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                                                                   EXHIBIT 10.16

                                           *** Text Omitted and Filed Separately
                                                Confidential Treatment Requested
                                          Under 17 C.F.R. Sections 200.80(b)(4),
                                                            200.83 and 240.24b-2

THIS FIFTH AMENDMENT is made the 27th day of October, 1997 to an Agreement dated
20 August 1996, by agreement between the parties ("the Agreement")

BETWEEN

COULTER PHARMACEUTICAL INC. of 550 California Avenue, Suite 200, Palo Alto, CA
94306, USA ("the Customer")

AND

LONZA BIOLOGICS PLC of 228 Bath Road, Slough, Berkshire, SL1 4DY, England
("Lonza")

WHEREAS:

A.      The parties entered into the Agreement pursuant to which Lonza agreed to
        provide Services to the Customer to develop a Process for production of
        Product which agreement has been previously amended; and

B.      Customer now wishes Lonza to perform certain additional Services in
        association with the above mentioned development and manufacture of
        Product; and

C.      Lonza is prepared to provide such additional Services on the terms and
        conditions set out herein.

NOW THEREFORE it is hereby agreed to amend the terms of the Agreement as
follows:

1.      A new Stage 16 shall be added to Schedule 2 as follows:

        "Stage 16 - Validation of [...***...]

        16.1   Objectives


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                                       1.
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               16.1.1 To obtain [...***...] data for each of the [...***...]
                      steps used in the manufacture of bulk Product.

               16.1.2 To investigate the [...***...] during the primary recovery
                      operations by analysing [...***...] extracted from samples
                      collected throughout the Process.

               16.1.3 To carry out [...***...] studies to determine the
                      [...***...].

        16.2   Activities

               16.2.1 Agree a GMP study protocol with the Customer.

               16.2.2 Extract and purify [...***...] from [...***...]. Determine
                      the [...***...].

               16.2.3 Collect samples from a [...***...] manufacturing batch of
                      Product. Samples will be collected throughout the primary
                      recovery operation and appropriate load samples taken
                      prior to each [...***...] step.

               16.2.4 Extract and purify [...***...] from samples collected
                      throughout the primary recovery Process and analyse the
                      [...***...] by [...***...].

               16.2.5 Carry out the scaled down Process for each of the
                      [...***...] without labelled [...***...] spike. Compare
                      the elution profile, Product yield and purity with the
                      full manufacturing Process.

               16.2.6 Repeat the scaled-down Process with each column load
                      sample spiked with [...***...]. Analyse load and eluate
                      samples.

               16.2.7 Calculate [...***...] factors for each column step.

               16.2.8 Issue a final study report to the Customer, copies of data
                      generated under Stage 16 if requested by the Customer and
                      deviations and amendments to the protocol agreed under
                      16.2.1. [...***...] during the course of this Stage 16 if
                      requested by the Customer (and more frequently if agreed
                      between the parties) Lonza will issue to the Customer
                      unaudited interim data 


                                       2.

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                      summaries provided such requests from the Customer are 
                      reasonable in nature.

        16.3   Timescale

               Stage 16 will be complete on issue of the final study report
               under activity 16.2.8. It is estimated that this final report
               will be issued [...***...] from commencement of Stage 16."

2. A new Stage 17b shall be added to Schedule 2 as follows:

        "Stage 17b Validation of [...***...] for Bulk Purified Product

        17b.1  Objective

               To validate these Product release assays to ensure their accuracy
               and reliability for Product release and for Product stability
               testing where this is relevant. Data will be generated by
               specific assay validation studies and also taken from assays
               carried out as part of the [...***...] of the Product. Product
               made by the manufacturing `Process will be used for the specific
               assay validation studies.

        17b.2  Activities

               17b.2.1 [...***...] (Reduced and Non-reduced)

                      Agree a GMP study protocol with the Customer. This will be
                      a Product-specific study; the following assay performance
                      areas will be defined:

                      o       [...***...]

                      o       [...***...]

                      o       [...***...]

                      o       [...***...]

                      o       [...***...]



