<PAGE>   1
                                                                   Exhibit 10.20

                                           *** TEXT OMITTED AND FILED SEPARATELY
                                                 CONFIDENTIAL TREATMENT REQUIRED
                                           UNDER 17 C.F.R. SECTIONS 20.80(b)(4),
                                                            200.83 AND 240.24b-2


                              FACILITIES AGREEMENT

        THIS FACILITIES AGREEMENT (the "Agreement") is made this 31st day of
August, 1998 between MDS NORDION INC., a Canadian corporation having a place of
business at 447 March Road, Kanata, Ontario, Canada, K2K 1X8 ("Nordion") and
COULTER PHARMACEUTICAL, INC., a Delaware corporation having a place of business
at 550 California Avenue, Suite 200, Palo Alto, California, U.S.A. ("Coulter").
Nordion and Coulter are sometimes referenced in this Agreement individually as a
"Party" and collectively as the "Parties".

                                    RECITALS

        WHEREAS, Coulter is the owner of certain data, information and
technology related to the B1 Antibody (as such term is defined below) and the
radiolabelling thereof; and

        WHEREAS, Nordion has expertise in designing and overseeing the
construction of facilities for the radiolabelling of pharmaceutical compounds;
and

        WHEREAS, the Parties have jointly developed a process for radiolabelling
of B1 Antibody with iodine ("131I") pursuant to an Agreement between the Parties
dated November 15, 1995 (the "Development Agreement"); and

        WHEREAS, the Parties are, simultaneous with the execution of this
Agreement, entering into a supply agreement pursuant to which Nordion will
radiolabel B1 Antibody with 131I on behalf of Coulter for clinical trials and
for commercialization of such product (the "Supply Agreement"); and

        WHEREAS, the Parties now desire to set forth the terms under which
Nordion will install and qualify an interim facility and a primary facility to
be used for the manufacture of such 131I-labelled B1 Antibody.

        NOW, THEREFORE, in consideration of the foregoing and the mutual
promises contained in this Agreement, the Parties agree as follows:

                                    ARTICLE 1

                                   DEFINITIONS

        As used in this Agreement, the following terms shall have the meanings
set forth below:

        1.1 "AFFILIATE" shall mean any entity or person which controls, is
controlled by or is under common control with either Party. For purposes of this
Section 1.1, "control" shall mean (a) in the case of corporate entities, the
direct or indirect ownership of more than one-half of the stock or participating
shares entitled to vote for the election of directors, and (b) in the case of a
partnership, the power to direct the management and policies of such
partnership.


                                       1.


<PAGE>   2
        1.2 "B1 ANTIBODY" shall mean the IgG2a anti-CD20 murine monoclonal
antibody, as supplied by Coulter to Nordion for purposes of this Agreement or
the Supply Agreement.

        1.3 "BATCH" shall mean a production batch of the Labelled Drug
manufactured under the Supply Agreement.

        1.4 "BATCH CAPACITY" shall mean the capacity of the Interim Facility or
the Primary Facility, as the case may be, to produce Labelled Drug.

        1.5 "CLASS 100 INSERT" shall mean both of the assets purchased by
Nordion by purchase order on behalf of Coulter as described in [...***...]

        1.6 "CLINICAL TRIALS" shall mean any trials for clinical development of
pharmaceutical products defined as "Phase I," "Phase II," "Phase III," or
"Treatment IND" in FDA regulations, as amended from time to time.

        1.7 "CURRENT GOOD MANUFACTURING PRACTICES" OR "CGMP" shall mean the good
manufacturing practices required by the FDA and set forth in the FD&C Act or FDA
regulations, policies, or guidelines in effect at a particular time for the
manufacture, testing and quality control of pharmaceutical materials as applied
to biologics, except to the extent that Canadian standards for the manufacture,
testing and quality control of pharmaceutical materials as applied to biologics
are higher or more stringent than those required by the FDA, in which case such
Canadian standards will apply. For Mexico, if any higher or more stringent
standards for the manufacture and testing of pharmaceutical materials as applied
to biologics are required, Coulter will notify Nordion and the Parties will
discuss in good faith how to respond to such requirements.

        1.8 "DECOMMISSION" OR "DECOMMISSIONING" shall mean to remove the
facility safely from service and render the facility environmentally safe and in
so doing reduce residual radioactivity to a level that permits release of the
facility for unrestricted use.

        1.9 "EFFECTIVE DATE" shall be the date set forth in Section 11.13.

        1.10 "FACILITY PROGRAM" shall mean the program for the construction of
the Interim Facility and the Primary Facility as described in Article 2 of this
Agreement.

        1.11 "FDA" shall mean the United States Food and Drug Administration.

        1.12 "FD&C ACT" shall mean the United States Federal Food, Drug and
Cosmetic Act, as amended.

        1.13 "HOT CELLS" shall mean the assets purchased by Nordion by purchase
order on behalf of Coulter pursuant to the letter agreement dated February 23,
1996 between Nordion and Coulter dated February 23, 1996 and September 30, 1997
(annexed as Exhibit 1.13), as installed [...***...] of Nordion's Kanata
production site.


                                       2.


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        1.14 "IND" shall mean an Investigational New Drug application and any
supplements thereto, as defined in FDA regulations, as amended from time to
time.

        1.15 "INSTALLATION" of the Interim Facility shall mean the point in time
when the Interim Facility is ready for the first dry run and Nordion's Nuclear
Safety Committee has approved the use of such facility for the radiolabelling of
B1 Antibody with 131I.

"Installation" of the Primary Facility shall mean the point in time when the
Primary Facility is erected and ready to receive the Moveable Equipment and
Nordion's Nuclear Safety Committee has approved the use of such facility for
radiolabeling of B1 Antibody with 131I.

Once a facility has undergone Installation, it shall be deemed to be
"Installed".

        1.16 "INSTALLATION QUALIFICATION" shall mean the program employed by
Nordion by which it is established that the process equipment in question and
ancillary systems are capable of consistently operating within established
limits and tolerances.

        1.17 "INTERIM FACILITY" shall mean the facility described in Section 2.1
which has been Installed for the production of Labelled Drug, pursuant to
Article 2.

        1.18 "LABELLED DRUG" shall mean a pharmaceutical product containing the
131I-labelled B1 Antibody in either dosimetric or therapeutic final dosage form
for use in Clinical Trials or for commercial sale, which has been, or is being,
processed, compounded, formulated, finished, filled, labeled, packaged and/or
shipped by Nordion pursuant to the Supply Agreement.

        1.19 "MAXIMUM ACTUAL CAPITAL COST" shall have the meaning set forth in
Section 2.6(b).

        1.20 "MOVEABLE EQUIPMENT" shall mean the equipment so identified in
Exhibit 1.20.

        1.21 "OPERATION QUALIFICATION" shall mean the program employed by
Nordion by which it is established that the process equipment in question and
ancillary systems are capable of producing Labelled Drug that consistently meets
Specifications.

