Exhibit 99.3

                            VOTING/SUPPORT AGREEMENT
                            ------------------------

                                        

     AGREEMENT, dated as of September 22, 1997 (this "Agreement"), among
McKesson Corporation, a Delaware corporation ("McKesson"), Patriot Acquisition
Corp., a Delaware corporation ("Merger Sub") and 399 Venture Partners, Inc. (the
"Stockholder").

     WHEREAS, the Board of Directors of McKesson and the Board of Directors of
AmeriSource (capitalized terms used but not defined herein having the respective
meanings given to them in the Merger Agreement) have approved an Agreement and
Plan of Merger dated as of even date herewith (the "Merger Agreement") providing
for the merger of a wholly owned subsidiary of McKesson with and into
AmeriSource;

     WHEREAS, the Stockholder is the record and beneficial owner of shares of
AmeriSource Class A Stock, AmeriSource Class B Stock and/or AmeriSource Class C
Stock in the amounts and of the class set forth opposite the Stockholder's name
on Annex A hereto (the "Shares");

     WHEREAS, Stockholder currently has certain registration rights in the
Shares pursuant to a Registration Rights Agreement dated March 30, 1995, by and
between Stockholder and AmeriSource (the "1995 Registration Rights Agreement");

     WHEREAS, McKesson and the Stockholder are entering into an agreement with
respect to the registration rights which will be accorded the shares of McKesson
Common Stock into which the Shares will be converted in the Merger (the
"Registration Rights Agreement"), which agreement upon its execution by the
parties will supersede the 1995 Registration Rights Agreement; and

     WHEREAS, as a condition to McKesson's entering into the Merger Agreement,
McKesson has required that the Stockholder agree, and the Stockholder has
agreed, to enter into this Agreement.

     NOW, THEREFORE, to induce McKesson to enter into the Merger Agreement and
the Registration Rights Agreement and in consideration of the premises herein
contained, the parties agree as follows:

     1.  Grant of Irrevocable Proxy.  (a)  Until this Agreement is terminated,
         --------------------------                                           
the Stockholder hereby irrevocably appoints Merger Sub, its officers, agents and

 
nominees, with full power of substitution, as proxy for and attorney in fact of
the Stockholder to act with respect to and vote the Shares owned by the
Stockholder for and in the name, place and stead of the Stockholder at any
annual, special or other meeting of the holders of shares of AmeriSource Common
Stock and at any adjournment or postponement thereof or pursuant to any written
consent in lieu of a meeting, to the fullest extent that the Shares are entitled
to be voted on any matter which may come before such meeting or which may be the
subject of such written consent, (i) in favor of the Merger, the Merger
Agreement and the transactions contemplated thereby (but not any Material
Adverse Amendments (as defined below) to the Merger Agreement), (ii) against any
Competing Transaction, (iii) against any action or agreement the purpose or
effect of which would be to impede, interfere with or attempt to discourage the
Merger, and (iv) against any action the taking of which would constitute a
breach by AmeriSource of any of its representations, warranties, covenants or
agreements contained in the Merger Agreement or in the AmeriSource Stock Option
Agreement; provided that such proxy may not be used to frustrate AmeriSource's
ability to terminate the Merger Agreement in accordance with the provisions of
Section 7.1(c) of the Merger Agreement.  In all other matters, the Shares shall
be voted by and in the manner determined by the Stockholder upon written notice
to Merger Sub.  The Stockholder hereby represents that it has not heretofore
granted any irrevocable proxy with respect to its Shares and hereby revokes any
and all proxies which may heretofore have been granted with respect to the
Shares.  As used herein, a "Material Adverse Amendment" is an amendment that (i)
materially and adversely affects the Stockholder and (ii) is approved by
AmeriSource's Board of Directors notwithstanding the fact that in such vote the
Stockholder's nominees on AmeriSource's Board of Directors voted against such
amendment.

