HBO & CO
SC 13D, 1998-10-27
COMPUTER INTEGRATED SYSTEMS DESIGN
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<PAGE>
 
                     SECURITIES AND EXCHANGE COMMISSION
                           WASHINGTON, D.C.  20549

                                SCHEDULE 13D

                  Under the Securities Exchange Act of 1934


                                HBO & Company
                        ----------------------------
                              (Name of Issuer)


                   Common Stock, par value $0.05 per share
                 ------------------------------------------
                       (Title of Class of Securities)


                                 0004041001
                    ------------------------------------
                               (CUSIP Number)

                               Nancy A. Miller
                        Vice President and Secretary
                            McKesson Corporation
                               McKesson Plaza
                               One Post Street
                      San Francisco, California  94104
                               (415) 983-8300

                               with a copy to:
                            Kenton J. King, Esq.
                  Skadden, Arps, Slate, Meagher & Flom LLP
                           Four Embarcadero Center
                      San Francisco, California   94111
                               (415) 984-6400
        -------------------------------------------------------------
                (Name, Address and Telephone Number of Person
              Authorized to Receive Notices and Communications)



                              October 17, 1998
                       -------------------------------
                        (Date of Event which Requires
                          Filing of this Statement)


If the filing person has previously filed a statement on Schedule 13G to report
 the acquisition which is the subject of this Schedule 13D, and is filing this
   schedule because of (S)(S) 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), 
                        check the following box [  ].
<PAGE>
 
                                  SCHEDULE 13D

CUSIP No.  0004041001
________________________________________________________________________________
(1)    NAME OF REPORTING PERSON
       S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
 
       McKesson Corporation (94-3207296)
________________________________________________________________________________
(2)    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:
                                                            (a)  (  )
                                                            (b)  (  )
________________________________________________________________________________
(3)    SEC USE ONLY

________________________________________________________________________________
(4)    SOURCE OF FUNDS

       WC, 00
________________________________________________________________________________
(5)    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
       PURSUANT TO ITEMS 2(d) or 2(e)                                 (  )

________________________________________________________________________________
(6)    CITIZENSHIP OR PLACE OF ORGANIZATION

       Delaware
________________________________________________________________________________
                              (7)   SOLE VOTING POWER
                                    85,865,517   (1)
      NUMBER OF     ____________________________________________________________
       SHARES                 (8)   SHARED VOTING POWER
    BENEFICIALLY
      OWNED BY                      0   (1)
        EACH        ____________________________________________________________
      REPORTING               (9)   SOLE DISPOSITIVE POWER
       PERSON
        WITH                        85,865,517   (1)
________________________________________________________________________________
                              (10)  SHARED DISPOSITIVE POWER
 
                                    0   (1)
________________________________________________________________________________
(11)   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

       85,865,517   (1)
________________________________________________________________________________

____________________
(1)  The shares of common stock of the Issuer covered by this report are
purchasable by the Reporting Person upon exercise of an option granted to the
Reporting Person on October 17, 1998, and described in Item 4 of this report.
Prior to the exercise of the option, the Reporting Person is not entitled to any
rights as a stockholder of the Issuer as to the shares covered by the option.
The option may only be exercised upon the occurrence of certain events referred
to in Item 4 and set forth in the related Stock Option Agreement attached to
this Schedule 13D as Exhibit C, none of which has occurred as of the date
hereof.  The Reporting Person expressly disclaims beneficial ownership of any of
the shares of common stock of the Issuer which are purchasable by the Reporting
Person upon exercise of the option until such time as the Reporting Person
purchases any such shares upon any such exercise.  The number of shares
indicated represents 19.9% of the total outstanding shares of common stock of
the Issuer as of October 17, 1998, excluding shares issuable upon exercise of
the option.

                                       2
<PAGE>
 
(12)   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
       SHARES                                   (  )
________________________________________________________________________________
(13)   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

       Approximately 16.6%  (2)
________________________________________________________________________________
(14)   TYPE OF REPORTING PERSON

       CO
________________________________________________________________________________



______________________
(2) After giving effect to the exercise of the option as described herein.

                                       3
<PAGE>
 
          ITEM 1.   SECURITY AND ISSUER.

          The class of equity securities to which this statement relates is the
common stock, $0.05 par value per share (the "HBOC Common Stock"), of HBO &
Company, a Delaware corporation ("HBOC" or the "Company") and the associated
preferred stock purchase rights issued pursuant to a Rights Agreement, dated as
of February 12, 1991, by and between HBOC and The Citizens and Southern Trust
Company (Georgia), N.A., as Rights Agent.  The principal executive offices of
HBOC are located at 301 Perimeter Center North, Atlanta, Georgia  30346.

          ITEM 2.   IDENTITY AND BACKGROUND.

          (a) - (c) and (f)

          This Schedule 13D is being filed by McKesson Corporation, a Delaware
corporation ("McKesson").  The principal business of McKesson is health care
supply management.  McKesson also develops and manages marketing programs for
pharmaceutical manufacturers and, through McKesson Water Products Company,
processes and markets pure drinking water.  The principal executive offices of
McKesson are located at McKesson Plaza, One Post Street, San Francisco,
California  94104.  Certain information concerning the executive officers and
directors of McKesson is set forth in Schedule A hereto and is incorporated
herein by this reference.

          (d) and (e)

          Neither McKesson nor, to the knowledge of McKesson as of the date
hereof, any of its executive officers or directors has during the past five
years (i) been convicted in a criminal proceeding (excluding traffic violations
or similar misdemeanors) or (ii) been a party to a civil proceeding of a
judicial or administrative body of competent jurisdiction that resulted in a
judgment, decree or final order enjoining future violations of, or prohibiting
or mandating activities subject to, federal or state securities laws or finding
any violation with respect to such laws.

          ITEM 3.   SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION.