        1.22 "PRIMARY FACILITY" shall mean the facility described in Section 2.1
which has been Installed for the production of Labelled Drug pursuant to Article
2.

        1.23 "PRIMARY FACILITY QUALIFICATION PLAN" shall have the meaning set
forth in Section 2.5(b).

        1.24 "PROCESS" shall mean the process of formulation and dispensing
described in Nordion's Standard Operating Procedure #960700.SOP.

        1.25 "PROCESS PERFORMANCE QUALIFICATION" shall mean the program by which
it is established that the process equipment, ancillary systems, and the Process
are effective and reproducible.


                                       3.


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        1.26 "QUALIFIED" shall mean the condition of having passed standards for
Installation Qualification, Operation Qualification and Process Performance
Qualification.

        1.27 "SPECIFICATIONS" shall have the meaning set forth in Section 1.31
of the Supply Agreement.

        1.28 "VALIDATION" shall mean the program mutually agreed to by the
Parties by which documented evidence provides a high degree of assurance that
the Process will consistently produce Labelled Drug that meets Specifications
and quality attributes as mutually agreed by the Parties.

                                    ARTICLE 2

                                FACILITY PROGRAM

        2.1 PURPOSE. The purpose of this Agreement is to provide for the
establishment of a facility at Nordion's manufacturing site in Kanata, Ontario,
Canada which Nordion will use to supply Labelled Drug to Coulter for Clinical
Trials and commercialization pursuant to the Supply Agreement. The facility to
be used for the foregoing activities initially will be [...***...] of Nordion's
plant located at Kanata, Ontario (the "Interim Facility"). Thereafter, the
foregoing activities will be conducted in [...***...] of Nordion's plant located
at Kanata, Ontario, in which will be installed or used (as appropriate) on
Coulter's behalf, the Hot Cells and the Moveable Equipment (the "Primary
Facility").

        2.2 PERFORMANCE CHARACTERISTICS. The Interim Facility and the Primary
Facility shall have the capacity to use the Process to radiolabel B1 Antibody in
accordance with cGMP and all other requirements set forth in this Agreement and
the Supply Agreement in those Batch Capacity amounts set forth in Section 2.1 of
the Supply Agreement.

               (a) It is acknowledged by the Parties that Installation of the
Interim Facility was completed on [...***...].

               (b) It is further acknowledged by the Parties that Installation
of the Primary Facility was completed by [...***...] (the "Primary Facility
Installation Date").


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        2.3 FACILITY EXCLUSIVITY; REMOVAL OF INTERIM FACILITY FROM SERVICE.

               (a) Until the date on which the Primary Facility is Qualified,
Nordion may not use the Interim Facility for any purpose other than producing
Labelled Drug for Coulter or for conducting additional experiments per the
instructions of Coulter and with the approval of Nordion. Additionally, for the
term of the Supply Agreement and any renewals thereof, Nordion may not use the
Primary Facility for any purpose other than producing Labelled Drug for Coulter
or for conducting additional experiments per the instructions of Coulter and
with the approval of Nordion.

               (b) The Interim Facility shall not be removed from service until
the later of (i) ten (10) weeks after the Primary Facility is Qualified or (ii)
the date upon which a fifth Batch of Labelled Drug which meets the
Specifications has been produced in the Primary Facility. Notwithstanding the
foregoing, Nordion may remove the Interim Facility from service if Coulter has
not requested Nordion to produce a fifth Batch of Labelled Drug by the ten (10)
week anniversary of the date on which the Primary Facility is Qualified.

        2.4 CHANGE ORDERS. If Coulter requests any modifications ("Change
Orders") to the Facility Program, Nordion will use reasonable efforts to carry
out such Change Orders. As set forth in Section 2.6, whether the Change Orders
affect the Interim Facility, the Primary Facility, or both, the Maximum Actual
Capital Cost will be increased, on the same basis for time and materials as is
set forth in Section 2.6(a).

        2.5 QUALIFICATION PLANS AND EXPENSES.

               (a) INTERIM FACILITY. It is acknowledged by the Parties that the
Interim Facility was Qualified by Nordion on or about [...***...].

               (b) PRIMARY FACILITY. Preparation and approval of Primary
Facility Qualification Plans shall be carried out by Nordion in collaboration
with Coulter as set forth in this Section 2.5(b). Nordion will prepare and
Coulter will approve, such approval not to be unreasonably withheld, a
qualification plan for the Primary Facility (the "Primary Facility Qualification
Plan") which will set forth the steps to be followed by Nordion in causing the
Primary Facility to be Qualified. The Parties will prepare a budget for
performance of the Primary Facility Qualification Plan in a manner consistent
with Nordion's current estimation process. Upon receipt of supporting
documentation from Nordion, Coulter shall pay in full, pursuant to Section
2.5(d), all amounts actually incurred by Nordion which are less than or equal to
the amounts budgeted for performance of the Primary Facility Qualification Plan.

               (c) EXCESS EXPENSES. Any reasonable expenses of performance under
the Primary Facility Qualification Plan which exceed the amounts budgeted
therefor ("Excess Expenses") shall be allocated as set forth in this Section
2.5(c). Excess Expenses related to performance under the Primary Facility
Qualification Plan derived primarily from errors committed by Nordion in
carrying out the Primary Facility Qualification Plan shall be borne solely by
Nordion. All other Excess Expenses related to performance under the Primary
Facility Qualification Plan shall be borne by Coulter, subject to Coulter's
prior written authorization.


                                       5.


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               (d) PAYMENTS. All payments due to Nordion under this Section 2.5
shall be paid by Coulter within thirty (30) days of Coulter's receipt of invoice
therefor, except that the final payment due to Nordion for performance of the
Primary Facility Qualification Plan shall be made by the later of (i) fifteen
(15) days after Coulter's acceptance of the Primary Facility Qualification
Report or (ii) thirty (30) days after Coulter's receipt of an invoice therefor.

               (e) PRIMARY FACILITY QUALIFICATION REPORT. At such time or times
as are set forth in the Primary Facilities Qualification Plan, and at such time
as the Primary Facility is Qualified, Nordion shall prepare and submit to
Coulter a report (the "Primary Facility Qualifications Report") summarizing the
activities performed pursuant to the Primary Facility Qualification Plan,
including but not limited to facility qualification, media fills, and the full
activity run contemplated by the Primary Facility Qualification Plan.

        2.6 CAPITAL COSTS OF THE FACILITY PROGRAM.

               (a) ACTUAL CAPITAL COST. The actual capital cost ("Actual Capital
Cost") of the Facility Program will be calculated on a time and materials basis
as follows: in performing the Facility Program, Nordion will provide labor at
the rate of [...***...] per hour ([...***...] per hour after November 1, 1997),
and will provide materials and equipment at Nordion's cost plus an additional
[...***...] administrative fee; provided, however, that no such
administrative fee shall be charged on any travel expenses Nordion incurs
pursuant to the Facility Program.

