Exhibit 10.28

                                  [Letterhead]

                                September 2, 2003

Bioenvision, Inc.
509 Madison Avenue, Suite 404
New York, New York  10022
Attn.:  David Luci

                 Re:    Letter Agreement For Co-Development Of An
                        Oral Clofarabine Formulation and First Amendment
                        to Co-Development Agreement dated March 12, 2001
                        between Bioenvision, Inc. and ILEX Products, Inc.
                        ("Agreement")
                        --------------------------------------------------

Dear Mr. Luci:


         Bioenvision,  Inc. ("Bioenvision") and ILEX Products, Inc. ("Ilex") are
desirous of  co-developing  an oral  formulation of Clofarabine.  The purpose of
this Letter  Agreement (the "Letter  Agreement") is to set forth the agreed upon
terms and conditions for co-developing  the oral Clofarabine.  As such terms and
conditions  may differ from those in the  Agreement,  this letter  shall also be
deemed to amend the relevant terms of the Agreement to the extent  applicable to
co-development  of  an  oral  Clofarabine  formulation.   Bioenvision  and  Ilex
understand and agree that except as expressly amended in this letter,  the terms
and  provisions  of the  Agreement  shall  remain in full  force  and  effect as
originally executed.

         Ilex and Bioenvision hereby agree that the following terms and
provisions shall apply to their co-development of an oral Clofarabine
formulation. Capitalized terms used, but not defined herein, shall have the
meanings ascribed thereto in the Agreement.
1. Co-Development of Oral Clofarabine

            (a)     Joint Development Plan. Notwithstanding Section 2 of the
                    Agreement, Ilex and Bioenvision shall jointly co-develop for
                    commercialization an oral Clofarabine formulation. Promptly
                    after execution of this Letter Agreement, the parties shall
                    discuss and agree upon a joint development plan for
                    co-development of an oral Clofarabine formulation, which
                    ILEX, on a day to day basis, shall manage, administer, and
                    coordinate, including, without limitation, assignment of
                    activities to be performed by the parties or subcontractors.
                    Contemplated activities include, without limitation,
                    analytical work to develop the oral formulation, and
                    performance of an oral bridging study. As of the date of
                    this Letter Agreement, ILEX has filed documents with the FDA
                    that are necessary to initiate human clinical trials of a
                    certain Clofarabine formulation, and based on such filing
                    expect to commence such clinical trials by September 30,
                    2003. The parties agree that if commencement of such
                    clinical trials is delayed by two (2) months, the parties
                    shall jointly manage, administer and coordinate all
                    subsequent activities for their co-development of oral
                    Clofarabine. The parties agree to work exclusively with each
                    other to co-develop oral Clofarabine and accordingly,
                    neither party shall engage in any activity for the
                    development of an oral Clofarabine formulation outside of
                    this Letter Agreement.

            (b)     Reporting Requirements. Each party shall provide to the
                    other party quarterly written progress reports describing
                    progress on its activities during the preceding quarter.
                    Such reports shall include all relevant data as well, as
                    identify and describe any Program Technology, as defined in
                    paragraph 3b, below.


<PAGE>


            (c)     Funding. Bioenvision and Ilex shall share equally all of the
                    costs and expenses incurred for the development of the oral
                    Clofarabine formulation. An initial agreed upon budget is
                    attached as Exhibit A hereto and made a part hereof. Such
                    initial budget, for Four Hundred Fifty-Five Thousand Three
                    Hundred Sixty United States Dollars ($455,360.00),
                    constitutes a good faith estimate of anticipated costs and
                    expenses, but is not a final budget. The parties acknowledge
                    and agree that it is reasonable to assume that there will be
                    additional costs not included in the initial budget, such as
                    for the conduct of stability studies and further
                    manufacturing campaigns. Any adjustments to the initial
                    budget shall be approved in advance in writing by both
                    parties.

                    It is expressly understood that except as to co-funding the
                    development of oral Clofarabine, all of the payment
                    provisions of the Agreement are unaffected by this Letter
                    Agreement.

            (d)     Payments and Invoicing. All costs and expenses for
                    performing the oral Clofarabine development activities shall
                    be incurred initially by Ilex. On a monthly basis, Ilex
                    shall invoice Bioenvision for fifty percent (50%) of the
                    preceding month's costs. Each such invoice shall contain
                    reasonable detail describing the nature and amount of the
                    incurred expenses. Bioenvision shall pay Ilex the amount of
                    each invoice within thirty (30) days after Bioenvision's
                    receipt of each such invoice. Payments shall be made in
                    United States dollars by wire transfer to an account
                    designated in writing to Bioenvision. Any disputes shall be
                    resolved in accordance with Section 22 (Dispute Resolution)
                    of the Agreement.

2.   Commercialization. Unless otherwise agreed in writing by the parties from
time to time, commercialization of oral Clofarabine in the Field shall be in
accordance with the Agreement.