          McKesson acquired an option to purchase 85,865,517 shares of HBOC
Common Stock (the "Option"), pursuant to a Stock Option Agreement dated October
17, 1998, between McKesson and HBOC (the "HBOC Option Agreement").  Pursuant to
the Option, McKesson has the right, upon the occurrence of certain events
specified in Section 2 of the HBOC Option Agreement, to purchase 85,865,517
shares of HBOC Common Stock (or such greater number of shares of HBOC Common
Stock equal to 19.9% of the then outstanding shares of HBOC Common Stock) for
$32.81 per share, subject to certain customary anti-dilution adjustments.
Notwithstanding any other provisions of the HBOC Option Agreement, the Total
Profit (as defined in the HBOC Option Agreement) that McKesson may realize from
the Option and pursuant to the termination fee under the Merger Agreement (as
defined below) may not exceed an amount equal to $220 million.  Upon the
occurrence of certain events specified in the HBOC Option Agreement, McKesson
may require HBOC to repurchase the Option.  In addition, the HBOC Option
Agreement grants certain registration rights to McKesson 

                                       4
<PAGE>
 
with respect to any shares of HBOC Common Stock acquired pursuant to the
Option. Under certain circumstances, HBOC is entitled to a right of first
refusal if McKesson desires to sell or otherwise transfer all or any part of
the Option or any shares of HBOC Common Stock acquired by McKesson pursuant to
the exercise of the Option. The HBOC Option Agreement is attached to this
Schedule 13D as Exhibit C and is incorporated herein by this reference. The
foregoing description of the terms of the HBOC Option Agreement is qualified
in its entirety by reference to the full text of the HBOC Option Agreement.

          It is anticipated that, should the Option become exercisable and
should McKesson determine to exercise the Option, McKesson would obtain the
funds therefor from working capital or by borrowing from parties whose identity
is not yet known.

          ITEM 4.   PURPOSE OF TRANSACTION.

          (a) - (g)

          On October 17, 1998, HBOC, McKesson and McKesson Merger Sub, Inc., a
Delaware corporation and a wholly owned subsidiary of McKesson ("Merger Sub"),
entered into an Agreement and Plan of Merger (the "Merger Agreement"), providing
for the merger of Merger Sub with and into HBOC such that HBOC will become a
wholly owned subsidiary of McKesson (the "Merger").  Upon consummation of the
Merger, HBOC will be the surviving corporation (the "Surviving Corporation"),
and the Certificate of Incorporation, with limited amendments, and the by-laws
of Merger Sub shall be the Certificate of Incorporation and by-laws of the
Surviving Corporation, the initial directors of the Surviving Corporation shall
be Charles W. McCall and Mark A. Pulido and the initial officers of the
Surviving Corporation shall be the officers of Merger Sub.  At the effective
time of the Merger, each outstanding share of HBOC Common Stock (other than
shares of HBOC Common Stock owned by McKesson, HBOC or Merger Sub) will be
converted into the right to receive .37 shares of common stock, par value $0.01
per share of McKesson ("McKesson Common Stock") (except that cash will be paid
in lieu of fractional shares).  If the Merger is consummated in accordance with
the terms and conditions of the Merger Agreement and upon the occurrence of
certain other events as set forth in the HBOC Option Agreement, the Option will
terminate.

          Concurrently with and as an inducement and condition to McKesson's
entering into the Merger Agreement, HBOC, as issuer, and McKesson, as grantee,
entered into the HBOC Option Agreement, pursuant to which HBOC granted McKesson
the Option.  Concurrently with and as an inducement and condition to HBOC's
entering into the Merger Agreement, McKesson, as issuer, and HBOC, as grantee,
entered into a Stock Option Agreement, dated October 17, 1998 (the "McKesson
Option Agreement"), pursuant to which McKesson granted HBOC an option, on
substantially identical terms as set forth in the HBOC Option Agreement, to
purchase from McKesson, upon the occurrence of certain events specified in
Section 2 of the McKesson Option Agreement, 19,759,717 shares of McKesson Common
Stock (or such greater number of shares equal to 19.9% of the then outstanding
shares of McKesson Common Stock) for $88.6875 per share, subject to certain
customary anti-dilution adjustments.  Notwithstanding any other provisions of
the McKesson Option Agreement, the Total Profit (as defined in the McKesson
Option Agreement) that HBOC may realize from the option granted pursuant thereto
and pursuant to the termination fee under the Merger Agreement may not exceed an
amount equal to $220 million.

                                       5
<PAGE>
 
          (h) - (j)

          Upon consummation of the Merger as contemplated by the Merger
Agreement, the HBOC Common Stock will be delisted from the Nasdaq National
Market, and the HBOC Common Stock will become eligible for termination of
registration pursuant to Section 12(g)(4) of the Securities Exchange Act of
1934, as amended.

          The preceding summary of certain provisions of the Merger Agreement,
the McKesson Option Agreement and the HBOC Option Agreement is qualified in its
entirety by reference to the full text of such documents, copies of which are
filed as Exhibits A, B and C, respectively, hereto, and which are incorporated
herein by this reference.

          ITEM 5.   INTEREST IN SECURITIES OF THE ISSUER.
 
          (a) - (b)

          The number of shares of HBOC Common Stock covered by the Option is
equal to 85,865,517, subject to certain adjustments for anti-dilution as set
forth in the HBOC Option Agreement, which constitutes (i) 19.9% of the issued
and outstanding shares of HBOC Common Stock on October 17, 1998, or (ii)
approximately 16.6% of the shares of HBOC Common Stock that would be outstanding
after giving effect to the exercise of the Option.

          Prior to the exercise of the Option, McKesson (i) is not entitled to
any rights as a stockholder of HBOC as to the shares covered by the Option, and
(ii) disclaims any beneficial ownership of the shares of HBOC Common Stock which
are purchasable by McKesson upon exercise of the Option because the Option is
exercisable only in the limited circumstances set forth in the HBOC Option
Agreement, none of which has occurred as of the date hereof.  If the Option were
exercised, McKesson would have the sole right to vote or to dispose of the
shares of HBOC Common Stock issued as a result of such exercise, subject to the
terms of the HBOC Option Agreement.

          (c)  Other than as set forth in this Item 5, to the knowledge of
McKesson as of the date hereof (i) neither McKesson nor any subsidiary or
affiliate of McKesson nor any of McKesson's executive officers or directors,
beneficially owns any shares of HBOC Common Stock, and (ii) there have been no
transactions in the shares of HBOC Common Stock effected during the past 60 days
by McKesson, nor to the knowledge of McKesson as of the date hereof, by any
subsidiary or affiliate of McKesson or any of McKesson's executive officers or
directors.

          (d)  No other person is known by McKesson to have the right to receive
or the power to direct the receipt of dividends from, or the proceeds from the
sale of, the HBOC Common Stock obtainable by McKesson upon exercise of the
Option.

