<PAGE>
 
                                                                   EXHIBIT 10.39


                             U.S. Bioscience, Inc.
                         Executive Severance Agreement
                              with C. Boyd Clarke


                               Table of Contents
                                                              Page
                                                              ----
           1.  Employment . . . . . . . . . . . . . . . . . .   2
 
           2.  Effective Date and Term. . . . . . . . . . . .   2
 
           3.  Position and Duties. . . . . . . . . . . . . .   4
 
           4.  Place of Performance . . . . . . . . . . . . .   4
 
           5.  Compensation and Benefits. . . . . . . . . . .   6

               (a)  Base Salary
               (b)  Compensation Elements
               (c)  Expenses
               (d)  Company Plans
               (e)  Fringe Benefits
               (f)  Vacations
               (g)  Acceleration of Outstanding Stock Options
               (h)  Changes Made Within 180 Days
                    Preceding the Effective Date

           6.  Unauthorized Disclosure . . . . . . . . . . . .  12

           7.  Termination of Employment . . . . . . . . . . .  13

               (a)  Death
               (b)  Disability
               (c)  Cause
               (d)  Termination by the Executive
               (e)  Notice of Termination

           8. Compensation Upon Company Termination Without Cause 
              or Executive Resignation for Good Reason . . . .  14

           9. Compensation Upon Termination of Employment Upon 
              Death or Disability, or During Disability, or
              for Cause or Upon Resignation . . . . . . . . . . 18
<PAGE>
 
               (a)  Death
               (b)  Disability
               (c)  Cause
               (d)  Resignation

                                                              Page
                                                              ----

          10. No Mitigation . . . . . . . . . . . . . . . . . . 20

          11. Successors; Binding Agreement . . . . . . . . . . 20

          12. Participation in Certain Plans. . . . . . . . . . 22

          13. Certain Expenses. . . . . . . . . . . . . . . . . 23
 
          14. Certain Agreements of the Executive . . . . . . . 23
 
          15. Section 280G of the Internal Revenue Code
              of 1986 . . . . . . . . . . . . . . . . . . . . . 26
 
          16. Arbitration . . . . . . . . . . . . . . . . . . . 30
 
          17. Definitions . . . . . . . . . . . . . . . . . . . 31
 
          18. Miscellaneous . . . . . . . . . . . . . . . . . . 39
 
<PAGE>
 
                             U.S. Bioscience, Inc.
                               One Tower Bridge
                               100 Front Street
                         West Conshohocken, PA  19428


                               September 3, 1996



C. Boyd Clarke
President and Chief Operating Officer
U.S. Bioscience, Inc.
One Tower Bridge
100 Front Street
West Conshohocken, PA  19428

Dear Mr. Clarke:

          U.S. Bioscience, Inc. (the "Company") considers that establishing and
maintaining a sound and vital management is essential to protecting and
enhancing the best interests of the Company and its stockholders.  In this
connection, the Company recognizes that, as is the case with many publicly held
corporations, the possibility of a change in control exists, and that it is of
the utmost importance to the Company and its stockholders, because of such
possibility, to provide for the continuity of management and its uninterrupted
attention and dedication to the affairs of the Company.  Accordingly, the Board
of Directors of the Company (the "Board") has determined that 
<PAGE>
 
appropriate steps should be taken to encourage the continued attention and
dedication of members of the Company's management, including yourself (referred
to in this agreement as the "Executive"), to their assigned duties without
distraction in the face of the potentially disturbing circumstances that could
arise from a change in control of the Company or rumors thereof.

          In order to induce you to remain in the employ of the Company, or any
present or future subsidiary (hereinafter included, as appropriate, in the term
"Company"), this agreement sets forth the terms on which your employment
relationship with the Company will continue in the event of a "Change in Control
of the Company" (this term and other terms capitalized in this agreement are
defined where first used and/or are defined in paragraph 17), and the severance
benefits that will be provided to you if your employment with the Company is
terminated on or after the Effective Date (a) by the Company for any reason
other than your death, your Disability or Cause, or (b) by you as a result of a
resignation for Good Reason (any of which events are defined as a
"Termination").

          1.   Employment.  The Company agrees to continue to employ the
               ----------                                               
Executive, and the Executive agrees to continue to serve the Company, on the
terms and conditions set forth in this agreement, for the period commencing on
the Effective Date and 

                                      -2-
<PAGE>
 
thereafter for the term provided in paragraph 2 unless sooner terminated in
accordance with the terms of this agreement.

          2.  Effective Date and Term.
              ----------------------- 
          (a) This agreement shall become binding upon the parties as of the
date of this agreement, provided that this agreement shall not become binding
unless it is executed by the Executive and delivered to the Company within the
time set forth on the last page of this agreement.  However, anything in  this
agreement to the contrary notwithstanding, neither this agreement nor any
provision thereof shall be operative, and neither the Company nor the Executive
shall have any rights or obligations under this agreement, until the Effective
Date.  This agreement shall define the respective rights and obligations of the
Company and the Executive for a period of three years (or longer where expressly
provided) commencing on the Effective Date (the "Employment Term"), without
further action by either party, unless this agreement is sooner terminated by
the Company or the Executive by mutual agreement or in accordance with
subparagraph (b) below.  This agreement, however, shall be of no further force
and effect upon (i) receipt by the Company of notice from the Executive of the
Executive's intent to terminate employment if such notice does not state
specifically that the Executive is terminating employment for Good Reason or
(ii) the cessation of 

                                      -3-
<PAGE>
 
the Executive's active employment relationship with the Company for any reason
prior to the Effective Date.

          (b) Unless otherwise provided in the writing referred to in paragraph
3, this agreement shall terminate upon any material diminution more than 180
days prior to the Effective Date in the positions, powers or duties of the
Executive as they exist as of the date of this agreement.

          3.   Position and Duties.  The Executive presently serves the Company
               -------------------                                             
as an executive officer of the Company, with such titles, powers and duties as
have been assigned to the Executive from time to time by the Board.  Unless
changed by the Board prior to the Effective Date, the Executive shall serve in
such positions and shall have such powers and duties during the Employment Term.
The Executive shall have such other powers and duties as may from time to time
be prescribed by the Board after the Effective Date, provided such powers and
duties are consistent with the Executive's powers and duties immediately prior
to the Effective Date.  The Executive shall devote substantially all of the
Executive's working time and efforts to the business and affairs of the Company
except for permitted vacation, illness or incapacity; provided, however, that
nothing in this agreement shall preclude the Executive from devoting reasonable
periods required for serving as a director or member of a committee of any
organization involving no conflict of 

                                      -4-
<PAGE>
 
interest with the interests of the Company, from engaging in charitable and
community activities and from managing the Executive's personal investments and
affairs, provided that such activities do not materially interfere with the
regular performance of the Executive's duties and responsibilities under this
agreement.