3.   Intellectual Property Matters

            (a)     Licenses to Bioenvision. Ilex hereby grants to Bioenvision
                    an exclusive with the right to sublicense, worldwide,
                    license, except for the United States and Canada, under the
                    Ilex Technology (royalty-free, subject only to the terms and
                    conditions set forth in the Agreement for Improvements) to
                    make, have made, use, offer for sale, sell, and import oral
                    Clofarabine in the Field. During the Term of the license,
                    ILEX also hereby grants to Bioenvision the option to obtain
                    from ILEX a non-exclusive license under ILEX technology to
                    make, have made, use, offer for sale, and import Clofarabine
                    for any human health related use outside of the Field, which
                    license shall be royalty-free outside of the United States
                    and Canada, and royalty-bearing inside the United States and
                    Canada at a reasonable royalty to be agreed upon.

                    For purposes hereof, Ilex Technology means and includes Ilex
                    Patents and Ilex Know-How. Ilex Patents means the patents
                    and patent applications listed on Exhibit B hereto and made
                    a part hereof, and any continuation, continuation-in-part,
                    division, provisional or any substitute application, any
                    patent anywhere in the world issued with respect to any such
                    patent applications, any reissue, reexamination, renewal or
                    extension (including any supplemental patent certificate) of
                    any of the foregoing, and any confirmation patent or
                    registration patent or patent of addition based on any such
                    patent, and all foreign counterparts of any of the
                    foregoing, as well as all patent applications anywhere in
                    the world claiming, or entitled to claim, priority to any of
                    the foregoing and any patents issuing thereon, including
                    without limitation, reissues, reexaminations and extensions
                    thereof. Ilex Know-How means any information, method,
                    material, know-how, trade secret, data or copyright that is
                    necessary or useful for oral Clofarabine existing as of the
                    date of this Letter Agreement or is discovered or acquired
                    by Ilex during the term of this Letter Agreement but outside
                    the conduct of its activities with Bioenvision hereunder.


<PAGE>


            (b)     Ownership. Ilex and Bioenvision shall jointly own any
                    invention, information, know-how, trade secret, data and
                    copyright, made or discovered by or for either party or both
                    parties, and any and all patentable inventions made, in the
                    conduct of their co-development activities for oral
                    Clofarabine formulation development hereunder ("Program
                    Technology") and such Program Technology shall be considered
                    to be jointly owned Improvements under Section 11.2 of the
                    Co-Development Agreement. The parties agree to joint
                    ownership of Program Technology irrespective of
                    inventorship. The parties further agree that any
                    subcontractor arrangement under this Letter Agreement shall
                    contain a provision under which the subcontractor assigns
                    all right and title in any invention, discovery,
                    information, know-how, trade secret, data, copyright and
                    patent ("Subcontractor Technology") made by the
                    subcontractor to both Ilex and Bioenvision as joint owners,
                    or if assigned solely to ILEX or Bioenvision, the party
                    receiving assignment shall promptly assign to the other
                    Party an undivided one-half ownership interest in such
                    Subcontractor Technology.

                    With respect to Program Technology, Ilex agrees to assign
                    and does hereby assign to Bioenvision an undivided one-half
                    ownership interest in any Program Technology made solely by
                    Ilex or jointly with Bioenvision; Bioenvision agrees to
                    assign and does hereby assign to Ilex an undivided one-half
                    ownership interest in any Program Technology made solely by
                    Bioenvision or jointly with Ilex. Ilex and Bioenvision agree
                    to cooperate with each other in the preparation, prosecution
                    and/or maintenance, as applicable, of all patents and patent
                    applications within Program Technology, including, without
                    limitation, execution of any and all papers as are necessary
                    or helpful in ensuring joint ownership.

                    The parties represent and warrant to each other that their
                    respective employees have executed agreements requiring
                    assignment to such party of all inventions made during the
                    course of and as a result of their association with such
                    party.

            (c)     Licenses to Program Technology. Bioenvision hereby grants to
                    Ilex an exclusive, royalty-free license, with the right to
                    sublicense, under Bioenvision's share of Program Technology
                    to make, have made, use, offer for sate, sell and import
                    oral Clofarabine in the Field, in the United States and
                    Canada. Ilex hereby grants to Bioenvision an exclusive,
                    royalty-free license, with the right to sublicense, under
                    Ilex's share of Program Technology to make, have made, use,
                    offer for sale, sell and import oral Clofarabine in the
                    Field, worldwide, except in the United States and Canada.

