<PAGE>
 
                                 EXHIBIT 10.12



                             EMPLOYMENT AGREEMENT
                             --------------------

          THIS EMPLOYMENT AGREEMENT, (the "Agreement") entered into on September
4, 1997, by and between AmeriSource Corporation, a Delaware corporation
("Company"), and R. David Yost (Executive"), effective as of August 1, 1997.

          WHEREAS, the Executive is presently employed by Company as President
and CEO;

          WHEREAS, the Executive is willing to continue to serve as President
and CEO, of the Company, and the Company desires to retain the Executive in such
capacity on the terms and conditions herein set forth;

          NOW THEREFORE, in consideration of the mutual covenants herein
contained, the parties hereto hereby agree as follows:

          1.   Employment.  The Company agrees to continue to employ the 
               ----------                                                 
Executive, and the Executive agrees to continue to be employed by the Company,
upon the terms and conditions hereinafter provided for a period commencing as of
August 1, 1997 and continuing until July 31, 2000 (the "Term"). On each
anniversary date of this Agreement, commencing August 1, 1998, on which
Executive is employed by the Company, the Term shall be automatically extended
for one additional year, provided, however, that the Term shall not extend
beyond Executive's 65th birthday unless affirmatively extended in writing by the
Company.

          2.   Position and Duties.  During the Term, the Company agrees to
               -------------------
employ the Executive to serve in such executive capacities at levels comparable
to or greater than his current position, and the Executive will have such powers
and duties as are commensurate with such positions and as may be assigned to him
by the Company's Board of Directors or other appropriate supervising officer
after consultation with the Executive. During the Term, and except for illness
or incapacity and reasonable vacation periods of no more than four weeks in any
calendar year (or such other period as shall be consistent with the Company's
policies for other key executives), the Executive shall devote all of his
business time, attention, skill and efforts exclusively to the business and
affairs of the Company and its subsidiaries and affiliates, provided, however,
that the Executive may serve on other boards as a director or trustee if such
service does not interfere with his ability to discharge his duties and
responsibilities to the Company.

<PAGE>
 
          3.   Compensation.  For all services rendered by the Executive in
               ------------
any capacity required hereunder during the Term, including, without limitation,
services as an executive, officer, director, or member of any committee of the
Company, or any subsidiary, affiliate or division thereof, the Executive shall
be compensated as follows:

               (a)  Base Salary. The Company shall pay the Executive a fixed
                    -----------
salary of $350,000 per annum or such higher annual amount as is being paid from
time to time pursuant to the terms hereof ("Base Salary"). The Base Salary shall
be subject to such periodic review (which shall occur in accordance with Company
policy) and such periodic increases as the Board of Directors shall deem
appropriate in accordance with the Company's customary procedures and practices
regarding the salaries of senior officers, provided, that the annual adjustment
                                           --------
shall be no less than the increase in the "Consumer Price Index" now known as
the Consumer Price Index for Urban Wage Earners and Clerical Workers, All Items,
Philadelphia, Pennsylvania (1967=100), as published by the Bureau of Labor,
Statistics, United States Department of Labor. Base Salary shall be payable in
accordance with the customary payroll practices of the Company, but in no event
less frequently than monthly.

               (b)  Bonus Awards.  The Executive shall be entitled to receive an
                    ------------
annual incentive cash compensation award under the Company's policy of providing
for the payment of incentive cash compensation to key officers based upon the
performance of the Company and the officer's individual performance.

               (c)  Stock Options.  The Executive shall be eligible to receive
                    -------------                                              
grants under the Company's stock option plan(s) at the sole discretion of the
plan's committee subject to such terms and conditions as such committee may
decide.

               (d)  Supplemental Insurance.  The Executive shall be entitled to
                    ----------------------
such split dollar life insurance policy as was in place between the Company and
the Executive prior to December 31, 1996.

