<PAGE>
 
                                                               EXHIBIT 10.24 (A)

                           NEXELL THERAPEUTICS INC.
                                  NINE PARKER
                           IRVINE, CALIFORNIA 92718


                                 May 28, 1998


Mr. L. William McIntosh
202 Somerset Court
Lansdale, PA 19446

Dear Bill:

     This letter agreement sets forth in all respects the agreement between you
and Nexell Therapeutics Inc. ("Nexell") as to the terms and conditions of your
employment by Nexell.

     1)   You will be employed as President & Chief Executive Officer of Nexell.
          You will report to the Board of Directors of Nexell. At the option of
          the Board of Directors of Nexell, you agree to serve, for no
          additional compensation, as a director and/or officer of any or all of
          Nexell's subsidiaries or affiliates throughout the term of your
          employment. The place of employment will be at the offices of Nexell
          in Irvine, California.

     2)   The term of your employment will commence effective March 1, 1998, and
          shall continue until terminated under the provisions of Paragraph 5
          below. You will be a full-time employee of Nexell and you agree to
          devote your business and professional time, energy and skills to the
          affairs of Nexell and its subsidiaries and affiliates and to serve
          Nexell faithfully and to the best of your ability.

               a)   As compensation for the services to be rendered by you
          hereunder, Nexell will pay you (i) a signing bonus of $25,000 payable
          on your execution and delivery of this letter agreement, and (ii) a
          base salary of $225,000 per annum, payable in installments in
          accordance with Nexell's regular payroll practices, and (iii) an
          annual cash bonus to be determined in accordance with the provisions
          of subparagraph 2(d).

               b)   As additional compensation, Nexell shall award you,
          effective upon commencement of your employment, Non-Incentive Stock
          Options to purchase 125,000 shares of Nexell common stock at an
          exercise price of $5.00 per share pursuant to Nexell's 1998 Non-
          Incentive Stock Option Plan (the "Plan"), subject to approval of the
          Board of Directors of Nexell and approval of the adoption of the 
<PAGE>
 
Mr. L. William McIntosh
May 28, 1998
Page 2

          Plan by the shareholders of Nexell and of VIMRx Pharmaceuticals Inc.
          ("VIMRx"). Such options will be granted pursuant to such terms and
          conditions as are set forth in the Plan and a Non-Incentive Stock
          Option Agreement substantially in the form of the Non-Incentive Stock
          Option Agreement attached to the Plan as Exhibit A. Such Non-Incentive
          Stock Option Agreement shall provide that (i) all of such options
          shall be vested and exercisable on and after January 1, 2002 and they
          shall become vested in accordance with the following vesting schedule:
          31,250 on January 1, 1999; 31,250 on January 1, 2000; 31,250 on
          January 1, 2001; and 31,250 on January 1, 2002, (ii) such options
          shall expire ten years after their grant (subject to earlier
          termination pursuant to the terms of the Plan and the Non-Incentive
          Stock Option Agreement), and (iii) pursuant to Section 7(a) of the
          Plan, or the corresponding provision of any amendment of the Plan,
          such options shall be exercisable for shares of VIMRx common stock at
          the rate of 3 shares of VIMRx common stock for each share of Nexell
          common stock purchasable under the options, at an exercise price per
          share of VIMRx common stock equal to one-third of the exercise price
          per share of the Nexell common stock set forth in the Non-Incentive
          Stock Option Agreement.

               c)   You will be eligible to participate in Nexell's medical,
          dental, life and long-term disability insurance and other benefit
          programs, including any 401(k) or other retirement plans, from time to
          time in effect for Nexell's senior executives, your participation in
          any such plans to be in accordance with their respective terms and
          conditions.

               d)   Your performance will be reviewed annually by Nexell's Board
          of Directors, in connection with which your annual cash bonus and
          possible increases in your base compensation for the future will be
          discussed, it being understood that any such decisions shall be within
          the discretion of Nexell's Board of Directors and/or its Compensation
          Committee (or other similar committee duly appointed by Nexell's Board
          of Directors). However, it is further understood that the annual cash
          bonus is initially targeted at an amount equal to at least 25 percent
          of base compensation, assuming satisfactory performance.