          4.   Place of Performance.  In connection with the Executive's
               --------------------                                     
employment by the Company, the Executive shall be based at the Company's
principal executive offices and shall not be required to be absent therefrom on
travel status or otherwise in any calendar year more than (i) forty-five days if
the Executive has worked less than 730 days for the Company preceding the
Effective Date or (ii) one-half the number of days the Executive was absent from
the office on travel status during the 730 days preceding the Effective Date.
The Company shall not relocate its principal executive offices to a location
which is more than 30 road miles from its principal executive offices on the
Effective Date and which is also both more than 25 road miles from the
Executive's then current residence and more than 10 road miles further from the
Executive's then current residence than the location of the principal executive
offices on the Effective Date.  If after the Effective Date the Company
relocates its principal executive offices to a new location contrary to its
agreement in this Paragraph 4, and the Executive waives the 

                                      -5-
<PAGE>
 
Executive's right to terminate this agreement for "Good Reason" by reason of the
failure of the Company to comply with its agreement in this Paragraph 4, then
the Company will promptly pay (or reimburse the Executive for) all reasonable
moving expenses incurred by the Executive relating to a change of the
Executive's then current residence to a new residence 10 or more road miles
closer to the new location of the Company's principal executive offices in
connection with any such relocation of the Company's principal executive
offices.

          5.  Compensation and Benefits.
              ------------------------- 
          (a) Base Salary.  The Executive shall receive a base salary at the
              -----------                                                   
annual rate in effect on the Effective Date, or at such greater annual rate as
the Board shall from time to time determine ("Base Salary"), payable in
accordance with the Company's normal payroll practices.  Commencing on the
January 1 next following the Effective Date, the Executive's Base Salary shall
be subject to increase (but not decrease) each calendar year during the
Employment Term by no less than the average percentage increase in base salary
for that particular year given to the Company's other executives (positions in
the Company which are titled as Vice President or higher as of the date of this
agreement and, in the event of any change or changes in titles in the future,
positions in the Company, regardless of the actual title assigned, which have
duties and responsibilities 

                                      -6-
<PAGE>
 
commensurate with those exercised by individuals acting as Vice President or
higher in the Company as of the date of this agreement). The annual Base Salary
increase will be calculated as follows: (i) the percentage increases for all
executives will be aggregated and divided by the number of executives; (ii) the
percentage increases for only those executives whose percentage increases are
greater than the average determined pursuant to subparagraph (i) will be
aggregated and divided by the number of those executives; (iii) each executive
whose percentage increase was less than the average percentage increase
determined pursuant to subparagraph (i) will have the Executive's percentage
increase increased to the average determined pursuant to subparagraph (ii).

          (b) Compensation Elements.  The Executive shall receive Compensation
              ----------------------                                          
Elements at the same level (in dollars or as a percentage of Base Salary,
depending on how historically calculated) as in effect immediately prior to the
Effective Date. "Compensation Elements" shall not include the Executive's Base
Salary and as used herein shall mean annual or other bonuses, incentive
compensation and other items of compensation of a nature to be reported to the
Internal Revenue Service as "wages, tips and other compensation", and amounts
deferred under a Company deferred compensation plan whether or not reportable to
the Internal Revenue Service as current income.

                                      -7-
<PAGE>
 
          (c) Expenses.  The Executive shall be entitled to receive prompt
              --------                                                    
reimbursement for all reasonable expenses incurred by the Executive in
performing services hereunder, in accordance with the reimbursement practices in
effect immediately prior to the Effective Date, provided that the Executive
accounts for them in accordance with Company policy.

          (d) Company Plans.  The Executive shall be and continue to be a full
              -------------                                                   
participant in the Company's stock option plans and any other plans in effect
immediately prior to the Effective Date, with at least the same reward
opportunities that have theretofore been provided.  Nothing in this agreement
shall preclude improvement of reward opportunities in such plans or other plans.

          (e) Fringe Benefits.  The Executive shall be entitled to participate
              ---------------                                                 
in or receive benefits under all the Company's employee benefit plans and
arrangements (collectively "Fringe Benefits") which are the greater of the
employee benefits and perquisites (i) then provided by the Company to any other
employee of the Company with comparable authority or duties (and in any event
not less than provided to any other executive or other employee of the Company
with junior authority or duties), or (ii) those applicable to the Executive in
effect immediately prior to the Effective Date, or plans or arrangements
providing the Executive with at least equivalent benefits.  In determining 

                                      -8-
<PAGE>
 
the equivalency of benefits with respect to any tax qualified pension or
retirement plan, if such plan is terminated or modified in a manner detrimental
to the Executive, the Executive shall receive in lieu thereof cash payments,
grossed up to offset the additional federal, state and local income tax
liabilities the Executive will incur as a result of receiving such payments, (i)
in the case of any defined benefit plan, in an amount equal to the difference
between the present value of the accrued benefit the Executive would have
received if the plan had not been terminated or modified to his detriment and
the present value of the accrued benefit the Executive is entitled to under such
plan as modified or terminated, (ii) in the case of any defined contribution
plan, other than with respect to any 401(k) cash or deferred arrangement feature
of such plan, in an amount equal to the Company contributions that would have
been required to be made to the plan for the Executive's benefit if the plan had
not been so modified or terminated, and (iii) in the case of a 401(k) cash or
deferred arrangement feature of a defined contribution plan, in an amount equal
to the Company matching contribution that would have been made for the
Executive's benefit if the plan had not been so modified or terminated and the
Executive had made the maximum salary reduction contribution permitted by the
plan on the Effective Date. With respect to Fringe Benefits other than those
attributable to tax qualified 

                                      -9-
<PAGE>
 
plans, the equivalent benefits to be provided to the Executive shall be in the
aggregate with respect to such Fringe Benefits rather than on a benefit by
benefit basis. "Fringe Benefits" as used herein shall mean employee benefits,
including, without limitation, any savings or retirement plan, disability,
medical, dental, executive medical reimbursement, life, accident, health,
umbrella liability and other insurance programs, vacation (including any accrued
vacation consistent with Company policy), floating holidays, car allowances and
purchases, office support, accommodations and staff and any other perquisites
provided by the Company. In lieu of the Executive's participation in or receipt
of benefits under any such plan or arrangement, the Executive may at the
Executive's request participate in or receive benefits under any pension plan,
profit-sharing plan, savings plan, life insurance or health-and-accident plan or
arrangement made available by the Company on or following the Effective Date to
its executives and key management employees, subject to and on a basis
consistent with the terms, conditions and overall administration of such plans
and arrangements.

          (f) Vacations.  Without limiting the generality of subparagraph (e)
              ---------                                                      
above, the Executive shall be entitled to the number of paid vacation days in
each calendar year determined by the Company from time to time for its executive
officers (prorated in any calendar year during which the Executive is 

                                      -10-
<PAGE>
 
employed hereunder for less than the entire such year in accordance with the
number of days in such calendar year during which the Executive is so employed),
consistent with the Executive's positions and length of service with the
Company. Any accrued but unused vacation time due the Executive on the Effective
Date shall remain the Company's obligation, to be paid to the Executive upon
termination of employment. The Executive shall also be entitled to all paid
holidays given by the Company to its executives.