               (e)  Additional Benefits.  Except as modified by this Agreement,
                    -------------------
the Executive shall be entitled to participate in all compensation or employee
benefit plans or programs (other than termination pay programs), and to receive
all benefits, perquisites and emoluments, for which any salaried employees of
the Company are eligible under any plan or program now or hereafter established
and maintained by the Company for senior officers, to the fullest extent
permissible under the general terms and provisions of such plans or programs and
in accordance with the provisions thereof, including group hospitalization,
health, dental care, life or other insurance, tax-qualified pension, savings,
thrift and profit-sharing plans, sick-leave plans, travel or accident insurance,
disability insurance, automobile allowance or automobile lease plans, and
executive contingent compensation plans, including, without limitation, capital

                                      -2-

<PAGE>
 
accumulation programs and stock purchase, restricted stock and stock option
plans.

Notwithstanding the foregoing, nothing in this Agreement shall preclude the
amendment or termination of any such plan or program, provided that such
amendment or termination is applicable generally to the senior officers of the
Company or any subsidiary or affiliate.

               (f)  Perquisites. Executive shall be entitled for each partial or
                    -----------
complete year this Agreement is in effect to reimbursement of tax planning and
tax preparation charges, not to exceed $5,000 annually, and to reimbursement of
club dues, not to exceed $10,000 annually.

          4.   Business Expenses.  The Company shall pay or reimburse the 
               -----------------                                            
Executive for all reasonable travel or other expenses incurred by the Executive
(and his spouse where there is a legitimate business reason for his spouse to
accompany him) in connection with the performance of his duties and obligations
under this Agreement, including, without limitation, expenses for entertainment,
travel (including automobile operating expenses), meals, hotel accommodations
and the like, in accordance with such rules and policies relating thereto as the
Company may from time to time adopt. Reimbursement shall be subject to the
Executive's presentation of appropriate vouchers in accordance with such
procedures as the Company may from time to time establish for senior officers
and to preserve any deductions for Federal income taxation purposes to which the
Company may be entitled.

          5.   Effect of Termination of Employment Other Than in Connection with
               -----------------------------------------------------------------
a Change in Control.
-------------------

               (a)  Certain Terminations. In the event the Executive's
                    --------------------
employment hereunder terminates due to either Permanent Disability, a Without
Cause Termination, or a Constructive Discharge, the Company shall, as severance
pay, continue, subject to the provisions of Section 7 below, to pay the
Executive's Base Salary (determined in accordance with the Company's usual
procedures) as in effect at the time of such termination until the expiration of
the Term or for one-year period beginning on the date of termination of
employment, if longer (the "Severance Period"), provided, that in the case of
                                                --------
Permanent Disability, such payments shall be offset by any amounts otherwise
paid to the Executive under the Company's disability program generally available
to other employees. In addition, earned but unpaid Base Salary as of the date of
termination of employment shall be payable in full. Group hospitalization,
health, dental care, life or other insurance, travel or accident insurance,
disability insurance and the perquisites set forth in Section 3(d) and Section
3(f) shall continue through the end of the Severance Period.

                                      -3-
<PAGE>
 
               (b)  Other Terminations. In the event that the Executive's
                    ------------------
employment hereunder terminates due to a Termination for Cause or Executive's
death, or the Executive voluntarily terminates employment with the Company for
reasons other than a Constructive Discharge or Permanent Disability (with
voluntary retirement being a voluntary termination for purposes of this
Agreement), earned but unpaid Base Salary as of the date of termination of
employment shall be payable in full. In the event of Executive's death, Company
shall also continue to pay Executive's Base Salary to his surviving spouse for a
period of six months (or to his estate, if he is not survived by a spouse), and
shall pay to her (or the estate) the bonus award in Section 3(b), prorated to
the date of Executive's death. However, no other payments shall be made, or
benefits provided, by the Company under this Agreement except for stock options
to the extent already exercisable hereunder, vested benefits payable under the
terms of the Pension Plan and Supplemental Employee Retirement Plan, and any
other benefits which the Executive is entitled to receive under the term of
employee benefit programs maintained by the Company or its affiliates for its
employees.