               (b) MAXIMUM ACTUAL CAPITAL COST. Except as set forth in Section
2.4, in no event will the Actual Capital Cost invoiced to Coulter exceed
[...***...] (the "Maximum Actual Capital Cost") without the prior written
authorization of Coulter. The Maximum Actual Capital Cost was derived from a
preliminary engineering estimate for a manually operated facility with capacity
to process [...***...] vials per Batch. Upon request of Coulter and as agreed by
Nordion, the scope of the Primary Facility shall be expanded to be capable of
processing [...***...] vials per Batch ("Scope Change"). The Scope Change shall
include the installation of a third Hot Cell, as well as the addition of
automated systems for dispensing, QC testing and packaging. Nordion shall obtain
the prior written authorization of Coulter, as set forth in Section 2.4 for all
incremental expenses resulting from the Scope Change.

               (c) AUDITS. Except as provided below, Coulter, at its sole
expense and through an independent certified public accountant reasonably
acceptable to Nordion, shall have the right to access the books and records of
Nordion relating to compensation due hereunder for the sole purpose of verifying
such statements. Such access shall be conducted after six (6) weeks prior
written notice to Nordion during ordinary business hours, shall not be more
frequent than once during each calendar year and shall not include books and
records which predate the date of the audit by more than three (3) years.
Coulter agrees to keep in strict confidence all information learned in the
course of such audit, except when it is necessary to reveal such information in
order to enforce its rights under this Agreement. Coulter's right to have such
records examined shall survive termination or expiry of this Agreement for a
period of two (2) years. In all events, Nordion shall promptly remit to Coulter
the amount of any overpayment. Coulter shall bear the 


                                       6.


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full cost of such audit unless such audit discloses an overpayment by Coulter
during the audit period in question (the "Audit Period") of more than
[...***...] of the amount actually due (provided that such overpayment is at
least [...***...] during such Audit Period), in which event Nordion shall
reimburse Coulter for the costs of such audit. In the event a given Audit Period
is more than one year, the [...***...] threshold amount set forth in the
preceding sentence shall be pro rated for such Audit Period by multiplying
[...***...] by a fraction, the numerator of which is the number of days in the
Audit Period and the denominator of which is 365.

        2.7 SCALE-UP AND VALIDATION PLAN. After the Primary Facility is
Installed, Nordion agrees, upon request from and in conjunction with Coulter, to
work diligently to develop a Primary Facility Scale-Up and Validation Plan
("Plan") that will allow the expansion of Batch Capacity in the Primary Facility
under cGMP to those amounts set forth in Section 2.1 of the Supply Agreement.
The costs of development and implementation of the Plan will be borne by Coulter
on a time and materials basis.

        2.8 REPAIRS AND MAINTENANCE OF FACILITIES. After each of the Interim
Facility and the Primary Facility are Installed, Nordion shall maintain each
such facility in satisfactory operating condition as required by the
Specifications, Process and cGMP and all other applicable laws, regulations,
rules or orders of the United States and Canada. In the event of any conflict
between the applicable laws, regulations, rules or orders of the United States
and the applicable laws, regulations, rules or orders of Canada, Nordion will
notify Coulter of such conflict and the Parties shall act in good faith to
resolve such conflict or to determine which laws, regulations, rules or orders
take precedence; provided, however, that at all times the Parties shall comply
with the requirements of cGMP. The cost of routine repairs, preventative
maintenance and service contracts for [...***...] shall be borne by [...***...].
[...***...] will bear all reasonable expenses for any Major Repairs to or
replacement of the Moveable Equipment. "Major Repair(s)" shall mean a repair to
a given asset entailing expenditures in excess of the lesser of (a) [...***...]
of the subject asset's purchase price as determined at the time of purchase by
the invoice price less any discounts received and (b) [...***...]. The maximum
amount [...***...] will be required to pay in any calendar year for routine
repairs, preventative maintenance and service contracts for the [...***...]
shall be [...***...]. Any reasonable amounts for routine repairs, preventative
maintenance and service contracts for the [...***...] in excess of [...***...]
in any calendar year will be borne by [...***...]. Preventative maintenance and
service contracts for the [...***...] in excess of [...***...] per year which
are approved in advance by [...***...] will be borne by [...***...]. All amounts
set forth in this Section 2.8 shall be based on Nordion's out-of-pocket costs
and Nordion's standard internal labor rates. Nordion shall coordinate with and
advise Coulter regarding the advisability of any Major Repair or replacement.
Notwithstanding anything in this Agreement, [...***...] shall be responsible for
bearing the costs for [...***...]. The only repairs, if any, to the [...***...]
which shall be borne by [...***...] are those [...***...].

        2.9 REPORTS. Nordion shall provide monthly written reports to Coulter
summarizing Nordion's progress under the Facility Program.


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                                    ARTICLE 3

                                 ASSET OWNERSHIP

        3.1 HOT CELLS.

               (a) Nordion has purchased, on behalf of Coulter, the Hot Cells,
which will be/have been installed in the Primary Facility as described above.
Subject to its obligations to [...***...] under circumstances as set forth in
this Agreement, Coulter shall [...***...]; provided, however, that usage thereof
shall belong exclusively to Nordion for the purpose of producing Labelled Drug
for Coulter. Coulter represents and warrants that the Hot Cells are not
encumbered, and shall, during the term of this Agreement, remain free and clear
of any and all encumbrances including, but not limited to, mortgages, charges
and liens and that no effective financing statement or other instrument similar
in effect covering all or any part of the Hot Cells is on file in any recording
office.

               (b) In partial consideration of the services to be performed
hereunder by Nordion, on or before the earlier of the natural expiration of the
Supply Agreement or the tenth (10th) anniversary of the Effective Date of this
Agreement, Coulter promises without further notice or demand to either:

                      (i) pay Nordion [...***...]; or

                      (ii) [...***...]

If at any time Coulter pays to Nordion the sum set out in (i) above and removes
the Hot Cells, Coulter shall be responsible for all costs associated with such
Decommissioning and removal and shall return the area at Nordion's facility
housing the Hot Cells to substantially its condition and state prior to the
removal of the Hot Cells.

               (c) Until such time as Coulter has made [...***...] as set out
elsewhere in this Agreement (the [...***...]), Nordion shall have, and is hereby
granted, [...***...]. The [...***...] shall [...***...] as of the Effective Date
and be [...***...] by Nordion, and shall [...***...] (notwithstanding any
material breach, founded or unfounded, alleged or attributable to Nordion).
Until the [...***...], Nordion at all times shall be entitled to the [...***...]
in accordance with the provisions set out in this Agreement.

               (d) Notwithstanding the applicable law set out in this Agreement,
the laws of Ontario shall govern the security interest set out in this Section
3.1.


