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                                                                    EXHIBIT 1.28

                          REGISTRATION RIGHTS AGREEMENT

      THIS AGREEMENT is made as of April 17, 2002;

BETWEEN:

            ALTAREX CORP., a corporation incorporated under the laws of Alberta
            (hereinafter referred to as the "Company")

                                     - and -

            UNITED THERAPEUTICS CORPORATION, a corporation incorporated under
            the laws of Delaware (hereinafter referred to as "United")

      WHEREAS:

      (a)   United wishes to purchase Purchased Securities from the Company;

      (b)   the Company has agreed to issue the Purchased Securities to United;
            and

      (c)   this Agreement is being entered into as a condition to the
            completion of the purchase of the Purchased Securities;

      NOW THEREFORE THIS AGREEMENT WITNESSES that in consideration of the
premises and the covenants and agreements herein contained, the parties hereto
agree as follows:

                           ARTICLE 1 - INTERPRETATION

1.1   DEFINITIONS

      In this Agreement, unless something in the subject matter or context is
inconsistent therewith:

      "affiliate" and "associate" have the respective meanings assigned thereto
      in the Securities Act (Ontario) as at the date hereof;

      "Agreement" means this agreement and all amendments made hereto by written
      agreement between the parties hereto;

      "business day" means a day other than a Saturday, Sunday or statutory or
      civic holiday in Toronto, Ontario;

      "Canadian Securities Laws" means the securities laws of any province or
      territory of Canada;

      "Common Shares" means the common shares of the Company;
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                                      -2-

      "Convertible Debentures" means, collectively, the First Convertible
      Debenture and Second Convertible Debenture;

      "Convertible Debenture Shares" means, collectively, the 100,000 Common
      Shares issued upon conversion of the First Convertible Debenture and the
      883,380 Common Shares issued upon the conversion of US $441,690 of the
      Second Convertible Debenture;

      "Expenses" means all expenses incident to the Company's performance of or
      compliance with this Agreement (including, without limitation, all filing
      fees, expenses of compliance with securities and "blue sky" laws, printing
      expenses and reasonable fees and disbursements of the Company's counsel,
      independent accountants, underwriters (but excluding underwriting
      discounts and selling commissions) and other persons retained by the
      Company) other than the Company's internal expenses (including, without
      limitation, all salaries and expenses of its officers and employees
      performing legal or accounting duties) which shall continue to be paid by
      the Company in any event;

      "First Convertible Debenture" means the First Convertible Debenture dated
      as of the date hereof between the Company and United;

      "Piggyback Qualification" has the meaning ascribed thereto in Section 3.1;

      "Prospectus" means a prospectus filed by the Company with a Canadian
      commission for a public offering and sale of securities of the Company;

      "Purchased Securities" means collectively the 4,900,000 Common Shares
      subscribed for by United pursuant to the Subscription and Debenture
      Purchase Agreement and the Convertible Debenture Shares, if any;

      "Qualifiable Shares" means (i) the Convertible Debenture Shares issuable
      upon conversion of the Convertible Debentures, as applicable, and (ii) any
      other Common Shares issuable in respect of such shares (because of share
      splits, share dividends, reclassifications, recapitalizations or similar
      events);

      "Second Convertible Debenture" means the Second Debenture (as such term is
      defined in the Subscription and Debenture Purchase Agreement);

      "Securities Acts" means the securities legislation of each of the
      provinces of Canada and all regulations, policy statements, orders, rules,
      rulings, communiques and interpretation notes issued thereunder or in
      relation thereto, as amended, re-enacted or replaced from time to time;
      and

      "Subscription and Debenture Purchase Agreement" the subscription and
      debenture purchase agreement dated as of the date hereof between the
      Company and United.

1.2   HEADINGS

      The division of this Agreement into Articles and Sections and the
insertion of headings are for convenience of reference only and shall not affect
the construction or interpretation of
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                                      -3-

this Agreement. The terms "this Agreement", "hereof", "hereunder" and similar
expressions refer to this Agreement and not to any particular Article, Section
or other portion hereof and include any agreement supplemental hereto. Unless
something in the subject matter or context is inconsistent therewith, references
herein to Articles and Sections are to Articles and Sections of this Agreement.