                                       3.


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                      o       [...***...]

                      Accuracy and precision of the method using the Product
                      will be determined for reduced and non-reduced
                      [...***...], with particular attention being paid to
                      [...***...] obtained as part of an estimate of the
                      precision of the method.

               17b.2.2 [...***...] Assay

                      o      Isolate [...***...] from the supernatant and cell
                             fractions from a large-scale fermenter harvest of
                             the Cell Line.

                      o      Purify the [...***...] from each fraction.

                      o      Assess the quantity and quality of the [...***...]
                             using appropriate procedures. Compare the
                             [...***...] by [...***...] to that of the Lonza
                             generic [...***...] standard. If the generic
                             standard and the [...***...] from the Cell Line are
                             not comparable then a new Cell Line-specific
                             standard will need to be prepared.

                      o      Determine the degree of cross-reactivity between
                             the [...***...] and the generic [...***...]
                             standard in the [...***...] assay.

                      o      Issue a report of activities containing a
                             recommendation on the most appropriate course of
                             action for routine testing to the Customer and
                             outline testing procedures. Issue copies of data
                             generated under activity 17b.2.2 if requested by
                             the Customer. [...***...] during the course of this
                             activity 17b.2.2 if requested by the Customer (and
                             more frequently if agreed between the parties)
                             Lonza will issue to the Customer interim data
                             summaries provided such requests from the Customer
                             are reasonable in nature.

                                       4.


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               17b.2.3 [...***...]

                      Agree a GMP study protocol with the Customer.

                      Carry out a study to compare the performance of the
                      Product in the [...***...] assay with results obtained for
                      a range of different monoclonal antibodies. Accuracy and
                      precision of the method using the Product will be
                      determined using the following criteria:

                      o      [...***...]

                      o      [...***...]

                      o      [...***...]

                      o      [...***...]

                      o      [...***...]

                      o      [...***...]

               17b.2.4 [...***...]

                      Agree a GMP study protocol with the Customer.

                      Carry out a Product specific study to compare the
                      performance of the Product on [...***...] using Lonza's
                      [...***...] method with results obtained for a range of
                      different monoclonals. Precision and limit of detection of
                      the assay for Product will be determined using the
                      following criteria:

                      o      [...***...]

                      o      [...***...]

                      o      [...***...]

                                       5.

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                      Validate [...***...]. Calculate [...***...] values of
                      major bands. Make a recommendation on the standardisation
                      of reporting of minor bands.

               17b.2.5 [...***...]

                      Agree a GMP study protocol with the Customer.

                      Validate the specificity and sensitivity of the assay for
                      testing of the Product.

               17b.2.6 [...***...] Assays

                      Agree GMP study protocols with the Customer.

                      Carry out validation of the assays for [...***...].
                      Accuracy, precision and storage times prior to assay will
                      be determined. The following studies will be performed:

                      o      [...***...].

                      o      [...***...].

                      o      [...***...].

                      o      [...***...].

               17b.2.7 [...***...]

                      Agree a GMP study protocol with the Customer.

                      Assay parameters to be validated will be accuracy, cross-
                      reactivity or specificity, limits of detection and
                      stability of [...***...] prior to testing.

                      Study the effect of the load on the sample band profile,
                      recovery of [...***...] in the presence of Product, within
                      and between assay precision and the limit of detection of
                      Product on [...***...].

                                       6.

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                      Analyse, by [...***...], all contaminant bands that
                      account for [...***...] or more of total protein, using
                      [...***...] and [...***...] that distinguish antibody
                      fragments.

                      Note:  It is understood that the Customer will provide
                             Lonza with a reagent expected to be suitable for
                             the detection of antibody light chain for this
                             Product. If this reagent is not available or not
                             suitable it is possible some contaminant bands may
                             not be identified.

               17b.2.8 Performance Review

                      Review performance of the assays during [...***...] of the
                      Product against the performance required to meet the
                      Specification for Product and defined in the validation
                      studies.

               17b.2.9 [...***...]