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        3.2 CLASS 100 INSERT. Nordion has purchased, on behalf of Coulter, one
or more Class 100 Insert(s). Coulter shall at all times hold all right, title
and interest in the Class 100 Insert(s). When the Interim Facility is removed
from service, Nordion shall Decommission and store the Class 100 Insert(s)
contained in the Interim Facility at its Kanata site in a manner which would
allow Nordion to use the Class 100 Insert(s) as an available backup to the
Primary Facility for manufacturing Labelled Drug, subject to the availability of
a suitable hot cell, until such time as the Parties mutually agree that the
Class 100 Insert(s) is (are) no longer useful for such purpose. Thereafter,
Nordion, at its option, will either purchase the Class 100 Insert(s) from
Coulter for a commercially reasonable price or deliver it (them) to a site
designated by Coulter, at Coulter's expense.

        3.3 OTHER ASSETS. Nordion shall own all assets purchased for
installation in the Interim Facility and the Primary Facility except as set
forth in Sections 3.1 and 3.2 and in the following sentence. In all events,
however, the Moveable Equipment will remain the property of Coulter. The parties
agree that certain backup equipment should be procured for use in the Interim
Facility and the Primary Facility and that such backup equipment may be used by
Nordion for purposes outside this Agreement, subject to the prior written
approval of Coulter.

                                    ARTICLE 4

                                    PAYMENTS

        Except as otherwise expressly provided, all payments due to Nordion
hereunder shall be paid within thirty (30) days of receipt of invoice by Coulter
therefor. All payment amounts set forth in this Agreement are in U.S. dollars
and all amounts due under this Agreement shall be paid in U.S. dollars.

                                    ARTICLE 5

                        LICENSES, PATENTS AND TECHNOLOGY

        5.1 LICENSE. Coulter hereby grants Nordion a [...***...] license during
the term of this Agreement to use the data, information and technology provided
by Coulter to Nordion which relates to B1 Antibody radiolabelling, for the
limited purpose of assisting Nordion in carrying out its obligations under this
Agreement.

        5.2 OWNERSHIP OF WORK PERFORMED.

               (a) For purposes of this section, "Background Technology" shall
mean all Nordion proprietary technology, including patents, know-how,
techniques, methods, processes and trade secrets which Nordion owns or which is
licensed to Nordion by a party other than Coulter and which is in existence in
the form of a writing, prototype or can otherwise be demonstrated to be the
property of Nordion, prior to the effective date of the Development Agreement.
The Background Technology shall remain the sole property of Nordion.


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               (b) Nordion agrees and Coulter acknowledges, that any and all
ideas, improvements, inventions and works of authorship conceived, written,
created or first reduced to practice in the performance of this Agreement,
except to the extent that it relates to or embodies the Background Technology or
improvements to the Background Technology, shall be the sole and exclusive
property of Coulter. Except as otherwise set out in this Section 5.2, Coulter is
and shall be vested with all right, title and interest, including patent,
copyright and trade secret rights, in such ideas, improvements, inventions and
works of authorship. Nordion hereby assigns to Coulter all right, title and
interest in and to any and all such ideas, improvements, inventions and works of
authorship.

               (c) Coulter hereby grants to Nordion a [...***...] license to
exploit all ideas, improvements, inventions and works of authorship assigned to
or vested in Coulter as described in Section 5.2(b) for any application, except
the manufacture of any pharmaceutical for the treatment of hematological
malignancy.

        5.3 COULTER PROPRIETARY INFORMATION. All data, information, or
technology supplied to Nordion by Coulter to assist Nordion in carrying out its
obligations hereunder shall remain the property of Coulter and shall be returned
by Nordion to Coulter upon termination of this Agreement.

        5.4 PATENT APPLICATIONS.

               (a) Nordion shall execute all papers, including patent
applications, invention assignments and copyright assignments, and otherwise
shall assist Coulter as reasonably required to perfect in Coulter the rights,
title and other interests in Nordion's work product expressly granted to Coulter
under this Agreement. Reasonable costs related to such assistance, if required,
shall be paid by Coulter.

               (b) Notwithstanding any other provision of this Agreement, in the
event that Nordion requests in writing that Coulter file, maintain and prosecute
a patent application pertaining to rights granted to Nordion pursuant to Section
5.2(c) hereof, then Coulter shall have thirty (30) days to inform Nordion
whether or not such application will be made. In the event that Coulter fails to
inform Nordion of its intent to proceed with the requested patent application
within such thirty (30) day period, then Nordion shall have the sole right to
proceed with the filing, prosecution and maintenance of such patent application,
at its sole expense. Any patent resulting therefrom shall be the sole property
of Nordion, subject only to Coulter's [...***...] right to use and sublicense
the use of such patent in connection with products based on the 131I-labelled B1
Antibody. Coulter shall execute all papers, including patent applications and
invention assignments, and otherwise shall assist Nordion as reasonably required
to perfect in Nordion the patent rights described in this Section 5.4(b).


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                                    ARTICLE 6

                                   TERMINATION

        6.1 TERMINATION FOR BREACH. This Agreement may be terminated by either
Party in the event of the material breach by the other Party of the terms and
conditions hereof; provided, however, the other Party shall first give to the
breaching Party written notice of the proposed termination of this Agreement (a
"Breach Notice"), specifying the grounds therefor. Upon receipt of such Breach
Notice, the breaching Party shall have [...***...] or [...***...] with respect
to a failure by Coulter to pay any amounts hereunder when due, other than with
respect to amounts which Coulter, in good faith, disputes are due to Nordion) to
respond by curing such breach. If the breaching Party does not cure such breach
within such cure period, the other Party may terminate the Agreement. Subject to
Section 11.3, termination of this Agreement pursuant to this Section shall not
affect any other rights or remedies which may be available to the non-breaching
Party.

        6.2 REMEDIES UPON TERMINATION. The remedies available to the Parties
upon early termination of this Agreement are set forth in Article 11 of the
Supply Agreement.

        6.3 BANKRUPTCY; INSOLVENCY. A Party may terminate this Agreement upon
the occurrence of either of the following:

               (a) The other Party is subject to the entry of a decree or order
for relief by a court having jurisdiction in the premises in respect of such
Party in an involuntary case under the Federal Bankruptcy Code in the United
States or the Bankruptcy and Insolvency Act in Canada, as now constituted or
hereafter amended, or any other applicable national federal or state insolvency
or other similar law and the continuance of any such decree or order unstayed
and in effect for a period of sixty (60) consecutive days; or

               (b) The filing by the other Party of a petition for relief under
the Federal Bankruptcy Code in the United States or the Bankruptcy and
Insolvency Act in Canada, as now constituted or hereafter amended, or any other
applicable federal or state insolvency or other similar law.

        6.4 CONSEQUENCES OF TERMINATION OR EXPIRATION. Upon expiration or
termination of this Agreement, the obligations of the Parties under Sections
2.6(c), 5.2, 5.4, 11.3 and Articles 3, 7 and 10 hereof, and any other article
which by its nature is to survive shall survive such expiration or termination.