          (g) Acceleration of Outstanding Stock Options. With respect to all
              -----------------------------------------                     
options held by the Executive, while serving the Company as a payroll employee
of the Company, on the Effective Date to purchase securities of the Company, or
of any other company that is, or is affiliated with, any successor company
described is paragraph 11, that become exercisable, in whole or in part, after
the Effective Date, the exercise date of all such options shall automatically
accelerate to the Effective Date.  [Note:  Before selling securities acquired
through the exercise of options, the Executive should consult the Executive's
counsel with respect to reporting obligations and potential short-swing profit
liability of the Executive pursuant to Section 16 of the Securities Exchange Act
of 1934.]
          (h) Changes Made Within 180 Days Preceding the Effective Date.  In the
              ------------------------------------------ --------------         
event that the Executive's Base Salary 

                                      -11-
<PAGE>
 
and/or Compensation Elements and/or Fringe Benefits are reduced, or there is any
material reduction in the positions, powers or duties of the Executive, within
180 days preceding the Effective Date, such Base Salary and/or Compensation
Elements and/or Fringe Benefits and/or positions, powers or duties, as the case
may be, shall upon the Effective Date be restored retroactively to their
previous levels, notwithstanding the Executive's agreement to such reduction
prior to the Effective Date. If the Executive's employment is terminated by the
Company other than by reason of the Executive's Disability or Cause, or if the
Executive shall terminate the Executive's employment for reasons equivalent to
Good Reason, within 180 days preceding a Change in Control of the Company, this
agreement shall become operative retroactively to the date of such termination
of employment, and such date of termination of employment shall be deemed to be
the Effective Date for purposes of this agreement. Under such circumstances, in
addition to the Executive's rights and obligations under paragraph 8 and
elsewhere in this agreement, the Executive shall be entitled, subject to the
provisions of paragraph 15, to a lump sum payment, payable when the amount
payable pursuant to paragraph 8(b) is payable, in an amount determined by
calculating the Fair Market Value on the date of the Change in Control of the
Company (not the date deemed to be the Effective Date) of all securities of the
Company which were subject to purchase options 

                                      -12-
<PAGE>
 
held by the Executive on the date of the Executive's termination of employment
but which were not exercisable following such termination of employment, and
subtracting from such amount the aggregate exercise price of all such options.

          6.   Unauthorized Disclosure.  During the Employment Term and
               -----------------------                                 
thereafter, the Executive shall not make any Unauthorized Disclosure; i.e. shall
not disclose to any person, other than an employee of the Company or a person to
whom disclosure is reasonably necessary or appropriate in connection with the
performance by the Executive of the Executive's duties as an executive of the
Company, any confidential information obtained by the Executive while in the
employ of the Company with respect to any of the Company's products,
improvements, formulas, processes, customers, methods of distribution or methods
of manufacture; provided, however, that confidential information shall not
include any information known generally to the public (other than as a result of
unauthorized disclosure by the Executive) or any information of a type not
otherwise considered confidential by persons engaged in the same business or a
business similar to that conducted by the Company.

          The obligations of the Executive under this paragraph 6 shall be in
addition to any other obligation of non-disclosure the Executive may have under
the law or under any other agreement with the Company in effect on the Effective
Date.

                                      -13-
<PAGE>
 
          7.   Termination of Employment.
               ------------------------- 
               (a) Death.  The Executive's employment shall terminate upon the
                   -----                                                      
Executive's death.
               (b) Disability.  The Company may terminate the Executive's
                   ----------                                            
employment upon the Executive's Disability.
               (c) Cause.  The Company may terminate the Executive's employment
                   -----                                                       
for Cause.
               (d) Termination by the Executive.  The Executive may terminate
                   ----------------------------  
the Executive's employment (i) for Good Reason, (ii) if the Executive's health
should become impaired to an extent that makes the continued performance of the
Executive's duties hazardous to the Executive's physical or mental health or
life, or (iii) by giving at least 30 days prior written notice to the Company of
the Executive's intention to terminate employment, for any reason at any time
after the third anniversary of the Effective Date or after the Executive reaches
age 65, whichever first occurs.
               (e) Notice of Termination.  Any termination by the Company 
                   ---------------------  
pursuant to subparagraph (b) ("Disability") or (c) ("Cause") above or by the
Executive pursuant to subparagraph (d) above shall be communicated by written
Notice of Termination to the other party hereto.

          8.   Compensation Upon Company Termination Without Cause or Executive
               ----------------------------------------------------------------
Resignation for Good Reason.  If the Company 
---------------------------

                                      -14-
<PAGE>
 
shall terminate the Executive's employment other than pursuant to paragraph 7(b)
("Disability") or paragraph 7(c) ("Cause"), or if the Executive shall terminate
employment for Good Reason, then
               (a) The Company shall pay the Executive the Executive's full Base
Salary at the rate in effect on the Date of Termination, Compensation Elements
(at the level in effect at Date of Termination) and Fringe Benefits, through the
Date of Termination.
               (b) In lieu of any further Base Salary and Compensation Elements
to be paid to the Executive for periods subsequent to the Date of Termination,
the Company shall pay as severance pay to the Executive, on or before the later
of the tenth day following the Date of Termination or the tenth day following
receipt of the determination of Independent Tax Counsel and the election of the
Executive referred to in paragraph 15(a), a lump sum equal to the product of
multiplying the highest annual compensation (Base Salary plus Compensation
Elements) paid or payable by the Company to the Executive with respect to each
of the three calendar years ending with the year in which the Date of
Termination occurs (annualized with respect to a partial calendar year), by the
number of years (including any fraction of a year) remaining in the Employment
Term subsequent to the Date of Termination.

                                      -15-
<PAGE>
 
          With respect to a Compensation Element not yet paid or accrued with
respect to any year (including the year in which the Date of Termination
occurs), such Compensation Element will, for purposes of this subparagraph (b),
be deemed to be payable to the Executive with respect to any such year in the
amount, or according to the formula, indicated in such Executive's engagement
letter to be expected with respect to such year or, if such amount or formula to
be expected is not indicated in an engagement letter, then in the average
annualized amount (in relation to Base Salary if relevant) most recently paid or
payable by the Company to other Company executives with the same status (e.g.
Executive Vice President, Senior Vice President, Vice President, etc.) or, if
there is no executive with the same status, in an amount equal to the average
annualized amount most recently paid by the Company to all executives one status
level above and one status level below the Executive's status level.