               e)   Until such time as you have relocated your residence to
          California or until the expiration of six months following the
          commencement of your employment hereunder, whichever occurs first,
          Nexell will pay or reimburse (i) all of your reasonable expenses for
          travel to and from California and all of your reasonable living
          expenses while in California, and (ii) the cost of up to three round
          trip economy airline tickets for travel to California by your fiancee.
          Nexell also will pay the reasonable expenses of moving your household
          and personal effects from your home in southeastern Pennsylvania to a
          new residence in California and the reasonable closing costs
          [including points?] of purchasing a new home in California. 
<PAGE>
 
Mr. L. William McIntosh
May 28, 1998
Page 3

          To the extent that any of the expenses paid or reimbursed by Nexell
          pursuant to this subparagraph 2(e) constitutes compensation income to
          you that is subject to personal income taxation, Nexell will pay you
          an additional amount of cash compensation equal to the aggregate
          federal, state, and local income taxes payable by you with respect to
          the compensation income attributable to the payment or reimbursement
          of such expenses by Nexell (including income taxes payable with
          respect to such additional cash compensation).

     3)   You will be entitled to take up to an aggregate of four weeks of
          vacation each calendar year as business conditions permit. Nexell
          shall not be required to provide any additional compensation to you
          for vacation time not utilized by you.

     4)   Nexell will reimburse you for all reasonable and documented business
          expenses incurred by you on behalf of Nexell during the term of your
          employment hereunder consistent with Nexell's expense reporting policy
          (as the same may be modified from time to time). Notwithstanding
          anything herein to the contrary, the provisions of this Paragraph 4
          shall survive the effective date of termination of this Agreement for
          a period of six months.

5)

               a)  Your employment hereunder may be terminated at any time by
          Nexell for cause (as such term is hereinafter defined) or, upon at
          least 60 days' prior written notice by you or by Nexell, without
          cause.

               b)  In the event your employment is terminated by Nexell without
          cause or is terminated by you in the circumstances described in
          Paragraph 5(f) below, this Agreement shall terminate immediately on
          the effective date of termination of your employment; provided,
          however, that:

                    i)     you will be paid twelve months' base salary as
                           severance in monthly installments (in arrears)
                           beginning the first full month following the
                           cessation of your employment with Nexell;

                    ii)    you will be entitled to receive any accrued but
                           unpaid salary earned by you through the effective
                           date of such termination.

                    iii)   in the event that your employment is terminated by
                           Nexell without cause or is terminated by you in the
                           circumstances described in Paragraph 5(f) below
                           within 30 months after the commencement of your
                           employment hereunder, Nexell will pay or reimburse
<PAGE>
 
Mr. L. William McIntosh
May 28, 1998
Page 4

                           reasonable expenses for moving your household and
                           personal effects from California to southeastern
                           Pennsylvania; and

                    iv)    in the event that your employment is terminated by
                           Nexell without cause or is terminated by you in the
                           circumstances described in Paragraph 5(f) below
                           following the date that is two years after the
                           commencement of your employment hereunder, all of
                           your then-vested options to purchase Nexell common
                           stock shall become exercisable for shares of VIMRx
                           common stock at the rate of 3 shares of VIMRx common
                           stock for each share of Nexell common stock
                           purchasable under such vested options, at an exercise
                           price per share of VIMRx common stock equal to one-
                           third of the exercise price per share of Nexell
                           common stock set forth in the Non-Incentive Stock
                           Option Agreement.

          c)  No severance shall be paid or payable to you in the event your
     employment is terminated for cause, or you voluntarily resign from your
     employment with Nexell, in which events this Agreement shall terminate
     immediately upon the effective date of termination of your employment or
     upon the effective date of your resignation, respectively; provided,
     however, that Nexell shall nonetheless be obligated to pay you any accrued
     but unpaid salary earned by you through the date of such termination.

          d)  For purposes of this Agreement, termination for "cause" shall mean
     termination due to any or more of the following:  (i) if you are indicted
     for committing a felony or a decision or determination is rendered by any
     court or governmental authority that you have committed any act involving
     fraud, willful misconduct, dishonesty, breach of trust or moral turpitude;
     (ii) if you willfully breach your duty of loyalty to, or commit an act of
     fraud or dishonesty upon, Nexell; (iii) if you demonstrate gross negligence
     or willful misconduct in connection with your employment; (iv) if, in the
     reasonable, good faith opinion of a majority of Nexell's whole Board of
     Directors (excluding yourself, if you shall then be a director of Nexell),
     you engage in personal misconduct of such a material nature as to render
     your presence as an officer of Nexell detrimental to Nexell or its
     reputation and you fail to cure the same within five days after notice
     thereof from Nexell; or (v) if you commit a material breach of or a default
     under any of the terms or conditions of this Agreement and you fail to cure
     such breach or default within ten days after prior written notice thereof
     from Nexell.