          (b) The Stockholder understands and acknowledges that McKesson is
entering into the Merger Agreement in reliance upon the Stockholder's execution
and delivery of this Agreement.  The Stockholder hereby affirms that the
irrevocable proxy set forth in this Section 1 is given in connection with and as
an inducement for the execution by McKesson of the Merger Agreement and the
Registration Rights Agreement and to secure the performance of the duties of the
Stockholder under this Agreement.  The Stockholder hereby further affirms that
the irrevocable proxy is coupled with an interest and may not be revoked.  The
Stockholder hereby ratifies and confirms all that such proxy may lawfully do or
cause to be done by virtue hereof.  This proxy is executed and intended to be
irrevocable in accordance with the provisions of Section 212 of the DGCL.

                                       2

 
     2.  Agreement Not to Transfer.  The Stockholder agrees that it will not at
         -------------------------                                             
any time during the term of this Agreement sell, transfer, assign or otherwise
dispose of ("Transfer") or pledge or otherwise encumber, or enter into any
contract, option or other arrangement with respect to the Transfer, pledge or
encumbrance of, any of the Shares, or grant or purport to grant to any person
any proxy or voting right or any right to acquire any of the Shares, or enter
into any voting agreement with any person with respect to any of the Shares, or
deposit any of the Shares into a voting trust.  The foregoing is in addition to
what will be the Stockholder's obligations under Section 3(d).

     3.  Additional Covenants of the Stockholder.  The Stockholder hereby
         ---------------------------------------                         
covenants and agrees with McKesson and Merger Sub that, until this Agreement
terminates:

          (a) The Stockholder will not at any time, directly or indirectly,
solicit, initiate, encourage or facilitate, or furnish or disclose non-public
information in furtherance of, any inquiries or the making of any proposal with
respect to any Competing Transaction, or authorize or permit any of its
affiliates that it controls ("Controlled Affiliates") or any of its or its
Controlled Affiliates' directors, officers, employees, agents or representatives
to so act, and will immediately cease all existing activities, discussions and
negotiations with any parties conducted heretofore with respect to any proposal
for a Competing Transaction.

          (b) The Stockholder shall take all actions necessary to call, or cause
AmeriSource to call, the AmeriSource Stockholders Meeting, in accordance with
the provisions of the Merger Agreement, and shall use its best efforts to cause
such meeting to be held and completed on the date scheduled for such meeting.

          (c) At the request of McKesson, the Stockholder shall take all actions
(including, without limitation, making or causing to be made all necessary
filings with all appropriate Governmental Authorities) necessary for the Shares
that are AmeriSource Class B Stock, or such portion thereof as McKesson may
request (but not in excess of the number of shares that would cause the
Stockholder to hold in excess of 19% of the outstanding voting securities of
AmeriSource), to be converted into AmeriSource Class A Stock.

          (d) The Stockholder will not, during the 30 days prior to the
Effective Time, sell, transfer or otherwise dispose of or reduce its risk (as
contemplated by the SEC Accounting Series Release No. 135) with respect to the
Shares or shares of Parent Common Stock that it may hold.  Provided that

                                       3

 
McKesson is not in breach of the provisions of Section 5(b), the Stockholder
will not sell, transfer or otherwise dispose of or reduce its risk (as
contemplated by SEC Accounting Series Release No. 135) with respect to any
Parent Common Stock received by it in the Merger or any other shares of Parent
Common Stock until after such time as combined financial results (including
combined sales and net income) covering at least 30 days of combined operations
of AmeriSource and Parent have been published by Parent, in the form of a
quarterly earnings report, an effective registration statement filed with the
Commission, a report to the Commission on Form 10-K, 10Q or 8-K, or any other
public filing or announcement which includes such combined results of
operations.

          (e) The Stockholder will deliver to McKesson at McKesson's request (i)
a written representation confirming, as of immediately prior to the Effective
Time, the accuracy of the representations and warranties contained in Section 4,
and (ii) such additional written representations as may be reasonably requested
by Dechert, Price & Rhoads or Fried, Frank, Harris, Shriver & Jacobson.