          (c) In lieu of any further Fringe Benefits (including any 401(k)
savings plan) to be paid to the Executive for periods subsequent to the Date of
Termination (except with respect to the Company's pension plan, as hereinafter
provided), the Company shall pay as severance pay to the  Executive, on or
before the later of the tenth day following the Date of Termination or the tenth
day following receipt of the 

                                      -16-
<PAGE>
 
determination of Independent Tax Counsel and the election of the Executive
referred to in paragraph 15(a), a lump sum equal to the product of multiplying
$30,000 by the number of years (including any fraction of a year) remaining in
the Employment Term subsequent to the Date of Termination. The foregoing payment
shall not affect any rights of the Executive under the Consolidated Omnibus
Budget Reconciliation Act (popularly known as COBRA) amendments to the
Employment Retirement Income Security Act, as amended.

          (d) The Company will pay as severance pay to the Executive, on or
before the later of the tenth day following the Date of Termination or the tenth
day following receipt of the determination of Independent Tax Counsel and the
election of the Executive referred to in paragraph 15(a), an amount (the
"Pension Payment") equal to the sum of (i) the amount, if any, by which the
Executive's nonforfeitable accrued benefit in the U.S. Bioscience, Inc. Money
Purchase Pension Plan (the "Pension Plan") on the Date of Termination,
determined with credit for service (solely for the purpose of determining the
Executive's nonforfeitable percentage of his accrued benefit) during the
Severance Period, exceeds such benefit determined without credit for service
during the Severance Period and (ii) the product of the Executive's Adjusted
Vesting Percentage and the Company contributions that would have been allocated
to the  Pension Plan 

                                      -17-
<PAGE>
 
on behalf of the Executive during the Severance Period in accordance with the
terms of the Pension Plan in effect on the Date of Termination and assuming the
Executive continued to earn the same annual compensation during the Severance
Period as the Executive was earning on the Date of Termination. The Company will
gross up the Pension Payment to offset the additional federal, state and local
income tax liabilities the Executive or the Executive's estate will incur as a
result of receiving the Pension Payment. "Adjusting Vesting Percentage" shall
mean the Executive's nonforfeitable percentage interest, if any, in the
Executive's accrued benefit under the Pension Plan on the Date of Termination
determined as if the Executive's Date of Termination occurred on the last day of
the Employment Term. "Severance Period" shall mean the period commencing on the
Date of Termination and ending on the last day of the Employment Term.

          (e) In lieu of receiving in a lump sum the payments provided in
subparagraphs (b) and (c) above, the Executive may, in a writing delivered to
the Company prior to the date that such amount would otherwise become payable,
elect to receive such amount on the next January 1 or in two or more, but not
more than five, substantially equal annual installments commencing on the next
January 1 and continuing until such amount has been paid in full.  The Company
shall not be obligated to establish any reserve or other fund with respect to
the payment 

                                      -18-
<PAGE>
 
of such amount, and, unless the Company shall fail to pay any such installment
within ten days of the date on which it is payable, the Executive shall have no
right to receive such amount other than as provided in the above election.

          9.   Compensation Upon Termination of Employment Upon Death or
               ---------------------------------------------------------
Disability, or During Disability, or For Cause or Upon Resignation.
------------------------------------------------------------------ 
          (a) Death.  If the Executive's employment shall be terminated by
              -----                                                       
reason of death, the Company shall pay to the Executive's designee identified in
a written designation filed by the Executive with the Company during the
Executive's lifetime or, if there be no such designation, to the  Executive's
estate, in a lump sum, an amount equal to the Executive's Base Salary and
Compensation Elements at the level in effect at date of death through the date
of death and for a period of 26 additional weeks.  This amount shall be
exclusive of and in addition to any payments the Executive's surviving spouse,
beneficiaries or estate may be entitled to receive pursuant to any pension or
employee Fringe Benefit maintained by the Company.

          (b) Disability.  During any period that the Executive fails to perform
              ----------                                                        
the Executive's duties hereunder as a result of incapacity due to physical or
mental illness or injury, the Executive shall continue to receive the
Executive's full Base Salary, Compensation Elements and Fringe Benefits until
the 

                                      -19-
<PAGE>
 
Executive's employment is terminated pursuant to the provisions of this
agreement. After termination, the Executive shall be paid 100% of the
Executive's Base Salary and Compensation Elements (at the level in effect at
Date of Termination) through the Date of Termination and for a period of 26
additional weeks (less any consecutive period of time immediately prior to the
Date of Termination during which the Executive failed to perform the Executive's
duties hereunder as a result of incapacity due to physical or mental illness or
injury, calculating such consecutive period of time starting upon expiration of
any sick leave to which the Executive would be entitled, independent of this
subparagraph (b), during such consecutive period of time), which payments shall
be made notwithstanding the death of the Executive during such payment period.
Thereafter the Executive shall be paid an annual amount equal to the amount that
the Executive would have received under any disability plan(s) of the Company in
effect on the Effective Date, but in no event for a shorter period of time than
five years; provided, that such payments shall terminate effective upon the
death of the Executive except to the extent that such payments would have
continued under any disability plan(s) of the Company in effect on the Effective
Date. All payments provided for in this subparagraph (b) shall be reduced by any
disability payments 

                                      -20-
<PAGE>
 
actually paid to the Executive pursuant to plans provided by the Company.

          (c) Cause.  If the Executive's employment shall be terminated for
              -----                                                        
Cause, the Company shall pay the Executive, subject to any offsetting claim
asserted by the Company in good faith, the Executive's full Base Salary through
the Date of Termination at the rate in effect at the time Notice of Termination
is given.

          (d) Resignation.  If the Executive terminates this agreement pursuant
              -----------                                                      
to paragraph 7(d)(iii), the Executive shall receive as severance pay on the
tenth day following the Date of Termination the amount of the Executive's Base
Salary through the Date of Termination at the rate in effect at the time Notice
of Termination is given.

          10.  No Mitigation.  The Executive shall not be required to mitigate
               -------------                                                  
the amount of any payment to be made to the Executive pursuant to paragraph 8 or
paragraph 9 by seeking employment or otherwise, and nothing earned by the
Executive following the termination of the Executive's employment under this
agreement shall affect the amount of any payment required to be made to or in
respect of the Executive pursuant to paragraph 8 or paragraph 9.

          11.  Successors; Binding Agreement.
               ----------------------------- 

                                      -21-
<PAGE>
 
          (a) The Company will require any successor (whether direct or
indirect, by purchase, merger, consolidation or otherwise) to all or
substantially all of the business and/or assets of the Company, to expressly
assume and agree to perform this agreement in the same manner and to the same
extent that the Company would be required to perform it if no such succession
had taken place.  Failure of the Company to obtain such agreement prior to the
effectiveness of such succession shall be a breach of this agreement and shall
entitle the Executive to compensation from the Company in the same amount and on
the same terms as the Executive would be entitled to hereunder if the Executive
terminated employment for Good Reason, except that for purposes of implementing
the foregoing, the date on which any such succession becomes effective shall be
deemed the Date of Termination.  As used in this agreement, "Company" shall mean
the Company and any successor to its business and/or assets which executes and
delivers the agreement provided for in this paragraph 11 or which otherwise
becomes bound by this agreement by operation of law.