               (c)  Definitions.  For purposes of this Agreement, the following
                    -----------
terms have the following meanings:

                    (i)  The term "Termination for Cause" means (A) termination
          of Executive's employment for willful or gross and repeated neglect of
          duties hereunder, or willful or gross and repeated misconduct in the
          performance of such duties, so as to cause material harm to the
          Company and its subsidiaries considered as a whole, determined in good
          faith by the Board of Directors; (B) termination following a judicial
          determination that Executive has committed fraud, misappropriation or
          embezzlement against the Company; or (C) termination due to
          Executive's having committed any felony or misdemeanor for which he is
          convicted and which, as determined in good faith by the Board of
          Directors, constitutes a crime involving moral turpitude and results
          in material harm to the Company and its subsidiaries considered as a
          whole.

                    (ii)  The term "Constructive Discharge" means a termination
          of the Executive's employment by the Executive due to a failure of the
          Company or its successors without the prior consent of the Executive
          to fulfill the obligations under this Agreement in any material
          respect, including (A) any failure of the Company or Parent to elect
          or reelect or of the Company to appoint or reappoint the Executive as
          an executive officer of the Company, or (B) any other material change
          by the Company in the functions, duties or responsibilities of the
          Executive's position with the Company which would, in Executive's
          reasonable judgment, reduce the ranking or level, dignity,
          responsibility, importance or scope of such

                                        -4-
<PAGE>
 
          position (other than a change after a Change in Control Event
          resulting solely from the fact that the Company is a subsidiary of
          another entity), or (C) any non-payment or reduction in the Base
          Salary then in effect or any material breach by the Company of this
          Agreement, or (D) following a Change in Control Event, any relocation
          of Executive's site of employment to a location more than 50 miles
          away from Executive's site of employment on the date of this
          Agreement.

                    (iii) The term "Without Cause Termination" means a
          termination of the Executive's employment by the Company, upon 30 days
          notice to the Executive, other than due to Permanent Disability,
          voluntary retirement or expiration of the Term, and other than a
          Termination for Cause.

                    (iv)  The term "Permanent Disability" means the inability of
          the Executive to work for a period of six full calendar months during
          any eight consecutive calendar months due to illness or injury of a
          physical or mental nature, supported by the completion by the
          Executive's attending physician of a medical certification form
          outlining the disability and treatment.

          6.   Effect of Termination of Employment in Connection with a Change
               ---------------------------------------------------------------
in Control.
---------- 

               (a)  Definitions.  For purposes of this Section 6, the
                    -----------
following terms shall have the following meanings:

                    (i)  The term "Change in Control Event" means any of the
          following events:

                         (A)  the acquisition by any "person" or "group" within
               the meaning of Section 13(d) and 14(d) of the Securities Exchange
               Act of 1934, as amended (the "1934 Act") (other than the Company,
               its affiliates and benefit plans sponsored by the Company or its
               affiliates, and other than 399 Venture Partners, Inc. ("VPI") and
               its Affiliates (as defined in Rule 12b-2 under the 1934 Act) of
               "beneficial ownership" of securities of Parent representing more
               than 35% of the total aggregate voting power of Parent's then
               outstanding securities entitled to vote generally in the election
               of directors, and such person or group owns more aggregate voting
               power of Parent's then outstanding securities entitled to vote
               generally in the election of directors than any other person or
               group.

                                      -5-
<PAGE>
 
                         (B)  consummation of a transaction following the
               approval by the stockholders of Company or Parent of (1) any
               consolidation, merger, or other similar type of transaction
               involving the Company or Parent in which all of the holders of
               voting stock of the Company or Parent immediately before the
               consolidation, merger, or similar transaction, will not own 50%
               or more of the voting shares of the continuing or surviving
               corporation immediately after such consolidation, merger, or
               similar transaction, or (2) any sale, lease, exchange or other
               transfer (in one transaction or a series of related transactions)
               of all or substantially all of the assets of Company or Parent;
               or

                         (C) a change of 25% (rounded to the next whole person)
               in the membership of the Board of Directors of Company or Parent
               or any successor within a 12-month period, unless the election or
               nomination for election by stockholders of each new director
               within such period was approved by the vote of 85% (rounded to
               the next whole person) of the directors then still in office who
               were in office at the beginning of the 12-month period.