            (d)     Patent Filing on Program Technology. The Party originating
                    and/or subcontracting for any given Program Technology shall
                    have the right to prepare, prosecute and maintain patent
                    applications and patents within Program Technology using
                    patent counsel of its choice, to which the other Party has
                    no reasonable objection. The parties shall consult and
                    cooperate with each other in good faith and adopt a
                    prosecution strategy by mutual agreement. The
                    non-prosecuting Party shall be provided with copies of all
                    documents related to the preparation, filing, prosecution
                    and maintenance of such patents and patent applications in
                    sufficient time to review such documents and comment
                    thereon, if desired by the non-prosecuting Party, prior to
                    filing, provided, however, that if the non-prosecuting Party
                    has not commented on such documents prior to the deadline
                    for filing a response with the relevant government patent
                    office, the prosecuting Party shall be free to respond
                    without consideration of Ilex's comments. The Parties agree
                    to sign, where appropriate, any necessary documents for the
                    preparation, filing, prosecution and maintenance of such
                    patents and patent applications. The costs, including
                    without limitation, reasonable attorney's fees, of
                    preparing, filing, prosecuting (including interferences and
                    oppositions) shall be shared equally by the parties.


<PAGE>


                    If either party elects not to file or continue to prosecute
                    or maintain any patent application or patent within Program
                    Technology, such party shall give written notice to the
                    other party within a reasonable time (i.e., with sufficient
                    time for the continuing party to take whatever action may be
                    necessary or desired) and such continuing party shall
                    thereafter have the right, at its sole discretion, to
                    prepare, file, prosecute and maintain such patent
                    application or patent in its own name and at its own
                    expense. In such case, the non-continuing party agrees to
                    assign and does hereby assign to the continuing party all
                    right, title and interest in and to any such patent
                    application or patent.

            (e)     Notices. Each party shall provide prompt written notice to
                    the other party of any Program Technology. In no event shall
                    such notice be later than thirty (30) days after any
                    invention within Program Technology is made.

            (f)     Enforcement. If either party becomes aware of any
                    infringement of patents within Program Technology, such
                    party shall notify the other party of the infringement in
                    writing and provide a summary of the relevant facts and
                    circumstances known to such party relating to such
                    infringement. Neither party shall notify a third party of
                    the infringement without first obtaining written consent of
                    the other party, which consent shall not be unreasonably
                    withheld or delayed. The parties shall cooperate in good
                    faith as to how to proceed against the alleged infringer. If
                    the parties agree to institute legal proceedings against
                    such infringer, unless otherwise agreed, the party
                    commercializing in the country where the infringement is
                    occurring shall have the right to prosecute, at its own
                    expense, and to recover, for its own account, any damages,
                    awards or settlements resulting therefrom. The
                    non-prosecuting party agrees that the other party may join
                    such non-prosecuting party as a party plaintiff in any such
                    suit, without expense to the non-prosecuting party. Neither
                    party shall settle nor compromise any such suit or enter
                    into any consent order for settlement or compromise that
                    adversely affects the relevant patent rights without the
                    prior written consent of the other party, which consent
                    shall not be unreasonably withheld or delayed.

                    The non-prosecuting party agrees to cooperate with the other
                    party, at such other party's request and expense, in
                    connection with the enforcement or defense of Ilex Patent or
                    patents within Program Technology. The granting of
                    sublicenses to any alleged infringer shall be in accordance
                    with the licenses granted herein.

4.   Termination.

            (a)     At Will. Either party may terminate its activities with
                    respect to co-development of oral Clofarabine upon sixty
                    (60) days written notice to the non-terminating party. The
                    licenses granted in Paragraphs 3a and 3c hereunder shall
                    survive such termination, and except for such licenses, the
                    provisions of the Agreement with respect to oral Clofarabine
                    shall remain in full force and effect.

            (b)     For Breach. If either party at any time defaults or
                    materially breaches any of the terms or provisions of this
                    Letter Agreement, and fails to remedy any such default or
                    material breach within sixty (60) days after receipt of
                    written notice thereof by the non-breaching party, this
                    Letter Agreement shall terminate effective as of the date of
                    such notice. Upon termination under this Paragraph 4b, the
                    breaching party hereby grants to the non-breaching party an
                    exclusive, worldwide, royalty-free license, with the right
                    to sublicense, to practice the Program Technology in the
                    Field.

5.   Conflict With Agreement. If any of the terms or provisions herein with
respect solely to oral Clofarabine conflict with any terms or provisions of the
Agreement, the terms and provisions of this Letter Agreement shall govern only
with respect to oral Clofarabine.


<PAGE>


               If the foregoing terms and conditions are acceptable to
Bioenvision, please indicate agreement by signing and dating the enclosed
duplicate original of this Letter Agreement and First Amendment to Agreement and
return one executed original to the undersigned, keeping one original for your
records.

                                       Very truly yours,
                                       Ilex Products, Inc.

                                       /s/ Al A. Jecminek
                                       ------------------

                                       Al A. Jecminek,
                                       Vice President, Licensing & Intellectual
                                       Property

Accepted and agreed on behalf of Bioenvision, Inc.

By:  /s/ David P. Luci
     --------------------------------------

Title: Director of Finance, General Counsel
       ------------------------------------