          (b) This agreement and all rights of the Executive hereunder shall
inure to the benefit of and be enforceable by the Executive's personal or legal
representatives. If the Executive should die while any amounts would still be
payable to the Executive hereunder if the Executive had continued 

                                      -22-
<PAGE>
 
to live, all such amounts, unless otherwise provided herein, shall be paid in
accordance with the terms of this agreement to the Executive's designee
identified in a written designation filed by the Executive with the Company
during the Executive lifetime or, if there be no such designation, to the
Executive's estate.

          12.  Participation in Certain Plans.    Nothing in this agreement
               ------------------------------                              
shall prevent or limit the Executive's continuing or future participation in any
benefit, bonus, incentive or other plans, programs, policies or practices
provided by the Company and for which the Executive may qualify, nor shall
anything herein limit or otherwise affect such rights as the Executive may have
under any stock option or other agreements with the Company. Amounts which are
(or become by virtue of this agreement) vested benefits or which the Executive
is otherwise entitled to receive under any plan, policy, practice or program of
the Company at or subsequent to the Date of Termination shall, unless otherwise
provided herein, be payable in accordance with such plan, policy, practice or
program.  Without limiting the foregoing, neither the termination of the
Executive's employment by the Company for any reason whatsoever (including
termination by the Company for Cause), nor termination of employment because of
the  Executive's death, shall result in a forfeiture, reduction or other
diminution of any of the Executive's entitlements or benefits 

                                      -23-
<PAGE>
 
which are vested as of such date or, in the absence of this agreement, would
continue to be exercisable by or otherwise be available to the Executive,
including, without limitation, funded benefits under any qualified plans of the
Company, vested stock options, unfunded benefits and entitlements under any
Company disability plan and other insurance programs of the Company.

          13.  Certain Expenses.    The Company agrees to pay, to the full
               ----------------                                           
extent permitted by law, all reasonable legal fees and expenses which the
Executive may reasonably incur as a result of any contest by the Company or
others of the validity or enforceability of, or liability under, any provision
of this agreement (including as a result of any contest by the Executive about
the amount of any payment pursuant to paragraph 13 hereof), provided that, in
the case of matters between the Executive and the Company, the Executive shall
have prevailed (after all appeals and appeal periods) in such litigation with
respect to such matters.  Such right to reimbursement of reasonable legal fees
and expenses shall be immediate upon the presentment, after the Executive has
prevailed in such litigation, by the Executive of written billings for such
reasonable fees and expenses.  The Executive shall be paid interest on any
payments of such expenses, or any other payments under this agreement, that are
overdue, at the rate per annum equal to the prime rate quoted 

                                      -24-
<PAGE>
 
from time to time by the principal New York City branch of The Bank of New York
or any successor thereto.

          14.  Certain Agreements of the Executive.
               ----------------------------------- 

          (a)  The Executive agrees that the Executive will not file any lawsuit
or administrative claim of any kind against the Company arising from the
Executive's employment by or termination of employment with the Company except
with respect to the matters referred to in subparagraphs (c)(i), (ii) or (iii)
hereof.  The Executive also agrees that the Executive will refrain from
disparagement of the Company and, except with respect to the matters referred to
in subparagraphs (c)(i), (ii) or (iii) hereof, from testifying against the
Company in any type of proceeding unless compelled to do so by appropriate
judicial order.  The Executive agrees to give testimony and otherwise cooperate
(including attending meetings) as reasonably requested by the Company regarding
any claim, lawsuit or action involving matters relating in any way to the
Executive's employment by the Company and the Company agrees to reimburse the
Executive for the Executive's reasonable expenses in so cooperating.
Notwithstanding the foregoing, nothing stated by the Executive under oath in a
judicial or administrative proceeding shall be deemed a violation of the terms
and provisions of this agreement.

          (b) The Company shall, to the full extent permitted by law, indemnify
the Executive and the Executive's 

                                      -25-
<PAGE>
 
personal and legal representatives to the extent provided in paragraphs A and B
of Article SIX of the Company's Certificate of Incorporation as it exists on the
date this agreement becomes binding.

          (c) The Executive, on behalf of the Executive and the Executive's
heirs and personal representatives, hereby unconditionally releases the Company
and its affiliates from and forever discharges them against any and all actions,
suits or claims in law or in equity arising from a termination of employment
which the Executive may have or will have, whether known or unknown, other than
any actions, suits or claims (i) which relate to injury, intentional or
otherwise, to the person or property, misrepresentation, defamation, the right
of privacy, misuse of legal procedure, survival or wrongful death, products
liability, or interference, unrelated to the Executive's acts as an employee,
officer or director of the Company, with contractual relations or prospective
advantage, or (ii) which relate to any amounts, benefits or service credits for
benefits which are owed or owing to the Executive or on the Executive's behalf,
as an employee, officer or director of the Company, and which have not been
received by the Executive, reimbursed to the Executive or paid or funded by the
Company on the Executive's behalf as of the Date of Termination, or (iii) which
arise under this agreement. Notwithstanding the foregoing, (A) nothing in this
agreement 

                                      -26-
<PAGE>
 
shall prevent the Executive from participating as a member of a class of
security holders of the Company in any litigation or proceeding brought against
the Company by such class of security holders, or prevent the Executive from
receiving any dividends or proceeds payable to members of any class of security
holders who instituted a proceeding or litigation against the Company or
otherwise payable to security holders of the Company generally and (B) the
Executive shall be entitled to receive an amount, based upon the Executive's
ownership of Company securities, equal to any award paid to any class of
security holders of the Company in any litigation or proceeding brought against
the Company by a class of security holders as if the Executive were a member of
such class (without duplication of payments and subject to the same burdens as
members of such class).