1.3   EXTENDED MEANINGS

      In this Agreement words importing the singular number only shall include
the plural and vice versa, words importing the masculine gender shall include
the feminine and neuter genders and vice versa and words importing persons shall
include individuals, partnerships, associations, trusts, unincorporated
organizations and corporations.

                       ARTICLE 2 - QUALIFICATION REQUEST

2.1   REQUEST FOR QUALIFICATION

      Subject to Section 2.2, United may at any time after the issue and sale of
the applicable Convertible Debenture, by written notice to the Company, request
(a "Qualification Request") the Company to (i) qualify for distribution
Qualifiable Shares issuable upon conversion of such Convertible Debenture under
the Securities Acts, or (ii) in the event the Company is offering Common Shares
to the public by way of a prospectus, to sell on a secondary basis the Purchased
Securities, such notice to specify the number of Common Shares requested to be
qualified or sold, as the case may be, and the intended method of disposition of
such Common Shares.

      Promptly upon receipt of a Qualification Request, the Company shall use
its best efforts to effect, as expeditiously as possible, the qualification for
distribution or resale under the Securities Acts of the Common Shares that the
Company has been requested to qualify by United to the extent necessary to
permit the disposition of the Common Shares to be qualified in accordance with
the intended methods of distribution.

2.2   RESTRICTIONS ON QUALIFICATION REQUEST

      The Company shall not be obligated to effect:

      (a)   any Qualification Request that requires the Company to qualify the
            Qualifiable Shares in any jurisdiction outside of Canada; or

      (b)   any Qualification Request unless United has requested either (i) the
            qualification for distribution of not less than 100% of the total
            number of Qualifiable Shares owned by United on the date of any such
            Qualification Request or (ii) the sale on a secondary basis of not
            less than 100% of the total number of Purchased Securities owned by
            United on the date of such Qualification Request.

2.3   QUALIFICATION REQUEST EXPENSES

      The Company will pay all Expenses relating to any Qualification Request:
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                                      -4-

      (a)   provided that with respect to the sale of Common Shares pursuant to
            a public offering in Canada, United shall pay its proportionate
            share (based on the number of Common Shares that United is selling
            of the total number of Common Shares being qualified for sale) of
            the Expenses to the extent required to be paid by United under
            Securities Acts and for greater certainty the expenses for which
            United would be responsible hereunder are limited to its
            proportionate proportion of the Canadian Securities Commission's
            regulated fees;

      (b)   provided, however, that if a qualification under Section 2.1 is
            withdrawn at the request of United (other than a withdrawal within
            10 days of the United becoming aware of, and as a result of,
            information concerning the business or financial condition of the
            Company which is made known to United after the date on which such
            qualification was requested), United shall pay its proportionate
            share (based on the number of Common Shares that United is selling
            of the total number of Common Shares being qualified for sale) of
            the Expenses to the extent required to be paid by United under
            Securities Laws.

2.4   UNDERWRITTEN OFFERINGS

      If any Qualification Request is an underwritten offering:

      (a)   United shall provide notice of this fact as part of its
            Qualification Registration;

      (b)   the Company, even in the case of an offering (a "Secondary
            Distribution") including United as a secondary offeror, shall have
            the right to select the investment banker(s) and manager(s) to
            underwrite any offering contemplated by a Qualification Request,
            subject, in the case of a Secondary Distribution, to the approval of
            United, such approval not to be unreasonably withheld; and

      (c)   United shall bear or pay its proportionate share of the underwriting
            discounts and selling commissions determined on the basis of the
            proportion that the number of United's Common Shares included in the
            Qualification Request bears to the total number of Common Shares
            qualified for distribution.