          (f) The Stockholder will not take any action which would jeopardize
qualification of the Merger as a reorganization within the meaning of Section
368(a) of the Code.

          (g) The Stockholder will execute the AmeriSource Affiliate Letter
promptly upon request therefor, which letter shall be in the form attached
hereto as Annex B.

     4.  Representations and Warranties of the Stockholder.  The Stockholder
         -------------------------------------------------                  
hereby represents and warrants to McKesson and Merger Sub that:

          (a) (i)  The Shares listed on Annex A attached hereto opposite the
Stockholder's name are the only shares of AmeriSource Common Stock owned of
record or beneficially by the Stockholder or in which the Stockholder has any
interest; (ii) such Shares are now and at all times during the term of this
Agreement will be owned by the Stockholder, free and clear of all liens, claims,
charges and encumbrances of any kind whatsoever except as contemplated by this
Agreement, and none of such Shares is subject to any voting trust or other
agreement or arrangement (except as created by this Agreement) with respect to
the voting of such Shares; and (iii) the Stockholder does not own any options to
purchase or rights to subscribe for or otherwise acquire any other shares of
AmeriSource Common Stock.

          (b) The Stockholder has full right, power and authority to execute and
deliver this Agreement and to perform all of its obligations hereunder and such

                                       4

 
execution, delivery and performance have been duly authorized by all requisite
corporate action of the Stockholder and no other corporate proceedings are
necessary therefor.

          (c) This Agreement has been duly and validly executed and delivered by
the Stockholder and represents a valid and legally binding obligation of the
Stockholder, enforceable against the Stockholder in accordance with its terms.

          (d) The execution, delivery and performance of this Agreement by the
Stockholder will not constitute a violation of, conflict with or result in a
default under (i) any contract, understanding or arrangement to which the
Stockholder is a party or by which the Stockholder is bound or require the
consent of any other person or any party pursuant thereto, (ii) any judgment,
decree or order applicable to the Stockholder, or (iii) any Applicable Law.

          (e) The Stockholder has no plan or present intention to sell, exchange
or otherwise dispose of a number of shares of McKesson Common Stock received in
the Merger which, together with sales, exchanges or other dispositions by other
stockholders of AmeriSource, would reduce the aggregate ownership of McKesson
Common Stock by stockholders of AmeriSource who receive McKesson Common Stock in
the Merger to a number of shares having a value, as of the Effective Time, of
less than fifty percent of the value of all of the shares of AmeriSource Common
Stock outstanding immediately prior to the Effective Time.

          (f) Based upon the representation set forth in Section 3.4 of the
Merger Agreement, and so long as such representation remains true and complete,
set forth on Annex C hereto is the maximum number of shares of AmeriSource Class
A Stock which the Stockholder may own, beneficially and of record, at any time
without violating any Applicable Laws.

     5.  Representations, Warranties and Covenants of McKesson and Merger Sub.
         --------------------------------------------------------------------  
(a)  McKesson and Merger Sub hereby represent and warrant to the Stockholder
that (i) McKesson and Merger Sub each have full corporate right, power and
authority to execute and deliver this Agreement and to perform its obligations
hereunder; (ii) such execution, delivery and performance have been duly
authorized by all requisite corporate action by McKesson and Merger Sub, and no
other corporate proceedings are necessary therefor; (iii) this Agreement has
been duly and validly executed and delivered by McKesson and Merger Sub and
represents a valid and legally binding obligation of McKesson and Merger Sub,
enforceable against McKesson and Merger Sub in accordance with its terms; and

                                       5

 
(iv) the execution, delivery and performance of this Agreement by McKesson and
Merger Sub will not constitute a violation of, conflict with or result in a
default under (A) any contract, understanding or arrangement to which either
McKesson or Merger Sub is a party or by which either is bound or require the
consent of any other person or any party pursuant thereto, (B) any judgment,
decree or order applicable to McKesson or Merger Sub, or (C) any Applicable Law.