          15.  Section 280G of the Internal Revenue Code of 1986.
               -------------------------------------------------         
          (a) Anything in this agreement to the contrary notwithstanding, in 
the event that it shall be concluded by independent tax counsel ("Independent
Tax Counsel") designated by the Executive and reasonably acceptable to the
Company that any payment or distribution by the Company to or for the
Executive's benefit (whether paid or payable or distributed or distributable
pursuant to the terms of this agreement or otherwise) (a "Payment") will be
nondeductible by the Company for federal income tax purposes because of Section
280G (or any successor

                                      -27-
<PAGE>
 
provision) of the Internal Revenue Code of 1986, as amended (the "Code"), then
the aggregate present value of amounts payable or distributable to or for the
Executive's benefit pursuant to this agreement shall be reduced (but not below
zero) to the "Reduced Amount." The "Reduced Amount" shall be an amount expressed
in present value that maximizes the aggregate present value of Payments without
causing any Payments to be nondeductible because of said Section 280G of the
Code. The determination of the Reduced Amount shall be made within 30 business
days after the Date of Termination by Independent Tax Counsel, who shall provide
detailed calculations thereof to the Company and to the Executive; provided,
however, that the Executive may, in the Executive's sole discretion, elect which
and how much of the amounts and/or benefits to be received by the Executive
shall be eliminated or reduced (as long as after such election the present value
of the amounts payable or distributable to the Executive hereunder equals the
Reduced Amount), and shall advise the Company in writing of such election within
30 business days of the Executive's receipt of Independent Tax Counsel's
determination. If no such election is made by the Executive within such 30-day
period, the Company may elect which of such amounts and/or benefits shall be
eliminated or reduced (as long as after such election the present value of the
amounts payable or distributable to the Executive hereunder equals the Reduced

                                      -28-
<PAGE>
 
Amount) and shall notify the Executive promptly of such election. For purposes
of this calculation, present value shall be determined in accordance with
Section 280G of the Code and any regulations thereunder. As promptly as
practicable following the Date of Termination, the Company shall pay to the
Executive or distribute for the Executive's benefit such amounts as are then due
to the Executive under this agreement or otherwise and shall promptly pay to the
Executive or distribute for the Executive's benefit in the future such amounts
as become due to the Executive under this agreement or otherwise.

          (b) Within 30 days after the giving of a Notice of Termination, the
Executive shall designate Independent Tax Counsel, and the Company shall
indicate whether such Independent Tax Counsel is acceptable to the Company as
soon as practicable thereafter.  The Company and the Executive shall promptly
provide such Independent Tax Counsel with such information as such Independent
Tax Counsel shall reasonably request in connection with its initial or any
subsequent determination.  Both the Company and the Executive shall promptly
inform the Independent Tax Counsel of any audits, claims, arbitrations, or
administrative or judicial proceedings which may affect the determination of
Independent Tax Counsel regarding the Reduced Amount or any Overpayment or
Underpayment, including, without limitation, any examination by the Internal
Revenue Service of 

                                      -29-
<PAGE>
 
the returns of the Company or the Executive, or any claim by the Company or the
Executive regarding the obligations of one to the other. Subsequent to the
initial determination of the Reduced Amount, if any, Independent Tax Counsel
shall not be required to consider whether an Overpayment or Underpayment has
been made unless requested to do so by the Company or the Executive. The Company
and the Executive may, by agreement, appoint a replacement or substitute
Independent Tax Counsel.

          (c) Because of the uncertainty in the application of Section 280G of
the Code, it is possible that amounts will have been paid or distributed by the
Company to the Executive or for the Executive's benefit pursuant to this
agreement which should not have been so paid or distributed ("Overpayment") or
that additional amounts which will have not been paid or distributed by the
Company to the Executive or for the Executive's benefit pursuant to this
agreement could have been so paid or distributed ("Underpayment"), in each case,
consistent with the calculation of the Reduced Amount hereunder.  Subject to the
last paragraph of this paragraph 15, in the event that Independent Tax Counsel
determines that an Overpayment has been made because of a mistake in fact or law
or because of a final determination involving the Payments by a court, or in
connection with an Internal Revenue Service proceeding which is no longer
subject to appeal, any such Overpayment shall be treated for all 

                                      -30-
<PAGE>
 
purposes as a loan to the Executive which the Executive shall repay to the
Company together with interest at the applicable federal rate provided for in
Section 7872(f)(2)(B) of the Code; provided, however, that no amount shall be
payable by the Executive to the Company if and to the extent such payment would
not reduce the amount which is subject to taxation under Section 4999 of the
Code. Subject to the last paragraph of this paragraph 15, in the event that
Independent Tax Counsel determines that an Underpayment has occurred because of
a mistake in fact or law or because of a final determination involving the
Payments by a court or in connection with an Internal Revenue Service
proceeding, any such Underpayment shall be promptly paid by the Company to the
Executive or for the Executive's benefit together with interest at the
applicable federal rate provided for in Section 7872(f)(2)(B) of the Code. In
calculating amounts required to be repaid by the Executive to the Company
pursuant to any Overpayment, the Overpayment shall be deemed to consist of the
most recent payments from the Company to Executive hereunder, in reverse order
of payment, and interest on such Overpayment shall run from the dates on which
the payments constituting the Overpayment were made.

          (d) The Executive and the Company agree that each will in all cases
file tax returns on a basis consistent with any determination made or conclusion
reached under this paragraph 15, 

                                      -31-
<PAGE>
 
and will defend such position to the extent practicable in the event a contrary
position is taken by the Internal Revenue Service. The Executive shall be
entitled to reimbursement of reasonable counsel fees and expenses in connection
with any such defense. In addition, the Company shall pay all of the reasonable
fees and expenses of Independent Tax Counsel.

          16.  Arbitration.  Any dispute arising under or in connection with
               -----------                                                  
this agreement, including claims by the Executive for violation of any federal,
state or local statute or ordinance, shall be settled exclusively by arbitration
held in the City of Philadelphia, Pennsylvania in accordance with the rules of
the American Arbitration Association then in effect or, if the parties so agree,
held in the City of Philadelphia, Pennsylvania in accordance with the rules of
procedure of Judicate, Inc. then in effect.  Judgment may be entered on the
award in any court having jurisdiction; provided, however, that the Company
shall be entitled to seek a restraining order or injunction in any court of
competent jurisdiction to prevent any continuation of any violation of paragraph
6 hereof.

          17.  Definitions.
               ----------- 
               (a) "Adjusted Vesting Percentage" as used herein shall have the
meaning set forth in paragraph 8(d).
               (b) "Base Salary" as used herein shall have the meaning set forth
in paragraph 5(a).

                                      -32-
<PAGE>
 
               (c) "Board" and "Board of Directors" as used herein shall mean
the Board of Directors of the Company.
               (d) The Company shall have "Cause" to terminate the Executive's
employment upon (i) the willful failure by the Executive to substantially
perform the Executive's duties hereunder, other than any such failure resulting
from the Executive's incapacity due to physical or mental illness or leaves of
absence to which the Executive is entitled, which failure is not remedied in a
reasonable time after receipt of written notice from the Company to do so, (ii)
an act or acts of fraud, misappropriation or criminal conduct of the Executive
involving or relating in any material way to the Company, or the willful
engaging by the Executive in gross misconduct or personal dishonesty materially
injurious to the Company, (iii) the willful violation by the Executive of the
provisions of paragraph 6 ("Unauthorized Disclosure"), provided that such
violation results in material injury to the Company, or (iv) engaging by the
Executive in any activity in competition with the Company while employed by the
Company (other than ownership of less than two percent of the stock of any class
of a publicly-owned corporation regardless of such corporation's line of
business).  For purposes of this paragraph, no act, or failure to act, on the
Executive's part shall be considered "willful" if it was done, or omitted to be
done, in good faith and with reasonable belief that such 

                                      -33-
<PAGE>
 
action or omission was in the best interest of the Company. The Executive shall
not be deemed to have been terminated for Cause unless and until there shall
have been delivered to the Executive a copy of the resolution, duly adopted by
the affirmative vote of not less than three-quarters of the entire membership of
the Board at a meeting of the Board called and held for the purpose (after
reasonable notice to the Executive and an opportunity for the Executive,
together with the Executive's counsel, to be heard before the Board prior to its
taking such action), finding that in the good faith opinion of the Board the
Executive engaged in conduct set forth above in clause (i), (ii), (iii) or (iv),
above, and specifying the particulars thereof.