                      ARTICLE 3 - PIGGYBACK QUALIFICATION

3.1   RIGHT TO PIGGYBACK

      If the Company proposes to (i) qualify for distribution any Common Shares
under any of the Securities Acts (other than in connection with any securities
exchange offer, dividend reinvestment plan or stock option or other employee
benefit plan) or (ii) sell by way of prospectus commons shares out of treasury
("Offering"), the Company shall give prompt written notice to United of its
intention to do so and, subject to Section 3.4, shall include in such
qualification all Qualifiable Shares and shall include as part of such offering
on a secondary basis the Purchased Securities (a "Piggyback Qualification") with
respect to which the Company has received from United a written request for
inclusion therein within ten Business Days after
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                                      -5-

the date of such notice. The Company has the right to postpone or withdraw any
qualification in its sole discretion.

3.2   PIGGYBACK EXPENSES

      The Company will pay all Expenses relating to any Piggyback Qualification.

3.3   UNDERWRITTEN OFFERINGS

      If any Piggyback Qualification is an underwritten offering:

      (a)   the Company shall be entitled to select the investment banker(s) and
            manager(s) to underwrite the offering contemplated thereby;

      (b)   United shall enter into an underwriting agreement in customary form
            with the underwriter or underwriters selected for the underwriting
            by the Company; and

      (c)   United shall bear or pay its proportionate share of the underwriting
            discounts and selling commissions determined on the basis of the
            proportion that the number of United's Common Shares included in the
            Piggyback Qualification bears to the total number of Common Shares
            qualified for distribution.

3.4   PRIORITY ON PRIMARY QUALIFICATION

      If any Piggyback Qualification includes an underwritten offering and the
managing underwriters advise the Company that the number of Common Shares that
the Company and United have requested to be included in such distribution
pursuant to Section 3.1 exceeds the number (in this Section 3.4, the "Maximum
Secondary Offering Size") which can be sold on a secondary basis in an orderly
manner in such offering within a price range acceptable to the Company (the
"Assessment"), the Company shall include in such distribution as many of the
Common Shares proposed to be sold by United on a secondary basis as will not
cause the offering to exceed the Maximum Secondary Offering Size which can be
sold on a secondary basis in an orderly manner in such offering within a price
range acceptable to the Company.

If United disagrees with the Assessment, the matter shall be decided by an
arbitrator mutually agreeable to the parties within ten calendar days whose
decision shall be final and binding upon the parties.

Subject to the foregoing, the Company shall use its reasonable efforts to
arrange for the underwriters to include the Piggyback Qualification as part of
the Common Shares to be distributed by or through such underwriters.
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                                      -6-

                      ARTICLE 4 - QUALIFICATION PROCEDURES

4.1   OBLIGATIONS OF THE COMPANY

      Whenever the Company receives a request for a Qualification Request or a
Piggyback Qualification, the Company shall use its best efforts to effect such
qualification for distribution and pursuant thereto the Company shall, as
expeditiously as possible, and to the extent necessary by virtue of the
requirements of the jurisdictions in which such qualification is to be effected:

      (a)   promptly prepare and file a preliminary prospectus in the relevant
            jurisdictions and such other related documents as may be necessary
            or appropriate relating to the proposed distribution or
            qualification and shall, as soon as possible after any comments of
            the relevant regulatory authorities have been satisfied with respect
            thereto, prepare and file under the Securities Acts a prospectus
            (provided that before filing a preliminary prospectus, prospectus or
            any amendments or supplements thereto, the Company shall furnish to
            United copies of such documents for their review) and obtain
            receipts therefor and shall take all other steps and proceedings
            that may be necessary in order to qualify the Common Shares for
            distribution under such Securities Acts by registrants who comply
            with the relevant provisions of such Securities Acts;

      (b)   prepare and file with the relevant regulatory authorities such
            amendments and supplements to such preliminary prospectus,
            prospectus as may be necessary to comply with the provisions of the
            Securities Acts with respect to the distribution of all Common
            Shares covered thereby until all of such Common Shares have been
            distributed in accordance with the intended method or methods of
            disposition;