          (b) McKesson hereby covenants that, if the Effective Time is less than
30 days prior to the end of McKesson's fiscal quarter or occurs during the first
30 days of McKesson's fiscal quarter, McKesson shall use reasonable efforts to
prepare and publicly release, as soon as practicable following the end of the
first month ending at least 30 days after the Effective Time, a report filed
with the Commission on Form 8-K or any other public filing, statement or
announcement which includes the combined financial results (including combined
sales and net income) of McKesson and Amerisource for a period of at least 30
days of combined operations of McKesson and Amerisource following the Effective
Time.

     6.  McKesson Board of Directors.  (a)  McKesson hereby covenants that, if
         ---------------------------                                          
the Merger is consummated, the Board of Directors of McKesson shall take all
action necessary immediately following the Effective Time to elect the following
persons to its Board of Directors:  (i) James Urry, who shall be assigned to the
class of directors whose term of office expires at McKesson's first annual
meeting of stockholders after the Effective Time, and (ii) Michael Delaney, who
shall be assigned to the class of directors whose term of office expires at
McKesson's second annual meeting of stockholders after the Effective Time.

          (b) If the Merger is consummated and if at any time the Stockholder
owns less than 60% of the shares of McKesson Common Stock received by it in the
Merger, then the Stockholder, upon the written request of McKesson, shall cause
James Urry promptly to resign from McKesson's Board of Directors, and (ii) if at
any time the Stockholder owns less than 30% of the shares of McKesson Common
Stock received by it in the Merger, then the Stockholder, upon the written
request of McKesson, shall cause Michael Delaney promptly to resign from
McKesson's Board of Directors.

     7.  Termination.  (a)  This Agreement, other than the obligations set forth
         -----------                                                            
in Sections 3(d), 3(f), 5(b) and 6, shall terminate at the Effective Time.

          (b) This Agreement shall terminate upon (i) the termination of the
Merger Agreement pursuant to its terms and (ii) the occurrence of a Material
Adverse Amendment.

                                       6

 
     8.  Severability.  Any term, provision, covenant or restriction contained
         ------------                                                         
in this Agreement held by a court or other Governmental Authority of competent
jurisdiction to be invalid, void or unenforceable shall be ineffective to the
extent of such invalidity, voidness or unenforceability, but neither the
remaining terms, provisions, covenants or restrictions contained in this
Agreement nor the validity or enforceability thereof in any other jurisdiction
shall be affected or impaired thereby.  Any term, provision, covenant or
restriction contained in this Agreement that is so found to be so broad as to be
unenforceable shall be interpreted to be as broad as is enforceable.

     9.  Expenses.  Each of the parties hereto shall pay all costs and expenses
         --------                                                              
incurred by it or on its behalf in connection with the transactions contemplated
hereunder, including fees and expenses of its own financial consultants,
investment bankers, accountants and counsel, except as otherwise provided
herein.

     10.  Entire Agreement.  This Agreement (including the documents and the
          ----------------                                                  
instruments referred to therein) constitutes the entire agreement between the
parties and supersedes all prior agreements and understandings, agreements or
representations by or between the parties, written and oral, with respect to the
subject matter hereof and thereof.

     11.  Successors; No Third Party Beneficiaries.  The terms and conditions of
          ----------------------------------------                              
this Agreement shall inure to the benefit of and be binding upon the parties
hereto and their respective successors and assigns.  Nothing in this Agreement,
expressed or implied, is intended to confer upon any party, other than the
parties hereto and their respective successors and assigns, any rights,
remedies, obligations, or liabilities under or by reason of this Agreement,
except as expressly provided herein.