                    (ii)   "Separation Period" means the three-year period
          beginning on the date of the Executive's Termination of Employment.

                    (iii)  "Termination of Employment" shall mean the
          termination of the Executive's actual employment relationship with the
          Company.

                    (iv)   "Termination upon a Change of Control" shall mean a
          termination of Employment upon or within fifteen months after a
          Change of Control Event either:

                         (A)  initiated by Company for any reason other than (1)
               the Executive's death, or (2) Termination for Cause, or

                         (B)  initiated by the Executive (i) due to a 
               Constructive Discharge during the one-year period following the
               Change in Control Event, or (ii) for any reason during the period
               beginning twelve months and one day following the Change in
               Control Event and ending 90 days thereafter.

                                      -6-
<PAGE>
 
               (b)  Payments for Termination upon a Change in Control.  Within
                    -------------------------------------------------
twenty days of the Executive's Termination upon a Change in Control, the Company
shall pay to the Executive in a single payment in cash and/or provide to the 
Executive, as applicable, the following:

                    (i)    the Executive's earned but unpaid Base Salary as of 
          the date of Termination of Employment;

                    (ii)   the benefits, if any, to which the Executive is 
          entitled as a former employee under the employee benefit programs and
          compensation plans and programs maintained for the benefit of the
          Company's officers and employees;

                    (iii)  continued group hospitalization, health, dental 
          care, life or other insurance, travel or accident insurance and
          disability insurance, for the Separation Period, with coverage
          equivalent to the coverage to which the Executive would have been
          entitled had the Executive continued working for the Company during
          the Separation Period at the highest annual rate of Base Salary
          achieved during the Executive's period of actual employment with the
          Company, provided, however, that the Executive may upon written notice
          elect to receive the present value of such coverage in cash in a lump
          sum, computed using a discount rate of 6% per year compounded monthly;
          and

                    (iv)   an amount equal to the Base Salary and annual bonus
          the Executive would have earned if the Executive had continued working
          for the Company during the Separation Period at the highest annual
          rate or Base Salary, and received the highest annual percentage bonus,
          achieved during the Executive's period of actual employment with the
          Company.

               (c)  In the event that, on or after the occurrence of a Change in
Control Event, the Company fails to make any payment or provide any coverage to 
Executive arising out of or relating in any way to this Agreement or to the 
Executive's employment by the Company (collectively, "Employment Rights"), 
then the Company shall pay to the Executive and reimburse the Executive for the 
Executive's full costs (including, without limitation, the fees and expenses of 
the Executive's attorneys and court and related costs) of enforcing the 
Executive's Employment Rights.

               (d)  Notwithstanding anything under this Agreement to the 
contrary, in the event that any payments or benefits under this Agreement 
(taking into account payments under any other agreement or arrangement with the 
Executive) would be considered "excess parachute payments" within the meaning of
Section 280G(b)(1) of 

                                      -7-
<PAGE>
 
the Internal Revenue Code of 1986, as amended (the "Code"), then the aggregate 
present value of benefits or amounts payable under this Agreement ("Agreement 
Payments") 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 Agreement Payments without causing any Agreement
Payments to be an excess parachute payment. For purposes of this Section 7, 
present value shall be determined in accordance with section 280G(d)(4) of the 
Code.

               (e)  All determinations under this Section 7 shall be made by a
national accounting firm selected by the Company (the "Accounting Firm"), which
shall provide detailed supporting calculations both to the Company and to the
Executive within 30 business days of the date the Executive's employment with
the Company terminates or such earlier time as is requested by the Company and
an opinion to the Executive that the Executive has substantial authority not to
report any excise tax on Executive's federal income tax return with respect to
Agreement Payments. Any such determination by the Accounting Firm shall be
binding on the Company and the Executive. Executive shall determine which of the
Agreement Payments shall be eliminated or reduced consistent with the
requirements of this Section 7, provided that, if the Executive does not make
such determination with 10 business days of the receipt of the calculation from
the Accounting Firm, the Company shall elect which and how much of the Agreement
Payments shall be eliminated or reduced consistent with the requirements of this
Section 7 and shall notify Executive promptly of such election. Within 5
business days thereafter, the Company shall pay to the Executive such amounts or
benefits that are then due to the Executive.