      (c)   furnish to United such number of copies of such preliminary
            prospectus, prospectus and any amendment and supplement thereto and
            such other relevant documents as such shareholder may reasonably
            request in order to facilitate the disposition of its Common Shares;

      (d)   furnish to United an opinion of counsel for the Company in a form
            that is customary at such time addressed to United and the
            underwriters and dated the effective date of such (final)
            prospectus;

      (e)   notify, as soon as possible, United of any circumstance or the
            happening of any event as a result of which the preliminary
            prospectus, prospectus as then in effect would include an untrue
            statement of material fact or would omit any fact that is required
            to be stated or that is necessary to make any statement therein not
            misleading, and at the request of United prepare and furnish to
            United a reasonable number of commercial copies of a supplement to
            or an amendment of the preliminary prospectus, prospectus as may be
            necessary so that, as thereafter delivered to the purchasers of such
            Common Shares, such document shall not include an untrue statement
            of material fact or omit to state any fact that is
<PAGE>
                                      -7-

            required to be stated or that is necessary to make any statement
            therein not misleading;

      (f)   otherwise comply with all applicable Securities Acts during the
            course of the distribution;

      (g)   enter into such customary agreements, including underwriting
            agreements, containing such representations and warranties by the
            Company and such other terms and provisions as are customary therein
            including, without limitation, rights of indemnity and contribution;

      (h)   in the event of the issuance of any order or ruling suspending the
            effectiveness of a prospectus receipt or of any order suspending or
            preventing the use of any prospectus or suspending the qualification
            of any Common Shares qualified by such prospectus for sale in any
            jurisdiction, use its best efforts promptly to obtain the withdrawal
            of such order or ruling;

      (i)   otherwise use its reasonable efforts to facilitate the offering of
            the Common Shares including, without limitation, causing management
            of the Company to participate in any road shows, sales meetings or
            other activities arranged by the underwriters;

      (j)   if the Company has delivered a Prospectus to United and after having
            done so the Prospectus is amended to comply with the requirements of
            the Securities Act, the Company shall promptly notify United and, if
            requested, United shall immediately cease making offers of
            Qualifiable Shares or Purchased Securities and return all
            Prospectuses to the Company. The Company shall promptly provide
            United with revised Prospectuses and, following receipt of the
            revised Prospectuses or United shall be free to resume making offers
            of the Qualifiable Shares or Purchased Securities; and

      (k)   in the event that, in the judgement of the Company, it is advisable
            to suspend use of a Prospectus due to pending material developments
            or other events that have not yet been publicly disclosed and as to
            which the Company believes public disclosure would be detrimental to
            the Company, the Company shall notify United to such effect, and,
            upon receipt of such notice, United shall immediately discontinue
            any sales of Qualifiable Shares or Purchased Securities pursuant to
            such Prospectus until United has received copies of a supplemented
            or amended Prospectus or until United is advised in writing by the
            Company that the then current Prospectus may be used and has
            received copies of any additional or supplemental filings that are
            incorporated or deemed incorporated by reference in such Prospectus.
            Notwithstanding anything to the contrary herein, the Company shall
            not exercise its rights under this Section 4.1(k) to suspend sales
            of Qualifiable Shares or Purchased Securities for a number of days
            in excess of 120 days in any 365-day period.
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                                      -8-

4.2   OBLIGATIONS OF UNITED

(1)   If in the reasonable opinion of counsel to the Company it is necessary or
appropriate in order to comply with any applicable Securities Acts, the
obligations of the Company under Articles 2 and 3 shall be conditional upon
United and any underwriter participating in such offering executing and
delivering to the Company an appropriate agreement, in a form reasonably
satisfactory to counsel for the Company, that such person shall comply with all
prospectus delivery requirements of all relevant Securities Acts and with
anti-stabilization, manipulation and similar provisions of the relevant
Securities Acts and shall furnish to the Company information about sales made in
such offering.