               (e) A "Change in Control of the Company" shall be deemed to have
occurred if
          (i) There has been a change in control of a nature that would be
required, if the Company would be subject to reporting requirements under the
Securities Exchange Act of 1934 (the "Exchange Act"), to be reported in response
to Item 6(e) of Schedule 14A of Regulation 14A promulgated under the Exchange
Act or Item 1, Form 8-K promulgated under the Exchange Act; or
          (ii) any person, entity or group (within the meaning of  Section
13(d)(3) or Section 14(d)(2) of the Exchange Act), other than any employee
benefit plan (or related trust) sponsored or maintained by the Company or any
subsidiary of the 

                                      -34-
<PAGE>
 
Company, is or becomes the beneficial owner, directly or indirectly, of
securities of the Company representing 30% or more of the combined voting power
in the election of directors; or

          (iii) during any period of two consecutive years, individuals
who at the beginning of such period constitute the Board cease for any reason to
have authority to cast at least a majority of the votes which all directors on
the Board are entitled to cast, unless the election, or the nomination for
election by the Company's stockholders, of each new director was approved by a
vote of at least two-thirds of the votes entitled to be cast by the directors
then still in office who were directors at the beginning of the period.
               (f) "Code" as used herein shall mean the Internal Revenue Code of
1986, as amended.
               (g) "Company" as used herein shall mean U.S. Bioscience, Inc. 
and, as appropriate, any present or future subsidiary of U.S. Bioscience, Inc.,
and shall have the meaning set forth in paragraph 11(a).
               (h) "Compensation Elements" as used herein shall have the meaning
set forth in paragraph 5(b).
               (i) "Date of Termination" as used herein shall mean (i) if the
Executive's employment is terminated by death, the date of death, (ii) if the
Executive's employment is terminated pursuant to paragraph 7(b) ("Disability"),
thirty days 

                                      -35-
<PAGE>
 
after Notice of Termination is given (provided that the Executive shall not have
returned to the performance of the Executive's duties on a full-time basis
during such thirty day period), (iii) if the Executive's employment is
terminated pursuant to paragraph 7(c) ("Cause") or paragraph (d)(iii), the date
specified in the Notice of Termination, (iv) if the Executive's employment is
terminated pursuant to paragraph 7(d)(i) ("Good Reason"), thirty days after
Notice of Termination is given, or the date specified in paragraph 11, whichever
comes first, and (v) if the Executive's employment is terminated for any other
reason, the date on which a Notice of Termination is given.
               (j) The Company's termination of the Executive's employment 
shall be deemed to be based upon the Executive's "Disability" if, (i) as a
result of the Executive's incapacity due to physical or mental illness or
injury, the Executive shall have been absent from, or unable to perform, the
Executive duties hereunder on a full time basis for at least 90 business days
within any period of 180 consecutive calendar days (even if such period of 180
days began prior to the Effective Date), and within 30 days after written Notice
of Termination is given shall not have returned to the performance of such
duties on a full time basis, or (ii) the Company's Board of Directors determines
in good faith that the Executive has become so seriously incapacitated that the
Executive's resumption of duties within 90 

                                      -36-
<PAGE>
 
calendar days is a practical impossibility and such determination is confirmed
in writing by a physician independent of the Company.
               (k) "Effective Date" as used herein shall mean the date on which
there shall have occurred a Change in Control of the Company, or on an earlier
date as set forth in Paragraph 5(h).
               (l) "Employment Term" as used herein shall have the meaning set
forth in paragraph 2(a).
               (m) "Exchange Act" as used herein shall have the meaning set
forth in paragraph 17(e)(i).
               (n) "Executive" as used herein shall have the meaning set forth
on the first page of this agreement.
               (o) Fair Market Value of a security of the Company shall be the
amount determined to be such by the Board of Directors of the Company acting in
good faith; provided, however, that if a security of the Company is, or was
within thirty days preceding the Effective Date, traded in a public market, then
the Fair Market Value of the security on the Effective Date shall be, if the
security is listed on a national securities exchange or included in the NASDAQ
National Market System, the last reported sale price thereof on the Effective
Date or the last date preceding the Effective Date that the security traded in a
public market, or, if the security is not so listed or included, the 

                                      -37-
<PAGE>
 
mean between the last reported "bid" and "asked" prices thereof on the Effective
Date or the last date preceding the Effective Date that the security traded in a
public market, as reported on NASDAQ, or, if not so reported, as reported by the
National Quotation Bureau, Inc. or, if not so reported, as reported in another
customary financial reporting service.
               (p) "Fringe Benefits" as used herein shall have the meaning set
forth in paragraph 5(e).
               (q) "Good Reason" shall as used herein mean (i) any significant
change in the nature or scope of the authority, powers, functions, duties,
responsibilities and status attached to the positions held by the Executive on
the Effective Date, (ii) any removal of the Executive from, or any failure to 
re-elect the Executive to, any of the offices (or failure to renominate the
Executive and seek the election of the Executive to any directorships) which the
Executive held on the Effective Date, except in connection with termination of
the Executive's employment for Cause, (iii) any failure by the Company to comply
with paragraph 5 ("Compensation and Benefits") which is not remedied within 30
days after receipt by the Company of written notice to such effect from the
Executive, (iv) any failure by the Company to comply with paragraph 4 ("Place of
Performance") which is not remedied within 30 days after receipt by the Company
of written notice to such effect from the Executive, (v) a

                                      -38-
<PAGE>
 
determination by the Executive made in good faith that as a result of a Change
in Control of the Company, and a change in circumstances which has thereafter
significantly affected the Executive's positions, the Executive is unable to
carry out the authority, powers, functions or duties attached to the Executive's
positions and the situation is not remedied within 30 days after receipt by the
Company of written notice to such effect from the Executive, (vi) failure of the
Company to obtain the assumption of and agreement to perform this agreement by
any successor as contemplated by paragraph 11, or (vii) a breach by the Company
of any provision of this agreement not embraced within the foregoing clauses (i)
through (vi) which is not agreed to in writing by the Executive or remedied
within 30 days after receipt by the Company of written notice to such effect
from the Executive.
               (r) "Independent Tax Counsel" as used herein shall have the
meaning set forth in paragraph 15(a).
               (s) "Legal representatives" as used herein shall include, without
limitation, attorney-in-fact and guardians.
               (t) "Notice of Termination" as used herein shall mean a notice 
which shall indicate the specific termination provision in this agreement relied
upon and shall set forth in reasonable detail the facts and circumstances
claimed to provide