                      Summarise [...***...] test performance during [...***...]
                      testing of Product and analyse the suitability of these
                      tests for Product.

               17b.2.10 Prepare final validation reports for activities 17b.2.1,
                      17b.2.3, 17b.2.4, 17b.2.5, 17b.2.6 and 17b.2.7 and submit
                      to the Customer together with details of the testing
                      procedures listed in Appendix 1 hereto and outline summary
                      procedures for the Customer Tests not included in that
                      list. Provide the Customer with copies of data generated
                      under Stage 17b if requested by the Customer and
                      deviations and amendments to the GMP protocols agreed
                      under Stage 17b. [...***...] during the course of each
                      assay validation if requested by the Customer (and more
                      frequently if agreed between the parties) Lonza will issue
                      to the Customer unaudited interim data summaries provided
                      such requests from the Customer are reasonable in nature.

                                       7.

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                      Note:  These Stage 17b reports will not review the
                             performance of the assays during the Process
                             validation studies that will be carried out under
                             other Stages in these Services. Such review will be
                             incorporated into the reports issued at the end of
                             the stages in question as appropriate.

                      Note:  Save for those particularised as being released to
                             the Customer pursuant to Stage 17.b.2.10 of the
                             Services, analytical procedures will not be issued
                             to the Customer as part of these Services. It is
                             Lonza's responsibility to ensure that the Services
                             are performed in accordance with the regulatory
                             requirements of this Agreement where applicable.

        17b.3  Timescale

               Each assay programme will be complete upon issue of the relevant
               report to the Customer. Stage 17b shall be complete upon the
               issue of the final report. It is estimated that Stage 1To will
               take approximately [...***...] to complete."

3.      The following shall be added to Clauses 1 and 2 of Schedule 3:

        "1.    Price

               In consideration for Lonza carrying out the Services as detailed
               in Schedule 2 (Stages 16 and 17b) the Customer shall pay Lonza as
               follows:

                 Stage 16 Evaluation of [...***...]                 [...***...]

                 Stage 17b  Validation  of  [...***...]  Assays
                 for Bulk Purified Product

                 17b.2.1 [...***...]                                [...***...]

                 17b.2.2 [...***...]                                [...***...]

                 17b.2.3 [...***...]                                [...***...]


                                       8.

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                 17b.2.5 [...***...]                                [...***...]

                 17b.2.6 [...***...]                                [...***...]

                 17b.2.6 [...***...]                                [...***...]

                 17b.2.6 [...***...]                                [...***...]

                 17b.2.7 [...***...]                                [...***...]

                 17b.2.9 [...***...]                                [...***...]


        2.     Payment

               Payment by the Customer of the Price for Stages 16 and 17b shall
               be made against Lonza's invoices as follows:

               2.1    For Stage 16

                      [...***...]   Upon commencement of Stage 16

                      [...***...]   Upon completion of Stage 16.

               2.2    For Stage 17b

                      [...***...] of the Price for each assay validation in
                      Stage 17b upon commencement of the Services on that assay
                      validation (total on commencement of all assays:
                      [...***...]).

                      [...***...] of the Price for each assay validation in
                      Stage 17b upon completion of the Services on that assay
                      validation (total on completion of all assays:
                      [...***...])."

4.      Save as expressly provided herein, the terms and conditions of the
        Agreement shall remain in full force and effect.


                                       9.

               *Confidential Treatment Requested


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        AS WITNESS the hands of the duly authorised representatives of the
parties hereto the day and year first above written.



Signed for and on behalf of         /s/
LONZA BIOLOGICS PLC                 -------------------------------------------

                                    Assistant Company Secretary
                                    -------------------------------------------
                                    TITLE


Signed for and on behalf of         /s/ William G. Hanis
COULTER PHARMACEUTICALS INC         -------------------------------------------

                                    Vice President and Chief Financial Officer
                                    -------------------------------------------
                                    TITLE


                                      10.


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                                   APPENDIX 1


                           TEST                 METHOD SOP
                           -------------------------------

                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]
                 [...***...]                                [...***...]


                                      11.


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