(2)   United shall not (until further notice) effect sales of any Common Shares
qualified by or included in a prospectus or deliver any prospectus in respect of
such sale after notification by the Company of any order or ruling suspending
the effectiveness of the receipt for such prospectus.

4.3   PREPARATION; REASONABLE INVESTIGATION

      In connection with the preparation and filing of any preliminary
prospectus, prospectus, or similar document pursuant to a qualification request,
United and the underwriters, if any, and its counsel, auditors and other
representatives, shall be given the opportunity to participate in the
preparation of such documents and each amendment thereof or supplement thereto
and there shall be inserted therein such material as is required under the
applicable Securities Acts or which in the reasonable judgment of United and its
counsel should be included. United and the underwriters shall also be given such
access to the books and records of the Company and such opportunities to discuss
the business of the Company with its officers and auditors as shall be necessary
in their respective opinions or in the opinion of their respective counsel, and
to conduct all due diligence which any of the foregoing persons may reasonably
require in order to conduct a reasonable investigation for purposes of
establishing a due diligence defence as contemplated by the Securities Acts and
in order to enable United and underwriters to execute the certificates required
to be executed by them pursuant to the Securities Acts for inclusion in each
such document.

4.4   EXPENSES

(1)   To the extent permitted under applicable Securities Acts, all Expenses
shall be borne by the Company. The Company shall pay its internal expenses
(including, without limitation, all salaries and expenses of its officers and
employees performing legal or accounting duties).

(2)   To the extent Expenses are not permitted by law to be paid by the Company,
United shall pay those Expenses allocable to the distribution or qualification
of the Qualified Shares or Purchased Securities owned by it as provided herein.
<PAGE>
                                      -9-

             ARTICLE 5 - ARTICLE - INDEMNIFICATION AND CONTRIBUTION

5.1   INDEMNIFICATION

(1)   The Company shall indemnify and hold United, each of its officers,
directors, legal counsel and accountants and each person which owns or controls
United from and against any and all expenses, claims, losses, damages and
liabilities (or actions, proceedings or settlements in respect thereof)
including costs of investigation and reasonable fees and expenses of legal
counsel arising out of or based upon:

      (a)   any liability pursuant to a provision of (including any indemnity
            in) any underwriting agreement, purchase agreement or other document
            relating to any Qualification Request or Piggyback Qualification and
            directly or indirectly arising out of or based upon any
            misrepresentation, breach of warranty, untrue statement or alleged
            untrue statement, whether of a material fact or otherwise, or any
            omission or alleged omission to state a fact, material or not,
            required to be stated or necessary to make a statement therein not
            misleading, in light of the circumstances in which it is made; or

      (b)   any misrepresentation or alleged misrepresentation, breach of
            warranty or untrue statement or alleged untrue statement, whether of
            a material fact or otherwise contained in any preliminary
            prospectus, prospectus registration statement or similar document
            (including any amendment or supplement thereto) relating to any
            Qualification Request or Piggyback Qualification, or in any
            underwriting agreement, purchase agreement or other document
            relating thereto, or arising out of or based upon any omission or
            alleged omission to state in any such preliminary prospectus,
            prospectus, registration statement or similar document (including
            any amendment or supplement thereto), or any such underwriting
            agreement, purchase agreement or other document, a fact, material or
            not, required to be stated therein or necessary to make a statement
            therein not misleading, in light of the circumstances in which it is
            made.

(2)   In the event of any qualification of any of the Qualifiable Shares or
Purchased Securities under the Securities Act pursuant to this Agreement, United
will indemnify and hold harmless the Company, each of its directors and officers
and each underwriter (if any) and each person, if any, who controls the Company
or any such underwriter within the meaning of the Securities Act, against any
losses, claims, damages or liabilities, joint or several, to which the Company,
such directors and officers, underwriter or controlling person may become
subject under Securities Act, or otherwise, insofar as such losses, claims,
damages or liabilities (or actions in respect thereof) arise out of or are based
upon any untrue statement or alleged untrue statement of a material fact
contained in any preliminary or final Prospectus under which such Qualifiable
Shares or Purchased Securities were qualified, or any amendment or supplement to
the Prospectus, or arise out of or are based upon any omission or alleged
omission to state a material fact required to be stated therein or necessary to
make the statements therein not misleading, if the statement or omission was
made in reliance upon and in conformity with information relating to United
furnished in writing to the Company by or on behalf of United specifically for
use in connection with the preparation of such Prospectus, amendment or
supplement; provided,
<PAGE>
                                      -10-