          (e)  Not applicable.

                                       6
<PAGE>
 
          ITEM 6.  CONTRACTS, ARRANGEMENTS, UNDERSTANDINGS OR RELATIONSHIPS WITH
                   RESPECT TO SECURITIES OF THE ISSUER.

          The responses to Items 3, 4 and 5 of this Schedule 13D and the
Exhibits to this Schedule 13D are incorporated herein by reference.

          ITEM 7.   MATERIAL TO BE FILED AS EXHIBITS.

          (A) Agreement and Plan of Merger, dated as of October 17, 1998 by and
              among HBOC, Merger Sub and McKesson.

          (B) Stock Option Agreement, dated October 17, 1998, by and between
              McKesson, as issuer, and HBOC, as grantee.

          (C) Stock Option Agreement, dated October 17, 1998, by and between
              HBOC, as issuer, and McKesson, as grantee.

                                       7
<PAGE>
 
                                   SIGNATURE

          After reasonable inquiry and to the best of its knowledge and belief,
the undersigned certifies that the information set forth in this statement is
true, complete and correct.



                                         McKESSON CORPORATION


Date:  October 27, 1998                 By: /s/ Nancy A. Miller
                                            --------------------------------
                                            Name: Nancy A. Miller
                                            Title: Vice President and Secretary

                                       8
<PAGE>
 
                                                                    SCHEDULE A

                        DIRECTORS AND EXECUTIVE OFFICERS

                            OF McKESSON CORPORATION


          The following table sets forth the name, business address and present
principal occupation or employment of each director and executive officer of
McKesson.  Unless otherwise indicated, each such person is a United States
citizen, and the business address of each such person is McKesson Plaza, One
Post Street, San Francisco, California  94104.


   NAME AND BUSINES                           PRESENT PRINCIPAL
       ADDRESS                                   OCCUPATION
- ---------------------             -----------------------------------------
Mark A. Pulido*                   President and Chief Executive Officer of
                                  McKesson
                        
William A. Armstrong              Vice President Human Resources and 
                                  Administration of McKesson
                        
Michael T. Dalby                  Vice President Strategic Planning of
                                  McKesson
                        
William J. Dawson                 Vice President Business Development of
                                  McKesson
                        
John H. Hammergren                Vice President of McKesson and Group
                                  President, McKesson Health Systems Group
                        
Richard H. Hawkins                Vice President and Chief Financial Officer
                                  of McKesson
                        
Nicholas A. Loiacono              Treasurer of McKesson
                        
_______________________________
*Director, McKesson Corporation

                                       9
<PAGE>
 
 NAME AND BUSINESS                            PRESENT PRINCIPAL
      ADDRESS                                    OCCUPATION
- ---------------------             -----------------------------------------
David L. Mahoney                  Vice President of McKesson and Group
                                  President, Pharmaceutical Services and 
                                  International Group
                        
Mark T. Majeske                   Vice President of McKesson and Group
                                  President, Customer Operations Group
                        
Ivan D. Meyerson                  Vice President and General Counsel of
                                  McKesson
                        
Nancy A. Miller                   Vice President and Secretary of McKesson
                        
Charles A. Norris                 Vice President of McKesson and President
                                  of McKesson Water Products Company
                        
Carmine J. Villani                Vice President and Chief Information Officer 
                                  of McKesson
                        
Heidi E. Yodowitz                 Controller of McKesson

Mary G.F. Bitterman*              President and Chief Executive Officer,
KQED, Inc.                        KQED, Inc.
2601 Mariposa Street
San Francisco, CA  94111
 
Tully M. Friedman*                Chairman and Chief Executive Officer,
Friedman Fleischer & Lowe, LLC    Friedman Fleischer & Lowe, LLC
One Maritime Plaza, Suite 1000
San Francisco, CA  94111
 
John M. Pietruski*                Chairman, Texas Biotechnology Corporation; 
One Penn Plaza, Suite 3408        Chairman and Chief Executive Officer,
New York, NY  10119               Retired, Sterling Drug Inc.

_______________________________
*Director, McKesson Corporation
 

                                       10
<PAGE>
 
 NAME AND BUSINESS                              PRESENT PRINCIPAL             
      ADDRESS                                       OCCUPATION                
- ---------------------               ----------------------------------------- 
David S. Pottruck*                  President, Co-Chief Executive Officer and 
The Charles Schwab Corporation      Chief Operating Officer, The Charles      
101 Montgomery Street               Schwab Corporation                        
San Francisco, CA  94104                                                      
                                                                              
Carl E. Reichardt*                  Chairman of the Board, Retired, Wells     
Wells Fargo & Company               Fargo & Company                           
420 Montgomery Street, 12th Floor                                             
San Francisco, CA  94104                                                      
                                                                              
Alan Seelenfreund*                  Chairman of the Board of McKesson         
                                                                              
Jane E. Shaw*                       Chairman of the Board and Chief Executive 
AeroGen, Inc.                       Officer, AeroGen, Inc.                    
1310 Orleans Drive                                                            
Sunnyvale, CA  94089                                                          
                                                                              
Robert H. Waterman, Jr.*            Chairman, The Waterman Group, Inc.         
The Waterman Group, Inc.
1777 Borel Place, Suite 410
San Mateo, CA  94402
                                        
_______________________________
*Director, McKesson Corporation

                                       11
<PAGE>
 
                                 EXHIBIT INDEX


Exhibit           Description
- -------           -----------

(A)               Agreement and Plan of Merger, dated as of October 17, 1998, by
                  and among HBOC, Merger Sub and McKesson.

(B)               Stock Option Agreement, dated October 17, 1998, by and between
                  McKesson, as issuer, and HBOC, as grantee.

(C)               Stock Option Agreement, dated October 17, 1998, by and between
                  HBOC, as issuer, and McKesson, as grantee.

                                       12

<PAGE>
 
                         AGREEMENT AND PLAN OF MERGER


                                 BY AND AMONG


                                HBO & COMPANY,

                           MCKESSON MERGER SUB, INC.