        6.5 SIMULTANEOUS TERMINATION OF SUPPLY AGREEMENT AND THIS AGREEMENT. If
either Party terminates the Supply Agreement or Facilities Agreement for any
reason, both Agreements shall be deemed to be terminated on the same day and
Article 11 of the Supply Agreement shall govern the effect and consequences of
the termination with respect to both agreements.


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                                    ARTICLE 7

                                    INDEMNITY

        7.1 INDEMNIFICATION BY COULTER. Subject to Nordion's compliance with
Section 7.4, Coulter agrees to indemnify, defend and hold Nordion and its
Affiliates, and their respective directors, officers, employees and agents
harmless from and against any damages, claims, liabilities and expenses
(including, but not limited to, reasonable attorneys' fees) resulting from any
third party claims or suits ("General Claims Against Nordion") arising out of
(a) Coulter's breach of any of its warranties or representations hereunder or
(b) Coulter's negligent acts or omissions or willful misconduct. Notwithstanding
the foregoing, Coulter will not be required to indemnify, defend and hold
Nordion or its Affiliates, and their respective directors, officers, employees
and agents harmless from and against any General Claims Against Nordion arising
out of (i) Nordion's breach of any of its warranties or representations
hereunder; (ii) Nordion's negligent acts or omissions or willful misconduct; or
(iii) except as otherwise directed by Coulter, any failure of Nordion to
construct the Interim Facility and/or the Primary Facility in accordance with
Sections 2.1 and 2.2 hereof and in accordance with cGMP and any other applicable
laws, regulations or other requirements of any applicable governmental entity in
the United States or Canada. Notwithstanding anything in this Section 7.1,
General Claims Against Nordion shall not include IP Claims Against Nordion (as
such term is defined below).

        7.2 INDEMNIFICATION BY NORDION. Subject to Coulter's compliance with
Section 7.4, Nordion agrees to indemnify, defend and hold Coulter and its
Affiliates, and their respective directors, officers, employees and agents
harmless from and against any damages, claims, liabilities and expenses
(including, but not limited to, reasonable attorneys' fees) resulting from any
third party claims or suits ("General Claims Against Coulter") arising out of
(a) Nordion's construction and/or use of the Interim Facility and the Primary
Facility; (b) Nordion's breach of any of its warranties or representations
hereunder; or (c) Nordion's negligent acts or omissions or willful misconduct.
Notwithstanding the foregoing, Nordion will not be required to indemnify, defend
and hold Coulter or its Affiliates, and their respective directors, officers,
employees and agents harmless from and against any General Claims Against
Coulter arising out of (i) Coulter's breach of any of its warranties or
representations hereunder or (ii) Coulter's negligent acts or omissions or
willful misconduct. Notwithstanding anything in this Section 7.2, General Claims
against Coulter shall not include IP Claims against Coulter (as such term is
defined below).

        7.3 INTELLECTUAL PROPERTY CLAIMS.

               (a) Subject to Coulter's compliance with Section 7.4, Nordion
agrees to indemnify, defend and hold Coulter and its Affiliates, and their
respective directors, officers, employees and agents harmless from and against
any damages, claims, liabilities and expenses (including, but not limited to,
reasonable attorneys' fees) resulting from any third party claims or suits
arising out of any proceeding instituted by or on behalf of a third party based
upon a claim that the construction or use of the Interim Facility or the Primary
Facility infringes a United States or Canadian patent or any other proprietary
rights of a third party ("IP Claims against Coulter"). Notwithstanding the
foregoing, Nordion will not be required to indemnify, defend 


                                      12.


<PAGE>   13
and hold Coulter or its Affiliates, and their respective directors, officers,
employees and agents harmless for and against any IP Claims Against Coulter
arising out of any data, information or technology provided by Coulter to
Nordion for purposes of this Agreement.

               (b) Subject to Nordion's compliance with Section 7.4, Coulter
agrees to indemnify, defend and hold Nordion and its Affiliates, and their
respective directors, officers, employees and agents harmless from and against
any damages, claims, liabilities and expenses (including, but not limited to,
reasonable attorneys' fees) resulting from any third party claims or suits
arising out of any proceeding instituted by or on behalf of a third party based
upon a claim that the use of any data, information or technology provided by
Coulter to Nordion for purposes of this Agreement infringes a United States or
Canadian patent or any other proprietary rights of a third party ("IP Claims
against Nordion").

        7.4 INDEMNIFICATION PROCEDURES. A Party (the "Indemnitee") which intends
to claim indemnification under this Article 7 shall promptly notify the other
Party (the "Indemnitor") in writing of any action, claim or other matter in
respect of which the Indemnitee or any of its Affiliates, or any of their
respective directors, officers, employees or agents intend to claim such
indemnification; provided, however, the failure to provide such notice within a
reasonable period of time shall not relieve the Indemnitor of any of its
obligations hereunder except to the extent the Indemnitor is prejudiced by such
failure. The Indemnitee shall permit, and shall cause its Affiliates, and their
respective directors, officers, employees and agents to permit, the Indemnitor,
at its discretion, to settle any such action, claim or other matter. The
Indemnitee agrees to the complete control of such defense or settlement by the
Indemnitor; provided, however, such settlement does not adversely affect the
Indemnitee's rights hereunder or impose any obligations on the Indemnitee in
addition to those set forth herein in order for it to exercise such rights. No
such action, claim or other matter shall be settled without the prior written
consent of the Indemnitor, and the Indemnitor shall not be responsible for any
attorneys' fees or other costs incurred other than as provided herein. The
Indemnitee, its Affiliates, and their respective directors, officers, employees
and agents shall cooperate fully with the Indemnitor and its legal
representatives in the investigation and defense of any action, claim or other
matter covered by this indemnification. The Indemnitee shall have the right, but
not the obligation, to be represented by counsel of its own selection and at its
own expense.

        7.5 INSURANCE. Nordion agrees, at Nordion's expense, to maintain general
liability insurance and property insurance covering loss or damage to (i) the
Interim Facility and the Primary Facility, (ii) any asset owned by Coulter in
the possession of Nordion under this Agreement, including, but not limited to,
[...***...], and (iii) Nordion's facility located at Kanata, Ontario. Such
insurance policy shall designate Coulter as payee in the event of any loss or
damage specifically as it relates to any asset owned by Coulter. Nordion agrees
that such insurance shall be replacement value insurance for all property owned
by Coulter. Nordion shall provide to Coulter a certificate of insurance
designating Coulter as payee in event of any loss or damage covered by clause
(ii) of the first sentence of this Section 7.5, provided that any proceeds so
received shall be used to repair or replace such damaged or destroyed assets,
including, but not limited to, [...***...]. Any insurance policy held by Nordion
pursuant to this Section 7.5 will provide that 


                                      13.


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<PAGE>   14
the insurer shall give Coulter thirty (30) days advance notice of any
termination or cancellation of such coverage by the insurer. Nordion shall
provide Coulter with thirty (30) days written notice prior to termination or
cancellation of such coverage by Nordion. Any proceeds received by Nordion for
the damage or destruction of the Primary Facility shall be used to repair or
replace such facility. This Section 7.5 shall survive termination of this
Agreement with respect to clause (ii) of the first sentence of this Section 7.5.