     7.   Other Duties of Executive During and After Term.
          -----------------------------------------------

          (a)  Confidential Information. Executive acknowledges that by reason 
               ------------------------
of his employment with the Company he has and will hereafter, from time to time
during the Employment Term, become exposed to an/or become knowledgeable about
proposals, plans inventions, practices, systems, programs, formulas, processes,
methods, techniques, research, records, supplier sources, customer lists, and
other forms of business information which are not known to the Company's
competitors and which are not recognized as being encompassed within standard
business or management practices and which are kept secret and confidential by
the Company (the "Confidential Information"). Executive therefore agrees that at
no time during or after the period of his employment by the Company will he
disclose or use the Confidential Information except as may be required in the
prudent course of business for the benefit of the Company; provided, that no
payment required to be made by the Company under the terms of this Agreement
after termination of the employment of Executive shall be subject to any right
of set-off, counterclaim, defense, abatement, suspension, deferment or reduction
by reason of any claim against Executive based upon breach of covenant

                                      -8-
<PAGE>
 
in this Section 7(a) other than upon execution of an unsatisfied judgment 
rendered by a court of competent jurisdiction.

          (b)  Non-Compete. In consideration of the mutual terms and agreements 
               -----------
set forth in this Agreement and the stock options that have been and may be 
granted under the AmeriSource Health Corporation 1995 Stock Option Plan and 1996
Stock Option Plan, Executive hereby agrees that (i) while Executive is employed 
during the Employment Term, (ii) during such time after the Employment Term as 
Executive is employed by the Company, (iii) while Executive is receiving 
payments of Base Salary or benefits pursuant to Sections 5(a) or 6 hereof, and 
(d) for a period of one year after Executive's termination of employment, he 
will not, unless authorized in writing to do so by the Company, directly or 
indirectly own, manage, operate, join, control or participate in the ownership, 
management, operation or control of, or be employed or otherwise connected in 
any substantial manner with any business which directly or indirectly competes 
to a material extent with any line of business of the Company or its 
subsidiaries; provided, that nothing in this paragraph shall prohibit Executive
              --------
from acquiring up to 5% of any class of outstanding equity securities of any 
corporation whose equity securities are regularly traded on a national 
securities exchange or in the "over-the-counter market." Executive agrees that 
for a period ending one year after Executive's termination of employment 
hereunder, Executive will not (x) recruit any employee of the Company or solicit
or induce, or attempt to solicit or induce, any employee of the company to 
terminate his or her employment with, or otherwise cease his or her relationship
with, the Company, or (y) solicit, divert or take away, or attempt to solicit, 
divert or take away, the business or patronage of any of the clients, customers 
or accounts, or prospective clients, customers or accounts, of the Company that 
were contracted, solicited or served by the Executive while employed by the 
Company.

          (c)  Remedies. The Company and Executive confirm that the restrictions
               --------
contained in Sections 7(a) and 7(b) hereof are, in view of the nature of the 
business of the Company, reasonable and necessary to protect the legitimate 
interests of the Company and that any violation of any provision of Section 7(a)
or 7(b) will result in irreparable injury to the Company. Executive hereby 
agrees that, in the event of any breach or threatened breach of the terms or 
conditions of this Agreement by Executive, the Company's remedies at law will be
inadequate and, in any such event, the Company shall be entitled to commence an
action for preliminary and permanent injunctive relief and other equitable 
relief in any court of competent jurisdiction. Executive further irrevocable 
consents to the jurisdiction of any Pennsylvania state court or federal court 
located in the Commonwealth of Pennsylvania over any suit, action or proceeding 
arising out of or relating to this Section 7(c) and hereby waives, to the 
fullest extent permitted by law, any objection that he may now or hereafter have
to such jurisdiction or to the laying or venue of any such suit, action or 
proceeding brought in such a court and any claim that such suit, action or 
proceeding has been brought in an inconvenient forum.