                                      AND

                             MCKESSON CORPORATION



                         DATED AS OF October 17, 1998
<PAGE>
 
                               TABLE OF CONTENTS
<TABLE>
<CAPTION>
                                                                                                    PAGE
<S>                                                                                                 <C>
ARTICLE I -- THE MERGER........................................................................       2

     SECTION 1.1    The Merger.................................................................       2
     SECTION 1.2    Closing....................................................................       2
     SECTION 1.3    Effective Time.............................................................       2
     SECTION 1.4    Effects of the Merger......................................................       2
     SECTION 1.5    Certificate of Incorporation and By-laws of the Surviving
                    Corporation and McKesson...................................................       2
     SECTION 1.6    Directors and Officers.....................................................       3

ARTICLE II -- EFFECT OF THE MERGER ON THE CAPITAL STOCK OF THE CONSTITUENT CORPORATIONS; 
  EXCHANGE OF CERTIFICATES....................................................................       3

     SECTION 2.1    Effect on Capital Stock....................................................       3
          (a)       Cancellation of Treasury Stock and McKesson-Owned Stock....................       3
          (b)       Conversion of HBO Common Stock.............................................       3
          (c)       Merger Sub Common Stock....................................................       4
          (d)       McKesson Common Stock......................................................       4
          (e)       Options....................................................................       4

     SECTION 2.2    Exchange of Certificates...................................................       5
          (a)       Exchange Agent.............................................................       5
          (b)       Exchange Procedures........................................................       5
          (c)       Distributions with Respect to Unexchanged Shares...........................       6
          (d)       No Further Ownership Rights in HBO Common Stock............................       6
          (e)       No Fractional Shares.......................................................       7
          (f)       Termination of Exchange Fund...............................................       7
          (g)       No Liability...............................................................       7
          (h)       Investment of Exchange Fund................................................       7
          (i)       Lost Certificates..........................................................       7

     SECTION 2.3    Certain Adjustments........................................................       7

ARTICLE III -- REPRESENTATIONS AND WARRANTIES..................................................       8

     SECTION 3.1    Representations and Warranties of McKesson and Merger Sub..................       8
          (a)       Organization, Standing and Corporate Power.................................       8
          (b)       Subsidiaries...............................................................       8
          (c)       Capital Structure..........................................................       8
          (d)       Authority; Noncontravention................................................      10
          (e)       SEC Documents; Undisclosed Liabilities.....................................      11
</TABLE>
<PAGE>
 
<TABLE>
<S>                                                                                                  <C>
          (f)       Information Supplied.......................................................      12
          (g)       Absence of Certain Changes or Events.......................................      12
          (h)       Compliance with Applicable Laws; Litigation................................      13
          (i)       Absence of Changes in Benefit Plans........................................      13
          (j)       Benefit Plans..............................................................      14
          (k)       Taxes......................................................................      16
          (l)       Voting Requirements........................................................      17
          (m)       State Takeover Statutes; Certificate of Incorporation......................      17
          (n)       Accounting Matters.........................................................      17
          (o)       Brokers....................................................................      17
          (p)       Opinion of Financial Advisors..............................................      17
          (q)       Ownership of HBO Common Stock..............................................      17
          (r)       Intellectual Property......................................................      18
          (s)       Certain Contracts..........................................................      20
          (t)       McKesson Rights Agreement..................................................      20
          (u)       Environmental Liability....................................................      21
          (v)       Insurance..................................................................      21
          (w)       Transactions with Affiliates...............................................      21
          (x)       Full Disclosure............................................................      21

     SECTION 3.2    Representations and Warranties of HBO......................................      21
          (a)       Organization, Standing and Corporate Power.................................      21
          (b)       Subsidiaries...............................................................      22
          (c)       Capital Structure..........................................................      22
          (d)       Authority; Noncontravention................................................      23
          (e)       SEC Documents; Undisclosed Liabilities.....................................      24
          (f)       Information Supplied.......................................................      25
          (g)       Absence of Certain Changes or Events.......................................      25
          (h)       Compliance with Applicable Laws; Litigation................................      26
          (i)       Absence of Changes in Benefit Plans........................................      26
          (j)       Benefit Plans..............................................................      27
          (k)       Taxes......................................................................      28
          (l)       Voting Requirements........................................................      29
          (m)       State Takeover Statutes; Certificate of Incorporation......................      29
          (n)       Accounting Matters.........................................................      29
          (o)       Brokers....................................................................      29
          (p)       Opinion of Financial Advisors..............................................      29
          (q)       Ownership of McKesson Common Stock.........................................      29
          (r)       Intellectual Property......................................................      29
          (s)       Certain Contracts..........................................................      32
          (t)       HBO Rights Agreement.......................................................      32
          (u)       Environmental Liability....................................................      32
          (v)       Insurance..................................................................      33
          (w)       Transactions with Affiliates...............................................      33
          (x)       Full Disclosure............................................................      33

ARTICLE IV -- COVENANTS RELATING TO CONDUCT OF BUSINESS........................................      33

     SECTION 4.1    Conduct of Business........................................................      33
          (a)       Conduct of Business by McKesson............................................      33
          (b)       Conduct of Business by HBO.................................................      36
</TABLE>
<PAGE>
 
<TABLE>
<S>                                                                                                  <C>
          (c)       Other Actions..............................................................      38
          (d)       Advice of Changes..........................................................      38

     SECTION 4.2    No Solicitation by McKesson................................................      38
     SECTION 4.3    No Solicitation by HBO.....................................................      40

ARTICLE V -- ADDITIONAL AGREEMENTS.............................................................      42

     SECTION 5.1    Preparation of the Form S-4 and the Joint Proxy Statement;
                    Stockholders' Meetings.....................................................      42
     SECTION 5.2    Letters of McKesson's Accountants..........................................      43
     SECTION 5.3    Letters of HBO's Accountants...............................................      43
     SECTION 5.4    Access to Information; Confidentiality.....................................      44
     SECTION 5.5    Commercially Reasonable Efforts............................................      44
     SECTION 5.6    Indemnification, Exculpation and Insurance.................................      45
     SECTION 5.7    Fees and Expenses..........................................................      45
     SECTION 5.8    Public Announcements.......................................................      46
     SECTION 5.9    Affiliates.................................................................      46
     SECTION 5.10   NYSE and PSE Listings......................................................      46
     SECTION 5.11   Tax Treatment..............................................................      46
     SECTION 5.12   Pooling of Interests.......................................................      47
     SECTION 5.13   Post-Merger Operations.....................................................      47
     SECTION 5.14   Conveyance Taxes...........................................................      47
     SECTION 5.15   Employee Benefits..........................................................      47