                                    ARTICLE 8

                         REPRESENTATIONS AND WARRANTIES

        8.1 COULTER WARRANTIES. Coulter warrants that:

               (a) all activities of Coulter under this Agreement will be
accomplished in accordance with the terms of this Agreement;

               (b) (i) to Coulter's best information and belief, it is the owner
or licensee of all data, information and technology supplied to Nordion by
Coulter to assist Nordion in carrying out its obligations hereunder (the
"Coulter IP"), (ii) to Coulter's best information and belief, the Coulter IP
does not infringe any patents, copyright or other industrial or intellectual
property rights of third parties, (iii) it has the right to grant the license
set forth in Section 5.1 and the right to permit Nordion to use the Coulter IP
to carry out Nordion's obligations as contemplated herein, and (iv) it has not
received any notice of adverse claim or infringement of any patent contained in
the Coulter IP; and

               (c) to Coulter's best information and belief, there are no
intellectual property rights, such as, but not limited to valid United States
patents, that would be infringed by the manufacture, use or sale of Labelled
Drug. Notwithstanding the preceding sentence, Coulter makes no warranty with
respect to intellectual property rights relating to (i) any processes performed
by Nordion hereunder, except for that portion of any process contributed by
Coulter, or (ii) the construction or maintenance of the Interim Facility or the
Primary Facility by Nordion.

        8.2 NORDION WARRANTIES. Nordion warrants that:

               (a) all activities of Nordion under this Agreement will be
accomplished in accordance with the terms of this Agreement;

               (b) (i) except for the Coulter IP, to Nordion's best information
and belief, it is the owner of all data, information and technology to be used
by Nordion in carrying out its obligations hereunder (the "Nordion IP"), (ii) to
Nordion's best information and belief, the Nordion IP does not infringe any
patents, copyright or other industrial or intellectual property rights of third
parties, (iii) it has the right to use the Nordion IP to carry out its
obligations as contemplated herein, and (iv) it has not received any notice of
adverse claim or infringement of any patent relating to the Nordion IP; and


                                      14.


<PAGE>   15
               (c) to Nordion's best information and belief, there are no
intellectual property rights, such as, but not limited to valid United States
patents, that would be infringed by (i) any processes performed by Nordion
hereunder, except for that portion of any process contributed by Coulter, or
(ii) the construction or maintenance of the Interim Facility or the Primary
Facility by Nordion.

               8.3 MUTUAL REPRESENTATIONS AND WARRANTIES. Each Party hereby
represents and warrants to the other Party that this Agreement is legal and
valid and the obligations binding upon such Party are enforceable in accordance
with their terms, and that the execution, delivery and performance of this
Agreement does not conflict with any agreement, instrument or understanding,
oral or written, to which such Party may be bound, nor violate any law or
regulation of any court, governmental body or administrative or other agency
having jurisdiction over it.

                                    ARTICLE 9

                                  FORCE MAJEURE

        Neither Party shall be liable to the other for loss or damage by virtue
of the occurrence of an event of Force Majeure (as defined in this Article 9).
In the event of Force Majeure, the Party affected shall promptly notify the
other and shall exert commercially reasonable efforts to eliminate, cure or
overcome such event and to resume performance of its obligations. Coulter
acknowledges that if Nordion is the Party affected by the Force Majeure event,
Nordion may first restore the production of its core products at Nordion's
facility prior to resuming its obligations under this Agreement, but Nordion
agrees that it will resume its obligations under this Agreement as soon as is
practicable thereafter. "Force Majeure" shall mean an occurrence which prevents,
delays or interferes with the performance by a Party of any of its obligations
hereunder, if such event occurs by reason of any act of God, flood, fire,
explosion, casualty or accident, or war, revolution, civil commotion, acts of
public enemies, blockage or embargo, or any law, order or proclamation of any
government, failure of suppliers to provide materials, equipment or machinery,
interruption of or delay in transportation, strike or labor disruption, or any
other cause whatsoever, whether similar or dissimilar to those above enumerated,
beyond the commercially reasonable control of such Party.

                                   ARTICLE 10

                                 CONFIDENTIALITY

        10.1 CONFIDENTIALITY AND EXCEPTIONS. Except as set forth below, all
information disclosed by one Party to the other Party in connection with the
work performed hereunder shall be deemed to be the disclosing Party's
"Confidential Information". Confidential Information shall include, but not be
limited to, information relating to the B1 Antibody and the structure of
Labelled Drug, any know-how relating to the manufacture of facilities for the
production of Labelled Drug, and the manufacturing cost and other financial
arrangements made pursuant to this Agreement. Each Party agrees that it will
take the same steps to protect the confidentiality of 


                                      15.


<PAGE>   16
the other Party's Confidential Information as it takes to protect its own
proprietary and confidential information, which shall in no event be less than
reasonable steps. Each Party, and its employees and agents, shall protect and
keep confidential and shall not use, publish or otherwise disclose to any third
party, except as permitted by this Agreement, or with the other Party's written
consent, the other Party's Confidential Information. For the purposes of this
Agreement, Confidential Information shall not include such information that:

               (a) was already known to the receiving Party at the time of
disclosure by the other Party, other than under an obligation of
confidentiality; or

               (b) was generally available to the public or was otherwise part
of the public domain at the time of disclosure or became generally available to
the public or otherwise part of the public domain after disclosure other than
through any act or omission of the receiving party in breach of this Agreement;
or

               (c) was lawfully disclosed to the receiving Party, other than
under an obligation of confidentiality, by a third party who had no obligation
not to disclose such information to others; or

               (d) was independently developed by or for the receiving Party
without the aid, application or use of Confidential Information; or

               (e) was disclosed to a third party by the disclosing Party
without an obligation of confidentiality; or

               (f) was independently developed by persons who did not have
access to the Confidential Information.

        10.2 AUTHORIZED DISCLOSURE. Each Party may disclose Confidential
Information hereunder to the extent such disclosure is reasonably necessary for
prosecuting or defending litigation, complying with applicable government laws
or regulations or conducting preclinical or Clinical Trials, provided that if a
Party is required by law or regulation to make any such disclosure of the other
Party's Confidential Information it will, except where impracticable for
necessary disclosures, for example in the event of medical emergency, give
reasonable advance notice to the other Party of such disclosure requirement and
will use its reasonable efforts to secure a protective order or confidential
treatment of such Confidential Information required to be disclosed. In
addition, upon prior approval of Nordion, which will not be unreasonably
withheld, Coulter may disclose, under a comparable binder of confidentiality,
and on a need-to-know basis, information received under this Agreement to its
other partners for the development or commercialization of Labelled Drug.
Neither Party shall disclose Confidential Information of the other Party in any
patent filings without the prior written consent of the other Party.