                                      -9-
<PAGE>
 
The Executive's Agreement as set forth in this Section 7 shall: (x) survive the 
termination of this Agreement, and continue throughout the duration of the 
Executive's employment with the Company, except as amended or modified by 
written agreement of the parties; and (y) survive the Executive's termination of
employment with the Company.

          (d)  Modification of Terms. If any restriction in this Section 7 of 
               ---------------------
the Agreement is adjudicated to exceed the time, geographic, service or other 
limitations permitted by applicable law in any jurisdiction, the Executive 
agrees that such may be modified and narrowed, either by a court or the Company,
to the maximum time, geographic, service or other limitations permitted by 
applicable law so as to preserve and protect the Company's legitimate business 
interest, without negating or impairing any other restrictions or undertaking 
set forth in the Agreement.

     8.   Withholding Taxes. The Company may directly or indirectly withhold 
          -----------------
from any payments made under this Agreement all Federal, state, city or other 
taxes as shall be required pursuant to any law or governmental regulation or 
ruling.

     9.   Vesting of Stock Options. In the event of Executive's Without Cause 
          ------------------------
Termination or Constructive Discharge after September 5, 1999 but prior to a 
Change in Control Event, then any portion of Executive's Company stock options 
which have been granted to Executive and are then outstanding but not then 
exercisable pursuant to the terms of such stock options shall become immediately
exercisable to the extent that such stock options would have been exercisable on
or before two years subsequent to the date of termination of employment, had 
such Without Cause Termination or Constructive Discharge not occurred, and such 
stock options shall remain exercisable after the applicable termination date for
a period of 30 days. Upon the occurrence of a Change in Control Event, each of 
Executive's Company stock options then outstanding shall become immediately 
exercisable to the full extent of the shares of Common Stock subject thereto in
accordance with the terms of the applicable stock option plans and agreement.

     10.  Consolidation, Merger, or Sale of Assets. Nothing in this Agreement 
          ----------------------------------------
shall preclude the Company from consolidating or merging into or with, or 
transferring all or substantially all of its assets to, another corporation 
which assumes this Agreement and all obligations and undertakings of the Company
hereunder. Upon such a consolidation, merger or transfer of assets and 
assumption, the term "Company" as used herein shall mean such other corporation 
and this Agreement shall continue in full force and effect.

     11.  Notices. All notices, requests, demands and other communications 
          -------
required or permitted hereunder shall be given in writing and shall be deemed to
have been duly given if delivered or mailed, postage prepaid, by same day or 
overnight mail as follows:

                                     -10-
<PAGE>
 
          (a)  To Company

               300 Chester Field Parkway
               Malvern PA 19355


          (b)  To the Executive:

               165 N. Stanbery Avenue               
               Columbus, OH 43209

or to such other address as either party shall have previously specified in 
writing to the other.

     12.  No Attachment. Except as required by law, no right to receive payments
          -------------
under this Agreement shall be subject to anticipation, commutation, alienation, 
sale, assignment, encumbrance, charge, pledge, or hypothecation or to execution,
attachment, levy, or similar process or assignment by operation or law, and any 
attempt, voluntary or involuntary, to effect any such action shall be null, void
and of no effect; provided, however, that nothing in this Section 12 shall 
                  --------  -------
preclude the assumption of such rights by executors, administrators or other 
legal representatives of the Executive or his estate and their assigning any 
rights hereunder to the person or persons entitled thereto.

     13.  No Mitigation. The Executive shall not be required to mitigate the
          -------------
amount of any payment or benefit provided for in this Agreement by seeking other
employment or otherwise, nor shall the amount of any payment or benefit provided
for in this Agreement be reduced by any compensation earned by other employment
or otherwise.