     12.  Notices.  All notices and other communications hereunder shall be in
          -------                                                             
writing and shall be deemed given if delivered personally, telecopied (which is
confirmed) or dispatched by a nationally recognized overnight courier service to
the parties at the following addresses (or at such other address for a party as
shall be specified by like notice):

               (i)  if to McKesson or Merger Sub, to

                    One Post Street
                    San Francisco, CA  94104
                    Attn.:  Ivan D. Meyerson, Esq.
                    Telecopy No.: 415-983-8826

                                       7

 
                    with copies to

                    Fried, Frank, Harris, Shriver & Jacobson
                    One New York Plaza
                    New York, New York  10004
                    Attn.:  Stephen Fraidin, P.C.
                    Telecopy No.:  212-859-6140
 
               (ii) if to the Stockholder,

                    to the address set forth opposite its
                    name on Annex A.

                    with a copy to

                    Kirkland & Ellis
                    153 East 53rd Street
                    Suite 3900
                    New York, New York  10022
                    Attn.:  Kirk A. Radke, Esq.
                    Telecopy No.:  212-446-4900

     13.  Counterparts.  This Agreement may be executed in counterparts, and
          ------------                                                      
each such counterpart shall be deemed to be an original instrument, but both
such counterparts together shall constitute but one agreement.

     14.  Specific Performance. The parties hereto agree that if for any reason
          --------------------                                                 
McKesson or the Stockholder shall have failed to perform its obligations under
this Agreement, then the party hereto seeking to enforce this Agreement against
such non-performing party shall be entitled to specific performance and
injunctive and other equitable relief, and the parties hereto further agree to
waive any requirement for the securing or posting of any bond in connection with
the obtaining of any such injunctive or other equitable relief.  This provision
is without prejudice to any other rights that any party hereto may have against
any other party hereto for any failure to perform its obligations under this
Agreement.

     15.  Governing Law.  This Agreement shall be governed by the laws of the
          -------------                                                      
State of Delaware, without giving effect to the conflict of laws principles
thereof.  Each party hereby irrevocably and unconditionally consents to submit
to the exclusive jurisdiction of the courts of the State of Delaware and of the
United States of America located in the State of Delaware, for any Action (and
agrees not 

                                       8

 
to commence any Action except in any such court), and further agrees that
service of process, summons, notice or document by U.S. registered mail to its
respective address set forth in Section 12 shall be effective service of process
for any Action brought against it in any such court. Each party hereby
irrevocably and unconditionally waives any objection to the laying of venue of
any Action in the courts of the State of Delaware or of the United States of
America located in the State of Delaware, and hereby further irrevocably and
unconditionally waives and agrees not to plead or claim in any such court that
any Action brought in any such court has been brought in an inconvenient forum.

     16.  Waiver and Amendment.  Any provision of this Agreement may be waived
          --------------------                                                
at any time by the party that is entitled to the benefits of such provision.
This Agreement may not be modified, amended, altered or supplemented except upon
the execution and delivery of a written agreement executed by the parties
hereto.

     17.  Additional Shares.  Notwithstanding the provisions of Section 16, in
          -----------------                                                   
the event that the Stockholder acquires any additional shares of AmeriSource
Common Stock, such shares shall, without further action of the parties, be
subject to the provisions of this Agreement (including, without limitation,
Sections 3(c) and 4(f)), and Annex A will be deemed amended accordingly.  If the
Stockholder acquires additional shares of AmeriSource Common Stock, it shall
promptly notify McKesson in writing of such acquisition.

                                       9

 
     IN WITNESS WHEREOF, the parties have caused this agreement to be duly
executed as of the date first above written.

                                McKESSON CORPORATION


                                By:  __________________________


                                PATRIOT ACQUISITION CORP.


                                By:  __________________________


                                399 VENTURE PARTNERS, INC.


                                By:  __________________________


175517

                                       10

 
                                    ANNEX A



                                                  Securities (in number of shares)
Record and                                        --------------------------------   Percentage
Beneficial Owner                                   Class A     Class B    Class C   Voting Power
- ----------------------------------------          ---------  -----------  --------  -------------
                                                                        
            1.  399 Venture Partners, Inc.        234,926    6,486,147    None          1.4%
                c/o Helene Shavin
                399 Park Avenue
                14/th/ Floor Zone 4
                NY, NY  10022-4614