ARTICLE VI -- CONDITIONS PRECEDENT.............................................................      47

      SECTION 6.1   Conditions to Each Party's Obligation to Effect the Merger.................      47
          (a)       Stockholder Approvals......................................................      47
          (b)       HSR Act....................................................................      47
          (c)       Governmental, Regulatory and Other Approvals...............................      47
          (d)       No Injunctions or Restraints...............................................      48
          (e)       Form S-4...................................................................      48
          (f)       NYSE and PSE Listings......................................................      48
          (g)       Tax Opinions...............................................................      48
          (h)       Pooling Letters............................................................      48

     SECTION 6.2    Conditions to Obligations of HBO...........................................      48
          (a)       Representations and Warranties.............................................      48
          (b)       Performance of Obligations of McKesson.....................................      49
          (c)       No Material Adverse Change.................................................      49
          (d)       McKesson Rights Agreement..................................................      49

     SECTION 6.3   Conditions to Obligations of McKesson.......................................      49
          (a)      Representations and Warranties..............................................      49
          (b)      Performance of Obligations of HBO...........................................      49
          (c)      No Material Adverse Change..................................................      49
          (d)      HBO Rights Agreement........................................................      49

ARTICLE VII -- TERMINATION, AMENDMENT AND WAIVER...............................................      49

     SECTION 7.1    Termination................................................................      49
     SECTION 7.2    Effect of Termination......................................................      51
     SECTION 7.3    Amendment..................................................................      52
     SECTION 7.4    Extension; Waiver..........................................................      52

ARTICLE VIII -- GENERAL PROVISIONS.............................................................      52

     SECTION 8.1    Nonsurvival of Representations and Warranties..............................      52
     SECTION 8.2    Notices....................................................................      53
     SECTION 8.3    Definitions................................................................      53
     SECTION 8.4    Interpretation.............................................................      54
     SECTION 8.5    Counterparts...............................................................      54
     SECTION 8.6    Entire Agreement; No Third-Party Beneficiaries.............................      55
     SECTION 8.7    Governing Law..............................................................      55
     SECTION 8.8    Assignment.................................................................      55
     SECTION 8.9    Consent to Jurisdiction....................................................      55
     SECTION 8.10   Headings, Etc..............................................................      55
     SECTION 8.11   Severability...............................................................      55
</TABLE>

EXHIBIT A    Form of McKesson Stock Option Agreement
EXHIBIT B    Form of HBO Stock Option Agreement
EXHIBIT C    McKesson Bylaw Amendments
EXHIBIT D    Directors and Officers
EXHIBIT E    McKesson Stock Plans
EXHIBIT F    HBO Stock Plans
EXHIBIT G    Form of Rule 145 Letter to be signed by McKesson Affiliates
EXHIBIT H    Form of Pooling Letter to be signed by McKesson and HBO Affiliates
<PAGE>
 
          AGREEMENT AND PLAN OF MERGER dated as of October 17, 1998, among
MCKESSON CORPORATION, a Delaware corporation ("McKesson"), HBO & COMPANY, a
Delaware corporation ("HBO"), and MCKESSON MERGER SUB, INC. ("Merger Sub"), a
Delaware corporation and a wholly-owned subsidiary of McKesson.

                              W I T N E S S E T H:

          WHEREAS, the respective Boards of Directors of HBO, Merger Sub and
McKesson have each approved the merger of Merger Sub with and into HBO (the
"Merger"), upon the terms and subject to the conditions set forth in this
Agreement, whereby each issued and outstanding share of common stock, par value
$.05 per share, of HBO ("HBO Common Stock," which reference shall be deemed to
include the associated HBO Rights (as defined in Section 3.2(c)), other than
shares owned by HBO or McKesson, will be converted into the right to receive the
Merger Consideration (as defined in Section 2.1(b)); and

          WHEREAS, the respective Boards of Directors of HBO and McKesson,
having carefully considered the long-term prospects and interests of HBO and
McKesson and their respective stockholders and having determined that the Merger
and the other transactions contemplated hereby are consistent with, and in
furtherance of, their respective business strategies and goals and are advisable
and in the best interests of their respective stockholders, have each approved
the transactions contemplated by this Agreement and the Option Agreements (as
hereinafter defined) and have each resolved to recommend to each party's
stockholders the approval and adoption of this Agreement and the Merger and the
consummation of the transactions contemplated hereby and thereby upon the terms
and subject to the conditions set forth herein; and

          WHEREAS, as a condition to the execution of this Agreement,
contemporaneously herewith HBO and McKesson will enter into a stock option
agreement (the "McKesson Option Agreement") attached hereto as Exhibit A and a
                                                               ---------      
stock option agreement (the "HBO Option Agreement" and, together with the
McKesson Option Agreement, the "Option Agreements") attached hereto as 
Exhibit B; and
- ---------

          WHEREAS, for federal income tax purposes, it is intended that the
Merger will qualify as a reorganization under the provisions of Section 368(a)
of the Internal Revenue Code of 1986, as amended, and the rules and regulations
promulgated thereunder (the "Code"), and this Agreement is intended to be and is
adopted as a plan of reorganization within the meaning of Section 368 of the
Code; and

          WHEREAS, for financial accounting purposes, it is intended that the
Merger will be accounted for as a pooling of interests transaction under United
States generally accepted accounting principles ("GAAP"); and

          WHEREAS, HBO and McKesson desire to make certain representations,
<PAGE>
 
warranties, covenants and agreements in connection with the Merger and also to
prescribe various conditions to the Merger;

          NOW, THEREFORE, in consideration of the representations, warranties,
covenants and agreements set forth herein and in the Option Agreements, the
parties agree as follows:

                                   ARTICLE I
                                  THE MERGER

     SECTION 1.1    The Merger.  Upon the terms and subject to the conditions
                    ----------                                               
set forth in this Agreement, and in accordance with the Delaware General
Corporation Law (the "DGCL"), Merger Sub shall be merged with and into HBO at
the Effective Time (as defined in Section 1.3). Following the Effective Time,
the separate corporate existence of Merger Sub shall cease and HBO shall be the
surviving corporation (the "Surviving Corporation") and shall succeed to and
assume all the rights and obligations of Merger Sub in accordance with the DGCL.