     14.  Source of Payment. All payments provided for under this Agreement 
          -----------------
shall be paid cash from the general funds of the Company. The Company shall not 
be required to establish a special or separate fund or other segregation of 
assets to assure such payments, and, if the Company shall make any investments 
to aid it in meeting its obligations hereunder, the Executive shall have no 
right, title or interest whatever in or to any such investments except as may 
otherwise be expressly provided in a separate written instrument relating to 
such investments. Nothing contained in this Agreement, and no action taken
pursuant to its provisions, shall create or be construed to create a trust of
any kind, or a fiduciary relationship, between the Company and the Executive or
any other person. To the extent that any person acquires a right to receive
payments from the Company hereunder, such right, without prejudice to rights
which employees may have, shall be no greater than the right of an unsecured
creditor of the Company.

     15.  Severability. If any provision of this Agreement of application 
          ------------
thereof to 

                                     -11-



         
<PAGE>
 
anyone or under any circumstances is adjudicated to be invalid or unenforceable 
in any jurisdiction, such invalidity or unenforceability shall not affect any 
other provision or application and shall not invalidate or render unenforceable 
such provision or application in any other jurisdiction.

     16.  Contents of Agreement. This Agreement supersedes all prior agreements 
          ---------------------
and sets forth the entire understanding among the parties hereto with respect to
the subject matter hereof and cannot be changed, modified, extended or 
terminated except upon written amendment approved by the parties hereto.

     17.  Governing Law. The validity, interpretation, performance, and 
          -------------
enforcement of this Agreement shall be governed by the laws of the Commonwealth 
of Pennsylvania, and Executive consents to the jurisdiction of the state and 
federal courts of Pennsylvania in any dispute arising under this Agreement.

     18.  Survival of Benefits. Any Section of this Agreement which provides a 
          --------------------
benefit to the Executive and which does not expressly provide for its 
termination upon the expiration of the Term shall survive the expiration of the 
Term and the obligation to provide benefits to the Executive as set forth in 
such Section shall remain binding upon the Company until such time as the 
Executive's employment relationship with the Company is terminated and the 
benefits provided under such Section are paid in full to the Executive.

     19.  Miscellaneous. All section headings are for convenience only. This 
          -------------
Agreement may be executed in any number of counterparts, each of which when
executed shall be deemed to be an original and all of which together shall be
deemed to be one and the same instrument. It shall not be necessary in marking
proof of this Agreement or any counterpart hereof to produce or account for any
of the other counterparts.

     20.  Arbitration. Except with respect to actions for preliminary and 
          -----------
permanent injunctive relief and other equitable relief under Section 7, any
controversy or claim arising out of or relating to this Agreement, or breach
thereof, shall be settled by arbitration in accordance with the rules any of the
American Arbitration Association, and judgment upon such award rendered by the
arbitrator(s) may be entered in any court having jurisdiction thereof. The
arbitration shall be held in Philadelphia, Pennsylvania, unless another location
shall be mutually agreed to by the parties at the time of the arbitration. If
Section 6(d) is not applicable, in any dispute between the parties as to which
Executive is sustained on the claim(s) by or against him, the Company shall pay
all legal fees incurred by Executive in connection with the dispute over such
claim(s). If more than one claim is involved in any dispute and if Executive is
sustained as to one or more of such claims but not as to all of such claims,
there shall be a reasonable

                                     -12-

<PAGE>
 
allocation of applicable legal expenses. The Company will reimburse Executive 
for those legal expenses determined by the arbitrator(s) or by the consent of 
the parties to be allocable to the claim or claims as to which Executive is 
upheld.

     IN WITNESS WHEREOF, and intending to be legally bound, the Company has 
caused this Agreement to the executed by its duly authorized officers and the 
Executive has signed this Agreement this 4th day of September, 1997, effective 
as of the day and year first above written.

                                        AMERISOURCE CORPORATION


                                        By: [SIGNATURE ILLEGIBLE]
                                           ---------------------------
                                        Name:_________________________
                                        Title:________________________



                                          /s/ R. David Yost
                                        ------------------------------  
                                         R. David Yost

                                     -13-