however, that the obligations of United hereunder shall be limited to an amount
equal to the net proceeds to United of Qualifiable Shares or Purchased
Securities sold in connection with such qualification.

(3)   Each party entitled to indemnification under this Section 5.1 (the
"Indemnified Party") shall give notice to the Company or United, as the case may
be, promptly after such Indemnified Party has actual knowledge of any claim as
to which indemnity may be sought, and shall permit the Company or United, as the
case may be, to assume the defence of any such claim or any litigation resulting
therefrom, provided that counsel for the Company or United, as the case may be,
who shall conduct the defence of such claim or any litigation resulting
therefrom, shall be approved by the Indemnified Party, and the Indemnified Party
may participate in such defence, and provided further that the failure of any
Indemnified Party to give notice as provided herein shall not relieve the
Company of its obligations under this Section 5.1. The Company or United, as the
case may be, shall not, in the defence of any such claim or litigation, except
with the consent of each Indemnified Party, consent to entry of any judgment or
enter into any settlement which does not include as an unconditional term
thereof the giving by the claimant or plaintiff to such Indemnified Party of a
release from all liability in respect to such claim or litigation.

5.2   CONTRIBUTION

      If the indemnification provided for in Section 5.1 is unavailable or
insufficient to hold harmless the Indemnified Parties in respect of any
expenses, claims, losses, damages or liabilities or actions in respect thereof,
then the Company or United, as the case may be, shall in lieu of indemnifying
the Indemnified Parties contribute to the amount paid or payable by the
Indemnified Parties as a result of such expenses, claims, losses, damages,
liabilities or actions in such proportion as is appropriate to reflect the
relative fault of the parties thereto, in connection with the statements or
omissions which resulted in such expenses, claims, losses, damages, liabilities
or actions as well as any other relevant equitable considerations; provided,
however, that no person guilty of fraudulent misrepresentation within the
meaning of Section 11(f) of the United States Securities Act of 1933, as
amended) shall be entitled to contribution from any person who was not guilty of
such fraudulent misrepresentation. The parties hereto agree that it would not be
just and equitable if contribution pursuant to this Section 5.2 were determined
by pro rata allocation or by any other method of allocation which did not take
account of the equitable considerations referred to above in this Section 5.2.
The amount paid or payable by the Indemnified Parties as a result of the
expenses, claims, losses, damages, liabilities or actions in respect thereof
referred to above in this Section 5.2 shall be deemed to include any legal or
other expenses reasonably incurred by the Indemnified Parties in connection with
investigating or defending any such action or claim.

                              ARTICLE 6 - GENERAL

6.1   INJUNCTIVE RELIEF

      The Company acknowledges and agrees that damages would be inadequate to
compensate for the breach of any of its obligations contained in this Agreement
and that other parties hereto would be seriously and irreparably injured if any
provision of this Agreement is
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                                      -11-

not performed by it in accordance with the specific terms and conditions of this
Agreement. Accordingly, the Company agrees, without prejudice to any additional
or alternative remedies the other parties hereto may have hereunder, that such
other parties shall be entitled:

      (a)   to an injunction to prevent any breach of this Agreement by the
            Company;

      (b)   to enforce specifically the terms and provisions hereof and any
            obligation in favour of such other parties, or any of them,
            contained in this Agreement; and

      (c)   to declaratory relief or injunctive relief in respect of anything
            done in breach of an obligation in favour of such other parties, or
            any of them, contained in this Agreement.