          e)  Your employment hereunder shall terminate immediately upon your
     death or "permanent disability" (as such term is hereinafter defined). In
     either such event,
<PAGE>
 
Mr. L. William McIntosh
May 28, 1998
Page 5


     this Agreement shall terminate immediately upon the cessation
     of your employment; provided, however, you (or your legal representative,
     as the case may be) will be entitled to receive any accrued but unpaid
     salary earned by you through the date of such termination, plus severance
     in monthly installments (in arrears), beginning the first full month
     following the date of such termination, in an aggregate amount equal to the
     positive difference, if any, between (x) the base salary you would have
     received hereunder for the six months immediately following such
     termination date had your employment continued for such six month period,
     and (y) the total monies paid or payable to you with respect to such six
     month period under the long-term disability insurance policy or policies
     maintained by Nexell for your benefit, if any.  For purposes of this
     Agreement, the term "permanent disability" shall have the meaning set forth
     in the long-term disability insurance policy or policies then maintained by
     Nexell for the benefit of its employees, or if no such policy shall then be
     in effect, or if more than one such policy shall then be in effect in which
     the term "permanent disability" shall be assigned different definitions,
     then the term "permanent disability" shall be defined for purposes hereof
     to mean any physical or mental disability or incapacity which renders you
     incapable of fully performing the services required of you in accordance
     with your obligations hereunder for a period aggregating 120 days during
     any twelve-month period.

          f)  In the event of occurrence of any of the following events, you
     shall have the right to terminate your employment with Nexell on at least
     60 days' notice. Subject to the foregoing provisions of this Paragraph 5,
     in the event such notice is given by you within 30 days of any one or more
     of such events, such termination of employment shall be deemed termination
     of your employment by Nexell without "cause" within the meaning of this
     Paragraph 5:

               i)   a material breach of or default under this Agreement by
                    Nexell which is not cured by Nexell within ten (10) days
                    after its receipt of written notice thereof from you;

               ii)  a material reduction in your duties by Nexell's Board of
                    Directors (not arising from any physical or mental
                    disability you may sustain) which would be inconsistent with
                    the position of President & Chief Executive Officer of
                    Nexell or such other executive position to which you may be
                    assigned and the same shall not have been alleviated by
                    Nexell's Board of Directors within ten (10) days after its
                    receipt of written notice thereof from you.

6)   You hereby agree that you shall not, directly or indirectly, during the
     term of your employment hereunder and until the expiration of six months
     after you cease to be 
<PAGE>
 
Mr. L. William McIntosh
May 28, 1998
Page 6

     so employed by Nexell, own, manage, operate, join, control or become
     employed by, or render any services of an advisory nature or otherwise, or
     participate in the ownership, management, operation or control of, or
     otherwise be connected in any manner with, any business competitive with
     the business of Nexell or any of its directly or indirectly, wholly or
     partially owned subsidiaries without Nexell's prior written consent.

7)

          a)  You further hereby covenant and agree that you will not at any
     time during, or (a) for a period of three (3) years following the
     termination of, your employment with Nexell, reveal, divulge or make known
     to any person or entity any secrets or confidential information (whether
     oral, written, or electronically encoded) whatsoever, of or concerning
     Nexell or any of its directly or indirectly, wholly or partially owned
     subsidiaries or its business or anything connected therewith, all of which
     is and shall remain the property of Nexell and shall be returned by you to
     Nexell (including all copies) immediately upon any termination of your
     employment (or earlier, if requested by Nexell), or (b) for a period of
     three (3) years following the termination of your employment with Nexell,
     directly or indirectly entice away from Nexell's employment, retain or
     otherwise engage, any employee of Nexell, or attempt to do any of the
     foregoing.

          b)  For purposes hereof, confidential information shall not include
     any information which: (i) is or becomes generally available to the public
     other than as a result of a wrongful disclosure by you or your
     representatives; (ii) was known by you on a non-confidential basis prior to
     its disclosure to you by Nexell or its representatives; (iii) becomes
     available to you from a source other than Nexell or its representatives,
     provided that such source is not bound by a confidentiality agreement with
     Nexell or its representatives and otherwise has a right to disclose the
     same; or (iv) is required to be disclosed by any governmental or judicial
     authority, provided, in such case, that you shall use your best efforts to
     notify Nexell immediately of any such requirement so that Nexell shall have
     an opportunity to contest it.