        10.3 CONFIDENTIALITY AND PUBLICITY. The Parties agree that, except as
may otherwise be required by applicable laws, regulations, rules, or orders, and
except as may be authorized elsewhere in this Article 10, no information
concerning this Agreement and the transactions contemplated herein shall be made
public by either Party without the prior written consent of the 


                                      16.


<PAGE>   17
other. In the event either Party decides to issue a press release announcing the
execution of this Agreement, it shall not do so without the prior written
approval of the other Party of the text of such press release, such approval not
to be unreasonably withheld..

        10.4 SURVIVAL OF CONFIDENTIALITY. All obligations of confidentiality and
non-use imposed upon the Parties under this Article 10 shall expire ten (10)
years after the expiration or termination of this Agreement.

                                   ARTICLE 11

                               GENERAL PROVISIONS

        11.1 NOTICE. All notices under this Agreement must be made in writing
and mailed or delivered to the following:

               Coulter:             Coulter Pharmaceutical, Inc.
                                    550 California Avenue, Suite 200
                                    Palo Alto, CA, U.S.A.
               Attention:           Chief Financial Officer

               Nordion:             MDS Nordion Inc.
                                    447 March Road
                                    Kanata, Ontario, CANADA

               Attention:           Vice President, Technology and
                                    Business Development



        11.2 NO IMPLIED LICENSE. It is agreed that disclosure of data,
information or technology by Nordion or Coulter during the term of this
Agreement shall not, except to the extent granted herein, constitute any grant,
option or license under any patent, technology or other rights held by Nordion
or Coulter.

        11.3 DISCLAIMER OF CONSEQUENTIAL DAMAGES. Except as set forth in the
Supply Agreement, in no event shall either Party be liable to the other Party or
its Affiliate for indirect, contingent, incidental, special or consequential
damages, including, but not limited to, any claim for damages based on lost
profits, cost of capital, loss of business opportunity or loss of time.

        11.4 ASSIGNMENT. Assignment of this Agreement may only be carried out in
conjunction with the Supply Agreement and for the purposes of assignment the
terms and conditions contained in Section 16.5 of the Supply Agreement shall
govern.

        11.5 APPLICABLE LAW. Except as set forth in Section 3.1(d), this
Agreement will be governed by the laws of the State of New York as applied to
contracts executed and performed in New York by New York residents.


                                      17.


<PAGE>   18
        11.6 DISPUTE RESOLUTION.

               (a) In the event that at any time during the term of this
Agreement a disagreement, dispute, controversy or claim should arise relating to
scientific or technical issues in connection with Nordion's performance under
this Agreement, the Parties will attempt in good faith to resolve their
differences for [...***...]. If, after [...***...], the Parties are unable to
resolve such dispute, the Parties shall refer the matter to a third party
consultant with expertise in the scientific or technical area of dispute for
[...***...]. In the event such consultant is unable to work out a resolution of
the issue with the Parties, either Party shall be free to take any action and
seek any remedy it may have at law or in equity, including specific performance
and injunctive relief.

               (b) In the event that at any time during the term of this
Agreement a disagreement, dispute, controversy or claim should arise out of or
relating to the interpretation of or performance under this Agreement, or the
breach, or invalidity thereof other than a dispute relating to scientific or
technical issues in connection with Nordion's performance under this Agreement,
the Parties will attempt in good faith to resolve their differences by referring
the matter to the Chief Executive Officers of the Parties (or their designees)
for [...***...], following which either Party shall be free to take any action
and seek any remedy it may have at law or in equity, including specific
performance and injunctive relief.

        11.7 SEVERABILITY. In the event that any one or more of the provisions
contained in this Agreement should be held invalid, illegal or unenforceable in
any respect, the validity, legality and enforceability of the remaining
provisions contained herein shall not in any way be affected or impaired
thereby, unless the absence of the invalidated provision(s) adversely affect the
substantive rights of the Parties. The Parties agree to replace any invalid
provision or parts thereof by new provision(s) which closely approximate the
economic and proprietary results intended by the Parties.

        11.8 ENTIRE AGREEMENT. This Agreement, together with the attached
Exhibits and the Supply Agreement and the Development Agreement, contains the
sole and entire understanding of the Parties related to its subject matter and
supersedes all prior or contemporaneous oral or written agreements concerning
the subject matter.

        11.9 MODIFICATION. This Agreement cannot be changed orally and no
modification of this Agreement will be recognized or have any effect, unless the
writing in which it is set forth is signed by Nordion and Coulter, nor will any
waiver of any of the provisions of this agreement be effective unless in writing
and signed by the Party to be charged therewith.

        11.10 COMPLIANCE. This Agreement shall be carried out in compliance with
all applicable laws, by-laws, rules, regulations and orders of federal, state,
provincial or municipal governments.


                                      18.


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<PAGE>   19
        11.11 NON-WAIVER. Failure by either Party to enforce at any time any of
the provisions of this Agreement shall not be construed as a waiver of its
rights hereunder. Any waiver of a breach of any provision hereof shall not
affect either Party's rights in the event of any additional breach.

        11.12 HEADINGS. The headings herein are for the purpose of convenience
of reference only and are not intended to define or limit the contents of this
Agreement.

        11.13 EFFECTIVE DATE. This Agreement shall be deemed effective as of
[...***...].

        IN WITNESS WHEREOF, the Parties have caused this Agreement to be
executed by the respective duly authorized officers as of the date first written
above.

                                            MDS NORDION INC.

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

                                       Printed Name:
                                                    ---------------------------


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


                                       COULTER PHARMACEUTICAL, INC.

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

                                       Printed Name:
                                                    ---------------------------

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


                                      19.


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<PAGE>   20
                                  EXHIBIT 1.13

                             LETTER AGREEMENTS DATED

                    FEBRUARY 23, 1996 AND SEPTEMBER 30, 1997



<PAGE>   21
Nordion International Inc.
447 March Road
Kanata, Ontario
Canada
K2K-1X8

Attention: David Evans, V.P. Technology and Business Development

Dear Sir,

     1. In preparation for the carrying out of the Facility Phase and
commercial supply of radiolabelled B-1 antibody with 1-131 as set out in the
agreement between Nordion International Inc. ("Nordion") and Coulter
Pharmaceutical, Inc. ("Coulter"), dated the 15th day of November, 1995 (the
"Agreement"), we hereby instruct Nordion and Nordion, in reliance on said
instruction, agrees to proceed with the purchase of the hot cells as Nordion
deems appropriate to carry out the Facility Phase and commercial supply of
radiolabelled B-1 antibody with I-131. Coulter acknowledges that Nordion shall
be entering into a purchase agreement (the "Purchase Agreement") with its
supplier Von Gahlen International Inc. ("Won Gahlen") for the purchase and
supply of such hot cells.