     SECTION 1.2    Closing.  The closing of the Merger (the "Closing") will
                    -------                                                 
take place on a date to be specified by the parties (the "Closing Date"), which
shall be no later than the second business day after satisfaction or waiver of
the conditions set forth in Article VI, unless another time or date is agreed to
by the parties hereto.  The Closing will be held at the offices of Jones, Day,
Reavis & Pogue, 3500 SunTrust Plaza, 303 Peachtree Street, N.E., Atlanta,
Georgia 30308-3242.

     SECTION 1.3    Effective Time.  Subject to the provisions of this
                    --------------                                    
Agreement, as soon as practicable on the Closing Date, the parties shall cause
the Merger to be consummated by filing a certificate of merger or other
appropriate documents (in any such case, the "Certificate of Merger") executed
in accordance with the relevant provisions of the DGCL and shall make all other
filings or recordings required under the DGCL.  The Merger shall become
effective at such time as the Certificate of Merger is duly filed with the
Secretary of State of Delaware, or at such subsequent date or time as HBO and
McKesson shall agree and specify in the Certificate of Merger (the time the
Merger becomes effective being hereinafter referred to as the "Effective Time").

     SECTION 1.4    Effects of the Merger.  The Merger shall have the effects
                    ---------------------                                    
set forth in Section 259 of the DGCL.

     SECTION 1.5    Certificate of Incorporation and By-laws of the Surviving
                    ---------------------------------------------------------
Corporation and McKesson.
- ------------------------ 

          (a)  At the Effective Time, the Certificate of Incorporation and the
by-laws of Merger Sub, as in effect immediately prior to the Effective Time,
shall be the Certificate of Incorporation and by-laws of the Surviving
Corporation, in each case until thereafter amended in accordance with applicable
law; provided, however, that Article First of the Certificate of Incorporation
of the Surviving Corporation shall be amended to read as follows:
<PAGE>
 
          The name of the Corporation (which is hereinafter referred to as the
"Corporation") is HBO & Company.

          (b)  At the Effective Time, Article 1 of the Certificate of
Incorporation of McKesson shall be amended to read as follows:

               The name of the corporation is McKesson HBOC, Inc.

          (c)  At the Effective Time, the by-laws of McKesson shall be amended
as provided in Exhibit C.
               --------- 

     SECTION 1.6    Directors and Officers.
                    ---------------------- 

          (a)  At the Effective Time, the Board of Directors, committees of the
Board of Directors, composition of such committees (including chairmen thereof)
and certain of the officers of McKesson shall be as set forth on Exhibit D
                                                                 ---------
hereto until the earlier of the resignation or removal of any individual listed
on or designated in accordance with Exhibit D or until their respective
                                    ---------                          
successors are duly elected and qualified, as the case may be.  If any officer
listed on or appointed in accordance with Exhibit D ceases to be a full-time
                                          ---------                         
employee of either HBO or McKesson as the case may be, the parties will agree
upon another person to serve in such person's stead.

          (b)  The directors set forth on Exhibit D shall be the initial
                                          ---------                     
directors of the Surviving Corporation, and the officers of Merger Sub
immediately prior to the Effective Time shall be the initial officers of the
Surviving Corporation, each to hold office in accordance with the Certificate of
Incorporation and By-Laws of the Surviving Corporation.

                                  ARTICLE II
                   EFFECT OF THE MERGER ON THE CAPITAL STOCK
                       OF THE CONSTITUENT CORPORATIONS;
                           EXCHANGE OF CERTIFICATES

     SECTION 2.1    Effect on Capital Stock.  As of the Effective Time, by
                    -----------------------                               
virtue of the Merger and without any action on the part of Merger Sub, HBO or
the holder of any shares of the following securities:

          (a)  Cancellation of Treasury Stock and McKesson-Owned Stock.  Each
               -------------------------------------------------------       
share of HBO Common Stock that is owned by McKesson, Merger Sub or HBO shall
automatically be canceled and retired and shall cease to exist, and no
consideration shall be delivered in exchange therefor.

          (b)  Conversion of HBO Common Stock.  Subject to Section 2.2(e), each
               ------------------------------                                  
issued and outstanding share of HBO Common Stock (other than shares to be
canceled in 
<PAGE>
 
accordance with Section 2.1(a)) shall be converted into the right to receive .37
(the "Exchange Ratio") validly issued, fully paid and nonassessable shares of
common stock, par value $.01 per share ("McKesson Common Stock"), of McKesson
(the "Merger Consideration"). As of the Effective Time, all such shares of HBO
Common Stock shall no longer be outstanding and shall automatically be canceled
and retired and shall cease to exist, and each holder of a certificate
representing any such shares of HBO Common Stock shall cease to have any rights
with respect thereto, except the right to receive the Merger Consideration and
any cash in lieu of fractional shares of McKesson Common Stock to be issued or
paid in consideration thereof upon surrender of such certificate in accordance
with Section 2.2(e).

          (c)  Merger Sub Common Stock.  Each share of common stock, par value
               -----------------------                                        
$0.01 per share, of Merger Sub ("Merger Sub Common Stock") issued and
outstanding immediately prior to the Effective Time shall be converted into one
validly issued, fully paid and nonassessable share of common stock of the
Surviving Corporation.

          (d)  McKesson Common Stock.  At and after the Effective Time, each
               ---------------------                                        
share of McKesson Common Stock issued and outstanding immediately prior to the
Closing Date shall remain an issued and outstanding share of common stock of
McKesson and shall not be affected by the Merger.

          (e)  Options.
               ------- 

               (i)  HBO and McKesson will take all action necessary such that,
at the Effective Time, each option granted by HBO to purchase shares of HBO
Common Stock which is outstanding immediately prior thereto shall cease to
represent a right to acquire shares of HBO Common Stock and shall be converted
into an option to purchase shares of McKesson Common Stock in an amount and at
an exercise price determined as provided below (and otherwise, in the case of
options, subject to the terms of the HBO Stock Plans (as defined in Section
3.2(c)) and the agreements evidencing grants thereunder) (the "Assumed
Options"):

               (1)  The number of shares of McKesson Common Stock to be subject
     to the new option shall be equal to the product of the number of shares of
     HBO Common Stock subject to the original option and the Exchange Ratio,
     provided that any fractional shares of McKesson Common Stock resulting from
     --------                                                                   
     such multiplication shall be rounded to the nearest whole share; and

               (2)  The exercise price per share of McKesson Common Stock under
     the new option shall be equal to the exercise price per share of HBO Common
     Stock under the original option divided by the Exchange Ratio, provided
                                                                    --------
     that such exercise price shall be rounded to the nearest whole cent.