8)   In the event of any breach or threatened breach by you of any one or more
     of the provisions of Paragraphs 6 (relating to non-competition) or 7
     (relating to non-disclosure and non-enticement of employees) above, Nexell
     will be entitled, in addition to any remedy hereunder or under any
     applicable law or in equity, to an injunction restraining the breach of
     such provisions hereof.

9)   You agree that Nexell may, in its discretion, apply for and take out in its
     name and at its own expense, and solely for its benefit, key man life
     insurance on you in any 
<PAGE>
 
Mr. L. William McIntosh
May 28, 1998
Page 7

     amount deemed advisable by Nexell to protect its interests, and you agree
     that you shall have no right, title or interest therein and further agree
     to submit to any medical or other examination and to execute and deliver
     any application or other instruments in writing reasonably necessary to
     effectuate such insurance.

10)  You represent and warrant that you are not under any obligation,
     restriction or limitation, including but not limited to confidentiality
     and/or non-competition restrictions, contractual or otherwise, to any other
     individual or entity which would prohibit or impede you from performing
     your duties and responsibilities hereunder and that you are free to enter
     into and perform the terms and provisions of this Agreement. Your
     employment agreement dated May 19, 1997 with VIMRx has been terminated by
     the mutual consent of the parties, effective February 28, 1998, and VIMRx
     has consented to your employment by Nexell.

11)  Notwithstanding anything herein to the contrary, the provisions of
     Paragraphs 6, 7, 8 and 10 hereof shall expressly survive the expiration or
     termination of this Agreement regardless of the reason for, or cause of,
     any such termination.

12)  All notices, requests, demands, and other communications provided for by
     this Agreement shall be in writing and shall be either personally delivered
     (including by couriers such as FedEx) or sent by pre-paid certified mail,
     return receipt requested, addressed to the address stated below of the
     party to which notice is given, or to such changed address as such party
     may have fixed by notice given in accordance with the terms hereof:

TO Nexell:            Nexell Therapuetics Inc.
                      Nine Parker
                      California 92718
                      Attn:  The Board of Directors

WITH A COPY TO:       VIMRx Pharmaceuticals Inc.
                      c/o Richard L. Dunning, President
                      2971 Centerville Road
                      Suite 210, Little Falls II
                      Wilmington, Delaware  19808

AND A COPY TO:        Lowell S. Lifschultz
                      Epstein Becker & Green, P.C.
                      250 Park Avenue
                      New York, New York  10177-0077
<PAGE>
 
Mr. L. William McIntosh
May 28, 1998
Page 8

   TO Mr. McIntosh:      L. William McIntosh
                         202 Somerset Court
                         Lansdale, PA 19446

  Any notice, sent as provided above, shall be deemed given upon receipt at the
  address provided for above (or, in the event delivery is refused, the first
  date on which delivery was tendered).

     13)  This Agreement contains the entire agreement and understanding between
          the parties relating to the subject matter hereof and supersedes any
          and all prior understandings, agreements and representations, written
          or oral, expressed or implied, with respect thereto.

     14)  This Agreement may not be amended, modified, altered or terminated
          (other than pursuant to its terms) except by an instrument in writing
          signed by the parties.

     15)  In case any one or more of the provisions of this Agreement shall be
          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 thereby.

     16)  This Agreement shall be governed by, construed and enforced in
          accordance with the laws of the State of Delaware applicable to
          contracts made and to be performed entirely therein (without giving
          effect to the conflict of law rules thereof) .

          Kindly indicate your agreement with the foregoing by countersigning
the enclosed duplicate copy of this letter agreement and returning it to me on
behalf of Nexell.

          On behalf of the Board of Directors of Nexell, we look forward to a
long and mutually rewarding relationship.

                                    Sincerely,

                                    NEXELL THERAPEUTICS INC.


                                    By: /s/ Richard L. Dunning
                                       ----------------------------- 
                                        Richard L. Dunning, Director
ACCEPTED AND AGREED TO THIS
28th DAY OF MAY, 1998

    /s/ L. William McIntosh
---------------------------   
     L. William McIntosh