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                                                                   EXHIBIT 10.55


                            NEXELL THERAPEUTICS INC.
                      NON-INCENTIVE STOCK OPTION AGREEMENT

                    ---------------------------------------

To:       Richard L. Casey

          We are pleased to notify you that by the determination of the Stock
Option Plan Committee (herein called the "Committee") a non-incentive stock
option to purchase 50,000 shares of the Common Stock of Nexell Therapeutics Inc.
(herein called the "Company") at a price of $1.375 per share has this 9th day of
November, 1999 been granted to you under the Company's 1997 Incentive and Non-
Incentive Stock Option Plan (herein called the "Plan"). This option may be
exercised only upon the terms and conditions set forth below.

          1.  Purpose of Option.
              -----------------

          The purpose of the Plan under which this non-incentive stock option
has been granted is to further the growth and development of the Company and its
subsidiaries by encouraging key employees, directors, consultants, agents,
independent contractors and other persons who contribute and are expected to
contribute materially to the Company's success to obtain a proprietary interest
in the Company through the ownership of stock, thereby providing such persons
with an added incentive to promote the best interests of the Company, and
affording the Company a means of attracting to its service persons of
outstanding ability.

          2.  Acceptance of Option Agreement.
              ------------------------------

          Your execution of this non-incentive stock option agreement will
indicate your acceptance of and your willingness to be bound by its terms; it
imposes no obligation upon you to purchase any of the shares subject to this
option. Your obligation to purchase shares can arise only upon your exercise of
the option in the manner set forth in paragraph 4 hereof.

          3.  When Option May Be Exercised.
              ----------------------------

          The option granted you hereunder shall be exercisable as follows:

              (i)   From and after November 9, 2000, options to purchase 12,500
                    shares shall become exercisable;

              (ii)  From and after November 9, 2001, options to purchase 25,000
                    shares (i.e., an additional 12,500 shares) shall become
                    exercisable;

              (iii) From and after November 9, 2002 options to purchase 37,500
                    shares (i.e., an additional 12,500 shares) shall become
                    exercisable; and

              (iv)  From and after November 9, 2003, options to purchase all
                    shares subject to this non-incentive stock option agreement
                    shall become exercisable.

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          This option may not be exercised for less than ten shares at any one
time (or the remaining shares then purchasable if less than ten) and expires at
the end of ten years from the date of grant whether or not it has been duly
exercised (hereinafter, the "Option Expiration Date"), unless sooner terminated
as provided in paragraphs 5, 6 or 7 hereof.

          4.  How Option May Be Exercised.
              ---------------------------

          This option is exercisable by a written notice signed by you and
delivered to the Company at its executive offices, signifying your election to
exercise the option. The notice must state the number of shares of Common Stock
as to which your option is being exercised, must contain a statement by you (in
a form acceptable to the Company) that such shares are being acquired by you for
investment and not with a view to their distribution or resale (unless a
Registration Statement covering the shares purchased has been declared effective
by the Securities and Exchange Commission) and must be accompanied by cash or a
check to the order of the Company for the full purchase price of the shares
being purchased, plus such amount, if any, as is required for withholding taxes.
Notwithstanding the foregoing, this option may also be exercised pursuant to the
following "cashless exercise" provision.

          In lieu of paying for the shares purchasable under this option by cash
or check, you may (i) deliver previously owned shares of Common Stock with a
fair market value equal to the full purchase price of the shares being purchased
under this option, or (ii) request that the Company withhold shares of Common
Stock issuable upon exercise of this option with a fair market value equal to
the full purchase price of the shares being purchased under this option (thereby
reducing the number of shares issuable upon exercise of this option). For
purposes of this option, unless the Committee determines otherwise, the "fair
market value" of a share of Common Stock as of a certain date shall be the
closing sale price of the Common Stock on The Nasdaq Stock Market or, if the
Common Stock is not then traded on The Nasdaq Stock Market, such national
securities exchange on which the Common Stock is then traded, on the trading
date immediately preceding the date fair market value is being determined. The
Committee may make such other determination of fair market value, based on other
factors, as it shall deem appropriate.

          If notice of the exercise of this option is given by a person or
persons other than you, the Company may require, as a condition to the exercise
of this option, the submission to the Company of appropriate proof of the right
of such person or persons to exercise this option.

          Certificates for shares of the Common Stock so purchased will be
issued as soon as practicable. The Company, however, shall not be required to
issue or deliver a certificate for any shares until it has complied with all
requirements of the Securities Act of 1933, the Securities Exchange Act of 1934,
any stock exchange on which the Company's Common Stock may then be listed and
all applicable state laws in connection with the issuance or sale of such shares
or the listing of such shares on said exchange. Until the issuance of the
certificate for such shares, you or such other person as may be entitled to
exercise this option shall have none of the rights of a stockholder with respect
to shares subject to this option.