               (ii) The adjustment provided herein with respect to any options
that are "incentive stock options" (as defined in Section 422 of the Code) shall
be and is intended to be effected in a manner that is consistent with Section
424(a) of the Code. The duration and other terms of the new options shall be the
same as the original options except that all references to HBO shall be deemed
to be references to McKesson.
<PAGE>
 
               (iii)  As soon as practicable following the Effective Time,
McKesson shall deliver, upon due surrender of the Assumed Options to holders of
Assumed Options appropriate option agreements representing the right to acquire
McKesson Common Stock on the same terms and conditions as contained in the
Assumed Options (except as otherwise set forth in this Section 2.1(e)). Except
as expressly contemplated herein, McKesson shall comply with the terms of the
HBO Stock Plans as they apply to the Assumed Options. McKesson shall take all
corporate action necessary to reserve for issuance a sufficient number of shares
of McKesson Common Stock for delivery upon exercise of the Assumed Options in
accordance with this Section 2.1(e). McKesson shall file a registration
statement on Form S-8 (or any successor form) or on another appropriate form,
and use commercially reasonable efforts to have such registration statement
declared effective reasonably promptly following the Effective Time, with
respect to McKesson Common Stock subject to the Assumed Options and shall use
commercially reasonable efforts to maintain the effectiveness of such
registration statement or registration statements (and maintain the current
status of the prospectus or prospectuses contained therein) for so long as the
Assumed Options remain outstanding and exercisable.

               (iv)   McKesson acknowledges and agrees that the consummation of
the Merger will have certain effects in respect of the Assumed Options as
reflected in Section 2.1(e)(iv) of the HBO Disclosure Schedule, and McKesson
agrees to act in accordance therewith and give full effect to same.

     SECTION 2.2    Exchange of Certificates.
                    ------------------------ 

          (a)  Exchange Agent.  As of the Effective Time, McKesson shall enter
               --------------                                                 
into an agreement with such bank or trust company as may be designated by
McKesson (the "Exchange Agent") which shall provide that McKesson shall provide
McKesson Common Stock (such shares of McKesson Common Stock, together with any
dividends or distributions with respect thereto with a record date after the
Effective Time, and any cash payable in lieu of any fractional shares of
McKesson Common Stock being hereinafter referred to as the "Exchange Fund")
issuable pursuant to Section 2.1 in exchange for outstanding shares of HBO
Common Stock.

          (b)  Exchange Procedures.  As soon as reasonably practicable after the
               -------------------                                              
Effective Time, the Exchange Agent shall mail to each holder of record of a
certificate or certificates which immediately prior to the Effective Time
represented outstanding shares of HBO Common Stock (the "Certificates") whose
shares were converted into the right to receive the Merger Consideration
pursuant to Section 2.1, (i) a letter of transmittal (which shall specify that
delivery shall be effected, and risk of loss and title to the Certificates shall
pass, only upon delivery of the Certificates to the Exchange Agent, and shall
otherwise be in customary form) and (ii) instructions for use in surrendering
the Certificates in exchange for the Merger Consideration. Upon surrender of a
Certificate for cancellation to the Exchange Agent, together with such letter of
transmittal, duly executed, and such other documents as may reasonably be
required by the Exchange Agent, the holder of such Certificate shall be entitled
to receive in exchange therefor a certificate representing that number of whole
shares of McKesson Common Stock which such holder has the right to receive
pursuant to the provisions of this Article II, certain dividends or other
distributions in accordance with Section 2.2(c) and cash in lieu of any
<PAGE>
 
fractional share of McKesson Common Stock in accordance with Section 2.2(e), and
the Certificate so surrendered shall forthwith be canceled.  Notwithstanding
anything to the contrary contained herein, no certificate representing McKesson
Common Stock or cash in lieu of a fractional share interest shall be delivered
to a person who is an affiliate of HBO for purposes of qualifying the Merger for
pooling of interests accounting treatment under Opinion 16 of the Accounting
Principles Board and applicable Securities and Exchange Commission ("SEC") rules
and regulations, unless such person has executed and delivered the agreement
described in the second sentence of Section 5.9(a) hereof. In the event of a
surrender of a Certificate representing shares of HBO Common Stock which are not
registered in the transfer records of HBO under the name of the person
surrendering such Certificate, a certificate representing the proper number of
shares of McKesson Common Stock may be issued to a person other than the person
in whose name the Certificate so surrendered is registered if such Certificate
shall be properly endorsed or otherwise be in proper form for transfer and the
person requesting such issuance shall pay any transfer or other taxes required
by reason of the issuance of shares of McKesson Common Stock to a person other
than the registered holder of such Certificate or establish to the satisfaction
of McKesson that such tax has been paid or is not applicable. Until surrendered
as contemplated by this Section 2.2, each Certificate shall be deemed at any
time after the Effective Time to represent only the right to receive upon such
surrender the Merger Consideration which the holder thereof has the right to
receive in respect of such Certificate pursuant to the provisions of this
Article II, certain dividends or other distributions in accordance with Section
2.2(c) and cash in lieu of any fractional share of McKesson Common Stock in
accordance with Section 2.2(e). No interest shall be paid or will accrue on any
cash payable to holders of Certificates pursuant to the provisions of this
Article II.

          (c)  Distributions with Respect to Unexchanged Shares.  No dividends
               ------------------------------------------------               
or other distributions with respect to McKesson Common Stock with a record date
after the Effective Time shall be paid to the holder of any unsurrendered
Certificate with respect to the shares of McKesson Common Stock represented
thereby, and, in the case of Certificates representing HBO Common Stock, no cash
payment in lieu of fractional shares shall be paid to any such holder pursuant
to Section 2.2(e), and all such dividends, other distributions and cash in lieu
of fractional shares of McKesson Common Stock shall be paid by McKesson to the
Exchange Agent and shall be included in the Exchange Fund, in each case until
the surrender of such Certificate in accordance with this Article II.  Subject
to the effect of applicable escheat or similar laws, following surrender of any
such Certificate there shall be paid to the holder of the certificate 
representing whole shares of McKesson Common Stock issued in exchange therefor,
without interest, (i) at the time of such surrender, the amount of dividends or
other distributions with a record date after the Effective Time theretofore paid
with respect to such whole shares of McKesson Common Stock and, in the case of
Certificates representing HBO Common Stock, the amount of any cash payable in
lieu of a fractional share of McKesson Common Stock to which such holder is
entitled pursuant to Section 2.2(e) and (ii) at the appropriate payment date,
the amount of dividends or other distributions with a record date after the
Effective Time and with a payment date subsequent to such surrender payable with
respect to such whole shares of McKesson Common Stock.