     2. In consideration of the purchase of the hot cells by Nordion,
Coulter hereby agrees to reimburse to Nordion the purchase price of [...***...],
plus an administration fee of [...***...] calculated thereon (hereafter the
"Aggregate Purchase Price"), payable as follows: Coulter shall pay to Nordion,
due within five (5) business days of execution of this letter agreement,
[...***...]. In addition Coulter shall make interim payments (the "Interim
Payments") to Nordion, (i) on a monthly basis within 15 days of receipt of
Nordion's invoice, and (ii) within 15 days of receipt of Nordion's invoice after
inspection/acceptance of the hot cells by Nordion at Von Gahlen's site, such
Interim Payments corresponding to the contract value of work performed to such
date by Von Gahlen, increased by the above mentioned [...***...] administration
fee. Coulter shall make a final payment (the "Final Payment") to Nordion within
30 days of receipt of Nordion's invoice after installation/acceptance of the hot
cells by Nordion at Nordion's facility. The [...***...] shall be retained by
Nordion and applied to the Final Payment when due, provided, for greater
certainty, that in the event of cancellation of the Purchase Agreement by
Nordion prior to Final Payment due to the acts or omissions of Coulter, such sum
shall be deemed forfeited by Coulter.

     3. Without the prior written consent of Coulter, under no circumstances
shall Coulter's payment obligations through the Interim Payment immediately
following inspection/acceptance by Nordion at Von Gahlen's site in the aggregate
exceed [...***...] nor shall Coulter's total payment obligations under this
agreement, subject to the indemnity provisions contained herein, exceed the
Aggregate Purchase Price. Notwithstanding the immediately preceding sentence and
without regard to the limitations set out therein, in the event of cancellation
of the Purchase Agreement due to the acts or omissions of Coulter the
cancellation payment terms set forth in paragraphs 5 and 8 shall govern.


[*] Confidential Treatment Requested
<PAGE>   22
     4. Accompanying each of Nordion's invoices shall be a report issued by Von
Gahlen setting out work performed and the contract value of such work performed
during the relevant period. With respect to the Interim Payment following
inspection/acceptance at Von Gahlen's site and with respect to the Final
Payment, Nordion's invoice shall be accompanied by its written representation
that Nordion has paid the amounts due to Von Gahlen following Nordion's
inspection/acceptance, pursuant to the terms of the Purchase Agreement. Within
30 days of installation/acceptance of the hot cells at Nordion's facility,
Nordion shall provide to Coulter a report stipulating that the work performed by
Von Gahlen substantially conforms to specification (which may be amended from
time to time) as set out in the Purchase Agreement and that the hot cells may be
used for the purpose for which they are intended.

     5. In the event of termination by Nordion of the purchase of the hot cells
from Von Gahlen, due to the acts or omissions of Coulter, including but not
limited to Coulter's failure to pay any sums due Nordion or Coulter's
discontinuation of the project, Coulter shall indemnify and hold harmless
Nordion for all losses, costs, expenses (including reasonable attorneys fees)
and damages that Nordion may incur as a result of any claim, action or suit
instituted by Von Gahlen against Nordion. Such indemnification on the pan of
Coulter does not extend to claims arising out of Nordion's gross negligence,
recklessness, or intentional wrongdoing.

     6. In addition, Nordion shall stipulate in the Purchase Agreement with Von
Gahlen that if Von Gahlen shall fail to complete installation of the hot cells
within one week of the installation date specified in the Purchase Agreement,
for reason other than an act or omission of Nordion or force majeure, in
addition to any other remedies Nordion may have, a late charge by way of
liquidated damages shall be payable by Von Gahlen to Nordion in the amount of
[...***...] per week of the total purchase price of the hot cells, for
[...***...]. Any such amounts collected by Nordion, provided Coulter is not in
default under this agreement, shall be paid over to Coulter.

     7. Nordion and Coulter shall use good faith efforts to negotiate and enter
into an agreement to carry out the Facility Phase and manufacturing obligations
for clinical trials and commercial supply of radiolabelled B-1 antibody with
1-131 which negotiations, notwithstanding the provision of right of first
negotiation set out in the Agreement, shall be completed prior to April 30,
1996. In the event that such good faith negotiations and an agreement are not
concluded by April 30, 1996, or such other date as agreed between the parties,
Nordion shall cancel the Purchase Agreement without recourse by Coulter.

     8. Upon cancellation of the Purchase Agreement due to the acts or omissions
of Coulter or failure by the parties to reach the agreement contemplated in
paragraph 7, within 15 days after receipt of Nordion's final invoice, Coulter
shall pay to Nordion the remaining unpaid contract value owing by Nordion to Von
Gahlen to the date of cancellation, as determined under provisions of the
Purchase Agreement, plus Nordion's [...***...] administration fee to the extent
not previously paid. At Coulter's request provided to Nordion within 20 days of
the effective date of termination and at Coulter's expense, Nordion shall take
possession of and/or assign such work performed, to Coulter. All shipping costs
incurred by Nordion for delivery to Coulter shall be borne by Coulter.


[*] Confidential Treatment Requested
<PAGE>   23
     9. Prior to installation of the hot cells at Nordion's facility, so long as
all payments then due have been received by Nordion from Coulter, title in and
to the work performed by Von Gahlen (including hot cells, components or parts
thereof, as between Nordion and Coulter, shall belong to Coulter. For greater
certainty, work performed by Von Gahlen shall not include Nordion drawings or
other Nordion proprietary information provided to Von Gahlen.

This letter agreement shall be governed and construed in accordance with the
laws of the Province of Ontario, Canada, without reference to its conflict of
laws.

Yours truly,

COULTER PHARMACEUTICAL, INC.          We acknowledge and concur with the 
                                      foregoing this 23rd day of February, 1996.

By:                                   NORDION INTERNATIONAL INC.
   --------------------------



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


[*] Confidential Treatment Requested
<PAGE>   24
[COULTER PHARMACEUTICAL LOGO]


To:      Graham Duncan - MDS- Nordion

From:    M. Buckley

Subject: Authorization of Expenditure for Third Hot Cell in Primary Facility

Date:    9/30/97

cc:     D. Shock, K. Iverson, B. Harris and C. David - MDS-Nordion

This letter is authorization for the expenditure of approximately [...***...]
for the design, construction and installation of a third hot cell in the primary
facility to house the automated QC, inspection, labelling and packaging
operations. This price includes the following general categories:

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

Installation is targeted to commence [...***...] with shipment from Von Gahlen
-- Netherlands on or before [...***...].

The terms of payment would be as follows:

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

The hot cell will be inspected at the Netherlands facility by Nordion and CPI
personnel and accepted [...***...].

This should be sufficient to allow the work to being on this project. Please
keep me apprised of any development that will effect either price or meeting
project timelines.

Best regards,

Michael Buckley -- Manager Product Development


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<PAGE>   25
                                  EXHIBIT 1.20

                               MOVEABLE EQUIPMENT

        131 1 ANTI-B1 FACILITY
        AS OF DECEMBER 31, 1997
        MOVEABLE FIXED ASSETS



[...***...]







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<PAGE>   26
[...***...]





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<PAGE>   27
[...***...]






[*] Confidential Treatment Requested