                                       11

 
ANNEX B


                      FORM OF AMERISOURCE AFFILIATE LETTER

McKesson Corporation
One Post Street
San Francisco, California  94104

Ladies and Gentlemen:

     I have been advised that as of the date of this letter I may be deemed to
be an "affiliate" of AmeriSource Health Corporation, a Delaware corporation
("AmeriSource"), as the term "affiliate" is (i) defined for purposes of
paragraphs (c) and (d) of Rule 145 of the rules and regulations (the "Rules and
Regulations") of the Securities and Exchange Commission (the "Commission") under
the Securities Act of 1933, as amended (the "Act"), or (ii) used in and for
purposes of Accounting Series, Releases 130 and 135, as amended, of the
Commission.  Pursuant to the terms of the Agreement and Plan of Merger dated as
of September 22, 1997 (the "Merger Agreement"), among McKesson Corporation, a
Delaware corporation ("McKesson"), Patriot Acquisition Corp., a Delaware
corporation and a wholly owned subsidiary of McKesson ("Merger Sub"), and
AmeriSource, Merger Sub will be merged with and into AmeriSource (the "Merger").

     As a result of the Merger, I may receive shares of common stock, par value
$.01 per share, of McKesson (the "McKesson Securities") in exchange for shares
owned by me of common stock, par value $.01 per share, of AmeriSource (or upon
the exercise of options for such shares).

     I hereby represent, warrant and covenant to McKesson that in the event I
receive any McKesson Securities as a result of the Merger:

     A.  I shall not make any sale, transfer or other disposition of the
McKesson Securities in violation of the Act or the Rules and Regulations.

B.  I have carefully read this letter and the Merger Agreement and discussed the
requirements of such documents and other applicable limitations upon my ability
to sell, transfer or otherwise dispose of the McKesson Securities, to the extent
I felt necessary, with my counsel or counsel for AmeriSource.

     C.  I have been advised that the issuance of McKesson Securities to me
pursuant to the Merger has been registered with the Commission under the Act on

                                       12

 
a Registration Statement on Form S-4.  However, I have also been advised that,
since at the time the Merger was submitted for a vote of the stockholders of
AmeriSource, (i) I may be deemed to have been an affiliate of AmeriSource and
(ii) the distribution by me of the McKesson Securities has not been registered
under the Act, I may not sell, transfer or otherwise dispose of the McKesson
Securities issued to me in the Merger unless (i) such sale, transfer or other
disposition has been registered under the Act, (ii) such sale, transfer or other
disposition is made in conformity with Rule 145 (as such rule may be hereafter
from time to time amended) promulgated by the Commission under the Act, or (iii)
in the opinion of counsel reasonably acceptable to McKesson, or a "no action"
letter obtained by me from the staff of the Commission, such sale, transfer or
other disposition is otherwise exempt from registration under the Act.

     D.  I understand that, except as may be provided in the Registration Rights
Agreement entered into by McKesson and me, dated as of September 22, 1997,
McKesson is under no obligation to register the sale, transfer or other
disposition of the McKesson Securities by me or on my behalf under the Act or to
take any other action necessary in order to make compliance with an exemption
from such registration available.

     E.  I also understand that stop transfer instructions will be given to
McKesson's transfer agents with respect to the McKesson Securities and that
there will be placed on the certificates for the McKesson Securities issued to
me, or any substitutions therefor, a legend stating in substance:

                  "THE SHARES REPRESENTED BY THIS CERTIFICATE WERE ISSUED IN A
                  TRANSACTION TO WHICH RULE 145 PROMULGATED UNDER THE SECURITIES
                  ACT OF 1933 APPLIES, AND MAY ONLY BE SOLD OR OTHERWISE
                  TRANSFERRED IN COMPLIANCE WITH THE REQUIREMENTS OF RULE 145 OR
                  PURSUANT TO A REGISTRATION STATEMENT UNDER THAT ACT OR AN
                  EXEMPTION FROM SUCH REGISTRATION."

