITORY A shall be borne [*] by SB and [*] by Coulter.

H.      COST SHARING BETWEEN THE PARTIES WITH RESPECT TO DEVELOPMENT COSTS

        Section 3.2.6 of the Agreement is hereby amended and restated to read in
full as follows:

        3.2.6 FUNDING OF DEVELOPMENT. Coulter shall bear the cost of all
DEVELOPMENT work for Territory A Trials (as defined in Section 3.2.3) and
Cross-Territory Trials (as defined in Section 3.2.3) until Coulter's aggregate
DEVELOPMENT COSTS (including, without limitation, its FTE expenses and its
OUT-OF-POCKET COSTS related to DEVELOPMENT work in TERRITORY A and the NON-USA
TERRITORY) reach [*], provided that it is understood that no expenses of Coulter
shall be included within the DEVELOPMENT COSTS used to determine the [*] unless
such work and related expenses are provided for in the then current JOINT
DEVELOPMENT PLAN or are within the category described in Section 1.15(ii).
Following the time at which the [*] has been reached.

                (a)     SB's FTES who support DEVELOPMENT work after the [*] has
been reached shall be included in DEVELOPMENT COSTS and handled as set forth in
Section 3.2.6(b); and

                (b)     SB will bear the SB Share of Development Costs (as
defined below) and Coulter will bear the Coulter Share of Development Costs (as
defined below). To the extent that one Pang has borne more of such DEVELOPMENT
COSTS than it is obligated to bear under this Section 3.2.6(b), the other Party
shall reimburse the first Party pursuant to Section 3.2.6(d) No expenses of a
Party shall be included within Development Costs except as provided in Section
1.15.

                (c)     The Parties agree that:

                        (i)     "Coulter Share of Development Costs" shall equal
A + B + C, where

                        A = [*] multiplied by all DEVELOPMENT COSTS
associated with Cross-Territory Trials (as defined in Section 3.2.3) incurred
after the [*] has been exceeded; and

                        B = [*] multiplied by all DEVELOPMENT COSTS
associated with Territory A Trials (as defined in Section 3.2.3) and incurred
after the [*] has been exceeded; and

* Confidential Treatment Requested

                                      -8-
<PAGE>

                        C = [*] multiplied by all costs related to development
of PRODUCT associated with Non-USA Territory Trials (as defined in Section
3.2.3) and incurred after June 30, 2000.

                        (ii)    "SB Share of Development Costs" shall equal D +
E + F, where

                        D = [*] multiplied by all DEVELOPMENT COSTS associated
with all Cross-Territory Trials (as defined in Section 3.2.3) incurred after the
[*] has been exceeded; and

                        E = [*] multiplied by all DEVELOPMENT COSTS associated
with Territory A Trials (as defined in Section 3.2.3) incurred after the [*] has
been exceeded; and

                        F = [*] multiplied by all costs related to development
of Product associated with Non-USA Territory Trials (as defined in Section
3.2.3) and incurred after October 1, 1998 and on or before June 30, 2000.

                        (iii)   Notwithstanding the foregoing, in the event that
the JDC uses data from a Non-USA Territory Trial to support a regulatory filing
to demonstrate the efficacy of the PRODUCT regarding a previously unapproved
indication for purposes of label expansion in TERRITORY A, then upon the date of
filing for such label expansion in TERRITORY A, SB shall reimburse to Coulter
[*] of the costs of such Non-USA Territory Trial; provided, however, that if all
or a portion of the cost of the Non-USA Territory Trial is paid by another
licensee of Coulter, then the portion of the total cost reimbursed by SB shall
be reduced [*]. SB shall not have any obligation to pay any reimbursement under
this clause (3) prior to the point at which Coulter has incurred aggregate
DEVELOPMENT COSTS equal to the [*], but in such case the amount that SB would
have been obligated to reimburse under this clause (3) shall be treated as a
DEVELOPMENT COST incurred by Coulter and credited against the [*].

                (d)     No later than thirty (30) days after the end of each
calendar quarter, each Party shall submit to the FINANCE SUBTEAM an accounting
of all DEVELOPMENT COSTS incurred by such Party during such quarter. The FINANCE
SUBTEAM shall promptly calculate whether any payments are due from one Party to
the other Party under this Section. In the event such payment is due, the Party
which owes the payment shall make such payment to the other Party within thirty
(30) days after it has received such a determination, in writing, from the
FINANCE SUBTEAM. The Party receiving the payment shall issue an invoice to the
Party making the payment which invoice shall be in the amount to be paid.

I.      INTEREST ON LATE PAYMENTS.

        Any late payments under the Agreement by either Party shall bear
interest at a rate equal to the prime rate as reported by Chase Manhattan Bank,
New York, calculated on the number of days such payment amount is delinquent.

* Confidential Treatment Requested

                                      -9-
<PAGE>

J.      LOAN

        SB agrees that it will promptly fund the loan as required by the terms
of the LOAN AGREEMENT, it being recognized by the Parties that final changes to
the Cash Collateral and Security Agreement, presently in draft form, remain to
be agreed, which agreement is expected within a few days.

K.      OTHER PROVISIONS OF THE AGREEMENT

        All other terms and conditions of the Agreement shall remain in full
force and effect. The parties shall promptly endeavor to produce an amended and
restated Agreement that shall incorporate the terms and conditions of this
Letter, as well as any remaining effect of SB's termination of its rights in
TERRITORIES B AND C.

        If Coulter agrees with the terms and conditions of this Letter, please
have an appropriate representative of Coulter sign and date the duplicate copes
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:

COULTER PHARMACEUTICAL

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

                                      -10-<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
<PAGE>

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

                                                                          Page 4

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