          The Company shall have the right to require you, or such other person
as may be permitted to exercise this option, to remit to the Company an amount
sufficient to satisfy federal,

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state and local withholding tax requirements prior to the delivery of any
certificate or certificates for shares of Common Stock issuable upon exercise of
this option.

          5.  Termination of Employment or Engagement.
              ---------------------------------------

          If your employment with the Company (or a subsidiary thereof) is
terminated for any reason other than by death or disability, or if you are not
an employee of the Company and your engagement by the Company (or a subsidiary)
is terminated for any reason, you may exercise that portion of this option which
was exercisable by you at the date of such termination, provided, however, that
such exercise occurs prior to the Option Expiration Date.

          6.  Disability.
              ----------

          If your employment with the Company (or a subsidiary thereof) is
terminated by reason of your disability, you may exercise, within twelve months
from the date of such termination, that portion of this option which was
exercisable by you at the date of such termination, provided, however, that such
exercise occurs prior to the Option Expiration Date.

          7.  Death.
              -----

          If you die while employed by the Company (or a subsidiary thereof) or
within six months after termination of your employment due to disability, that
portion of this option which was exercisable by you at the date of your death
may be exercised by your legatee or legatees under your Will, or by your
personal representatives or distributees, within twelve months from the date of
your death, but in no event after the Option Expiration Date.

          8.  Non-Transferability of Option.
              -----------------------------

          This option shall not be transferable except by Will or the laws of
descent and distribution, and may be exercised during your lifetime only by you.

          9.  Adjustments upon Changes in Capitalization.
              ------------------------------------------

          If at any time after the date of grant of this option, the Company
shall, by stock dividend, split-up, combination, reclassification or exchange,
or through merger or consolidation, or otherwise, change its shares of Common
Stock into a different number or kind or class of shares or other securities or
property, then the number of shares covered by this option and the price of each
such share shall be proportionately adjusted for any such change by the
Committee, whose determination shall be conclusive.

          10.  Acceleration of Exercisability Upon Change in Control.
               -----------------------------------------------------

          Upon the occurrence of a "change in control" of the Company (as
defined below), this option shall become immediately fully exercisable. For
purposes of this option, a "change in control" of the Company shall mean (i) the
acquisition at any time by a "person" or "group" (as such terms are used in
Sections 13(d) and 14(d)(2) of the Exchange Act of beneficial ownership (as
defined in Rule 13d-3 under the Exchange Act), directly or indirectly, of
securities

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representing 50% or more of the combined voting power in the election of
directors of the then outstanding securities of the Company or any successor of
the Company; (ii) the termination of service of directors, for any reason other
than death, disability or retirement from the Board of Directors, during any
period of two consecutive years or less, of individuals who at the beginning of
such period constituted a majority of the Board of Directors, unless the
election of or nomination for election of each new director during such period
was approved by a vote of at least two-thirds of the directors still in office
who were directors at the beginning of the period; (iii) approval by the
stockholders of the Company of any merger, consolidation, or statutory share
exchange as a result of which the Common Stock shall be changed, converted or
exchanged (other than a merger, consolidation or share exchange with a wholly-
owned Subsidiary) or liquidation of the Company or any sale or disposition of
80% or more of the assets or earning power of the Company; or (iv) approval by
the stockholders of the Company of any merger, consolidation, or statutory share
exchange to which the Company is a party as a result of which the persons who
were stockholders immediately prior to the effective date of the merger,
consolidation or share exchange shall have beneficial ownership of less than 50%
of the combined voting power in the election of directors of the surviving
corporation; provided, however, that no change in control shall be deemed to
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have occurred if, prior to such time as a change in control would otherwise be
deemed to have occurred, the Company's Board of Directors deems otherwise.

          11.  Subject to Terms of the Plan.
               ----------------------------

          This non-incentive stock option agreement shall be subject in all
respects to the terms and conditions of the Plan and in the event of any
question or controversy relating to the terms of the Plan, the decision of the
Committee shall be conclusive.

          12.  Tax Status.
               ----------

          This option does not qualify as an "incentive stock option" under the
provisions of Section 422A of the Internal Revenue Code of 1986, as amended, and
the income tax implications of your receipt of a non-incentive stock option and
your exercise of such an option should be discussed with your tax counsel.

                                    Sincerely yours,

                                    NEXELL THERAPEUTICS INC.

                                    By:  /s/ William A. Albright
                                         --------------------------------
                                         Name:   William A. Albright
                                                 ------------------------
                                         Title:  Chief Financial Officer
                                                 ------------------------
Agreed to and accepted this
9/th/ day of November, 1999

 /s/ Richard L. Casey
--------------------------
Richard L. Casey, Optionee

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