6.2   TERMINATION

      This Agreement shall terminate (i) in the case of the Common Shares
constituting a portion of the Purchased Securities, four months from the date
hereof, and (ii) in the case of the Qualifiable Shares, four months from the
date of issue and sale of the Convertible Debentures.

6.3   FURTHER ASSURANCES

      Each of the parties hereto shall from time to time execute and deliver all
such further documents and instruments and do all acts and things as the other
parties may reasonably require to effectively carry out or better evidence or
perfect the full intent and meaning of this Agreement.

6.4   TIME OF THE ESSENCE

      Time shall be of the essence of this Agreement.

6.5   BENEFIT OF THE AGREEMENT

      This Agreement shall enure to the benefit of and be binding upon the
respective successors and permitted assigns of the parties hereto.

6.6   ENTIRE AGREEMENT

      This Agreement, the Subscription and Debenture Purchase Agreement and the
Convertible Debenture constitutes the entire agreement between the parties
hereto with respect to the subject matter hereof and cancels and supersedes any
other understandings and agreements between the parties hereto with respect
thereto. There are no representations, warranties, terms, conditions,
undertakings or collateral agreements, express, implied or statutory, between
the parties other than as expressly set forth in this Agreement.

6.7   AMENDMENTS AND WAIVER

      No modification of or amendment to this Agreement shall be valid or
binding unless set forth in writing and duly executed by the parties hereto and
no waiver of any breach of any term
<PAGE>
                                      -12-

or provision of this Agreement shall be effective or binding unless made in
writing and signed by the party purporting to give the same and, unless
otherwise provided, shall be limited to the specific breach waived.

6.8   ASSIGNMENT

      This Agreement may not be assigned by any party hereto without the written
consent of the other parties hereto.

6.9   NOTICES

      Any demand, notice or other communication to be given in connection with
this Agreement shall be given in writing and shall be given by personal delivery
or by facsimile transmission addressed to the recipient as follows:

      (a)   to the Company:

            AltaRex Corp.
            610 Lincoln Street
            Waltham, Massachusetts 02451

            Telecopier: 781-466-8740
            Attention:  Edward Fitzgerald

      (b)   to United:

            United Therapeutics Corporation
            1735 Connecticut Avenue, N.W
            Washington, D.C.  20009

            Telecopier: (202) 483-7000
            Attention:  Paul A. Mahon, General Counsel

or to such other address, individual or facsimile number as may be designated by
notice given by any party to the others. Any demand, notice or other
communication given by personal delivery shall be conclusively deemed to have
been given on the day of actual delivery thereof and, if given by facsimile
transmission, on the day of transmittal thereof.

6.10  GOVERNING LAW

      This Agreement shall be governed by and construed in accordance with the
laws of the Province of Ontario and the laws of Canada applicable therein.

6.11  ATTORNMENT

      In relation to any legal proceedings or action to enforce this Agreement,
the courts of the Province of Ontario shall have jurisdiction to entertain any
action arising under this Agreement.
<PAGE>
                                      -13-

Each of the parties hereto hereby irrevocably attorns to the jurisdiction of the
courts of the Province of Ontario and waives any objections to proceedings in
such courts on the grounds of venue or on the grounds that the proceedings have
been brought in an inconvenient forum. This Section 6.10 shall not affect the
right of any party to take proceedings in any other jurisdiction nor shall the
taking of proceedings in any jurisdiction preclude any party from taking
proceedings in any other jurisdiction.

6.12  COUNTERPARTS

      This Agreement may be executed in any number of counterparts, each of
which shall be deemed to be an original and all of which shall constitute one
and the same agreement.

      IN WITNESS WHEREOF the parties have executed this Agreement.

                                    ALTAREX CORP.

                                    Per:  /s/ Richard E. Bagley
                                        --------------------------------

                                    Per:  President and Chief Executive Officer



                                    UNITED THERAPEUTICS CORPORATION

                                    Per:  /s/ Paul Mahon
                                        --------------------------------

                                    Per:  SVP and General Counsel
                                        --------------------------------