                                      -39-
<PAGE>
 
a basis for termination of the Executive's employment under the provision so
indicated.
               (u) "Overpayment" as used herein shall have the meaning set forth
in paragraph 15(c).
               (v) "Payment" as used herein shall have the meaning set forth in
paragraph 15(a).
               (w) "Pension Plan" as used herein shall have the meaning set
forth in paragraph 8(d).
               (x) "Personal representatives" as used herein shall include,
without limitation, executors and administrators.
               (y) "Reduced Amount" as used herein shall have the meaning set
forth in paragraph 15(a).
               (z) "Severance Period" as used herein shall have the meaning set
forth in paragraph 8(d).
               (aa) "Termination" as used herein shall have the meaning set
forth on the second page of this agreement.
               (bb) "Unauthorized Disclosure" as used herein shall have the
meaning set forth in paragraph 6.
               (cc) "Underpayment" as used herein shall have the meaning set
forth in Paragraph 15(c).
          18.  Miscellaneous.
               ------------- 
          (a)  Indulgences, Etc.  Neither the failure nor any delay on the part
               ----------------                                                
of either party to exercise any right, remedy, power or privilege under this
agreement shall operate as 

                                      -40-
<PAGE>
 
a waiver thereof, nor shall any single or partial exercise of any right, remedy,
power or privilege preclude any other or further exercise of the same or of any
other right, remedy, power or privilege, nor shall any waiver of any right,
remedy, power or privilege with respect to any occurrence be construed as a
waiver of such right, remedy, power or privilege with respect to any other
occurrence. No waiver shall be effective unless it is in writing and is signed
by the party asserted to have granted such waiver.

          (b)  Controlling Law.  This agreement and all questions relating to
               ---------------                                               
its validity, interpretation, performance and enforcement (including, without
limitation, provisions concerning limitations of actions), shall be governed by
and construed in accordance with the laws of the Commonwealth of Pennsylvania,
notwithstanding any conflict-of-laws doctrines of such state or other
jurisdiction to the contrary, and without the aid of any canon, custom or rule
of law requiring construction against the draftsman.

          (c)  Notices.  All notices, requests, demands and other communications
               -------                                                          
required or permitted under this agreement shall be in writing and shall be
deemed to have been duly given, made and received only when personally
delivered, or on the day specified for delivery when deposited with a courier
service such as Federal Express for delivery to the intended addressee, or two

                                      -41-
<PAGE>
 
days following the day when deposited in the United States mails, by registered
or certified mail, postage prepaid, return receipt requested, addressed as set
forth below:

                    (i)  If to the Executive:

                    C. Boyd Clarke
                    U.S. Bioscience, Inc.
                    One Tower Bridge
                    100 Front Street
                    West Conshohocken, PA  19428

                    (ii) If to the Company:

                    U.S. Bioscience, Inc.
                    One Tower Bridge
                    100 Front Street
                    West Conshohocken, PA 19428
                    Attention:  Secretary

          In addition, notice by mail shall be by air mail if posted outside of
the continental United States.

          Any party may alter the address to which communications or copies are
to be sent by giving notice of such change of address in conformity with the
provisions of this paragraph for the giving of notice.

                                      -42-
<PAGE>
 
          (d)  Binding Nature of Agreement; No Assignment.
               ------------------------------------------ 
This agreement shall be binding upon and inure to the benefit of the parties
hereto and their respective heirs, personal representatives, successors and
assigns, except that neither party may assign or transfer his, her or its rights
or obligations under this agreement without the prior written consent of the
other party hereto, except to the extent set forth in paragraph 11 hereof.

          (e)  Provisions Separable.  The provisions of this agreement are
               --------------------                                       
independent of and separable from each other, and no provision shall be affected
or rendered invalid or unenforceable by virtue of the fact that for any reason
any other or others of them may be invalid or unenforceable in whole or in part.

          (f)  Entire Agreement.  This agreement contains the entire
               ----------------                                     
understanding between the parties hereto with  respect to the subject of the
Executive's employment and severance upon a Change in Control of the Company,
and supersedes all prior and contemporaneous agreements and understandings,
inducements or conditions, express or implied, oral or written, except as herein
contained.  The express terms hereof control and supersede any course of
performance and/or usage of the trade inconsistent with any of the terms hereof.
This agreement may not be modified or amended other than by an agreement in
writing.  Upon becoming 

                                      -43-
<PAGE>
 
operative this agreement shall supersede the provisions of any employment
agreement, engagement letter or arrangement (collectively "existing employment
arrangements") then in effect between the Company and the Executive to the
extent that this agreement and the existing employment arrangements are
inconsistent; the existing employment arrangements shall not be terminated in
their entirety upon this agreement becoming operative unless inconsistent with
this agreement in their entirety.

          (g)  Paragraph Headings.  The paragraph headings in this agreement are
               ------------------                                               
for convenience only; they form no part of this agreement and shall not affect
its interpretation.

          (h)  Gender, Etc.  Words used herein, regardless of the number and
               -----------                                                  
gender specifically used, shall be deemed and construed to include any other
number, singular or plural, and any other gender, masculine, feminine or neuter,
as the context indicates is appropriate.

          (i)  Number of Days.  In computing the number of days for purposes of
               --------------                                                  
this agreement, all days shall be counted, including Saturdays, Sundays and
holidays; provided, however, that if the final day of any time period (including
the effective date of a notice or other communication given hereunder) falls on
a Saturday, Sunday or holiday on which federal banks are or may 

                                      -44-
<PAGE>
 
elect to be closed, then the final day shall be deemed to be the next day which
is not a Saturday, Sunday or such holiday.

          (j) Withholding.  The Company shall have the right, in connection with
              -----------                                                       
any payments made to the Executive pursuant to this agreement, to withhold, or
require the Executive to pay to the Company, an amount sufficient to provide for
taxes required to be withheld by law.

          The Company, intending to be legally bound hereby, has duly executed
and delivered this agreement in the Commonwealth of Pennsylvania as of the date
first written above.  If you are in agreement with all of the foregoing, please
execute and deliver the enclosed copy of this agreement to the Company no later
than ten days from the date hereof.

                              U.S. BIOSCIENCE, INC.


                              By /s/ Robert I. Kriebel
                                ----------------------------
                                    Sr. Vice President
                                    Finance and Administration

          I agree, intending to be legally bound hereby.


                              /s/ C. Boyd Clarke
                              ------------------------------
                              C. Boyd Clarke

                                      -45-