     F.  I also understand that unless the transfer by me of my McKesson
Securities has been registered under the Act or is a sale made in conformity
with the provisions of Rule 145, McKesson reserves the right to put the
following legend on the certificates issued to my transferee:

                  "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
                  REGISTERED UNDER THE SECURITIES ACT OF 1933 AND WERE ACQUIRED
                  FROM A PERSON WHO 

                                       13

 
                  RECEIVED SUCH SHARES IN A TRANSACTION TO WHICH RULE 145
                  PROMULGATED UNDER THE SECURITIES ACT OF 1933 APPLIES. THE
                  SHARES HAVE BEEN ACQUIRED BY THE HOLDER NOT WITH A VIEW TO, OR
                  FOR RESALE IN CONNECTION WITH, ANY DISTRIBUTION THEREOF WITHIN
                  THE MEANING OF THE SECURITIES ACT OF 1933 AND MAY NOT BE SOLD,
                  PLEDGED OR OTHERWISE TRANSFERRED EXCEPT IN ACCORDANCE WITH AN
                  EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THE SECURITIES
                  ACT OF 1933."

     It is understood and agreed that the legends set forth in paragraphs E and
F above shall be removed by delivery of substitute certificates without such
legend if such legend is not required for purposes of the Act or this Agreement.
It is understood and agreed that such legends and the stop orders referred to
above will be removed if (i) evidence or representations satisfactory to
McKesson that the McKesson Securities represented by such certificates are being
or have been sold in a transaction made in conformity with the provisions of
Rule 145(d) (as such rule may be hereafter from time to time amended) or (ii)
McKesson has received either an opinion of counsel, which opinion and counsel
shall be reasonably satisfactory to McKesson, or a "no action" letter obtained
by me from the staff of the Commission, to the effect that the restrictions
imposed by Rule 145 under the Act no longer apply to me.

     I further represent to and covenant with McKesson that I will not, during
the 30 days prior to the Effective Time (as defined in the Merger Agreement),
sell, transfer or otherwise dispose of or reduce my risk (as contemplated by the
SEC Accounting Series Release No. 135) with respect to AmeriSource shares or
shares of the capital stock of McKesson that I may hold and, furthermore,
provided that McKesson is not in breach of Section 5(b) of the Voting/Support
Agreement between McKesson and me dated as of September 22, 1997, I will not
sell, transfer or otherwise dispose of or reduce my risk (as contemplated by SEC
Accounting Series Release No. 135) with respect to any McKesson Securities
received by me in the Merger or any other shares of the capital stock of
McKesson until after such time as combined financial results (including combined
sales and net income) covering at least 30 days of combined operations of
AmeriSource and McKesson have been published by McKesson, in the form of a
quarterly earnings report, an effective registration statement filed with the
Commission, a report to the Commission on Form 10-K, 10-Q or 8-K, or any other
public filing or announcement which includes such combined results of
operations. If the Effective Time is less than 30 days prior to the end of
McKesson's fiscal quarter, McKesson shall use reasonable efforts to prepare and

                                       14

 
publicly release such combined financial results as soon as practicable
following the end of the first month ending at least 30 days after the Effective
Time and shall notify the "affiliates" of the publication of such results.

     Execution of this letter should not be considered an admission on my part
that I am an "affiliate" of AmeriSource as described in the first paragraph of
this letter, or as a waiver of any rights I may have to object to any claim that
I am such an affiliate on or after the date of this letter.

                                 Very truly yours,


                                 --------------------------------------  
                                 Name:

Accepted this ____ day of
__________________________, 199_ by

McKESSON CORPORATION


By: ___________________________
    Name: _____________________
    Title: ____________________

                                       15

 
                                    ANNEX C

    The maximum number of shares of AmeriSource Class A Stock which the
Stockholder may own, beneficially or of record, without violating Applicable
Law, is 3,974,513 shares, assuming 16,943,975 shares of AmeriSource Class A
Stock are owned by persons other than the Stockholder.

                                       16