          (d)  No Further Ownership Rights in HBO Common Stock.  All shares of
               -----------------------------------------------                
McKesson Common Stock issued upon the surrender for exchange of Certificates in
accordance 
<PAGE>
 
with the terms of this Article II (including any cash paid pursuant to this
Article II) shall be deemed to have been issued (and paid) in full satisfaction
of all rights pertaining to the shares of HBO Common Stock theretofore
represented by such Certificates, subject, however, to the Surviving
Corporation's obligation to pay any dividends or make any other distributions
with a record date prior to the Effective Time which may have been declared or
made by HBO on such shares of HBO Common Stock which remain unpaid at the
Effective Time, and there shall be no further registration of transfers on the
stock transfer books of the Surviving Corporation of the shares of HBO Common
Stock which were outstanding immediately prior to the Effective Time. If, after
the Effective Time, Certificates are presented to McKesson, the Surviving
Corporation or the Exchange Agent for any reason, they shall be canceled and
exchanged as provided in this Article II, except as otherwise provided by law.

          (e)  No Fractional Shares.    Notwithstanding anything to the contrary
               --------------------                                             
contained herein, no certificates or scrip representing fractional shares of
McKesson Common Stock shall be issued upon the surrender for exchange of
Certificates, no dividend or distribution of McKesson shall relate to such
fractional share interests and such fractional share interests will not entitle
the owner thereof to vote or to any rights of a stockholder of McKesson.  In
lieu of the issuance of such fractional shares, McKesson shall pay each former
holder of HBO Common Stock an amount in cash equal to the product obtained by
multiplying (A) the fractional share interest to which such former holder would
otherwise be entitled by (B) the average closing price per share (or if there is
no sale on such date then the average between the closing bid and ask prices on
any such day) for shares of McKesson Common Stock as reported by the New York
Stock Exchange ("NYSE") (as reported in The Wall Street Journal, or, if not
reported therein, any other authoritative source) during the ten trading days
preceding the fifth trading day prior to the Closing Date (such average, the
"Average McKesson Price").

          (f)  Termination of Exchange Fund.  Any portion of the Exchange Fund
               ----------------------------                                   
which remains undistributed to the holders of the Certificates for six months
after the Effective Time shall be delivered to McKesson, upon demand, and any
holders of the Certificates who have not theretofore complied with this Article
II shall thereafter look only to McKesson for payment of their claim for Merger
Consideration, any dividends or distributions with respect to McKesson Common
Stock and any cash in lieu of fractional shares of McKesson Common Stock.

          (g)  No Liability.  None of HBO, McKesson, Merger Sub, the Surviving
               ------------                                                   
Corporation or the Exchange Agent shall be liable to any person in respect of
any shares of McKesson Common Stock, any dividends or distributions with respect
thereto, any cash in lieu of fractional shares of McKesson Common Stock or any
cash from the Exchange Fund, in each case delivered to a public official
pursuant to any applicable abandoned property, escheat or similar law.

          (h)  Investment of Exchange Fund.  The Exchange Agent shall invest any
               ---------------------------                                      
cash included in the Exchange Fund, as directed by McKesson, on a daily basis.
Any interest and other income resulting from such investments shall be paid to
McKesson.

          (i)  Lost Certificates.  If any Certificate shall have been lost,
               -----------------                                           
stolen or destroyed, upon the making of an affidavit of that fact by the person
claiming such Certificate to 
<PAGE>
 
be lost, stolen or destroyed and, if required by the Surviving Corporation, the
posting by such person of a bond in such reasonable amount as the Surviving
Corporation may direct as indemnity against any claim that may be made against
it with respect to such Certificate, the Exchange Agent shall issue in exchange
for such lost, stolen or destroyed Certificate the Merger Consideration and, if
applicable, any unpaid dividends and distributions on shares of McKesson Common
Stock deliverable in respect thereof and any cash in lieu of fractional shares,
in each case pursuant to this Agreement.

     SECTION 2.3    Certain Adjustments.  If between the date hereof and the
                    -------------------                                     
Effective Time, the outstanding shares of McKesson Common Stock or of HBO Common
Stock shall be changed into a different number of shares by reason of any
reclassification, recapitalization, split-up, combination or exchange of shares,
or any dividend payable in stock or other securities shall be declared thereon
with a record date within such period, the Exchange Ratio shall be adjusted
accordingly to provide to the holders of HBO Common Stock the same economic
effect as contemplated by this Agreement prior to such reclassification,
recapitalization, split-up, combination, exchange or dividend.

                                  ARTICLE III
                        REPRESENTATIONS AND WARRANTIES

     SECTION 3.1    Representations and Warranties of McKesson and Merger Sub.
                    ---------------------------------------------------------  
Except as disclosed in the Disclosure Schedule delivered by McKesson and Merger
Sub to HBO prior to the execution of this Agreement (the "McKesson Disclosure
Schedule") and making reference to the particular subsection of this Agreement
to which exception is being taken, McKesson and Merger Sub jointly and severally
represent and warrant to HBO as follows:

          (a)  Organization, Standing and Corporate Power.    Each of McKesson
               ------------------------------------------                     
and its subsidiaries (as defined in Section 8.3) is a corporation or other legal
entity duly organized, validly existing and in good standing (with respect to
jurisdictions which recognize such concept) under the laws of the jurisdiction
in which it is organized and has the requisite corporate or other power, as the
case may be, and authority to carry on its business as now being conducted,
except, as to subsidiaries, for those jurisdictions where the failure to be so
organized, existing or in good standing individually or in the aggregate would
not have a material adverse effect (as defined in Section 8.3) on McKesson.
Each of McKesson and its subsidiaries is duly qualified or licensed to do
business and is in good standing (with respect to jurisdictions which recognize
such concept) in each jurisdiction in which the nature of its business or the
ownership, leasing or operation of its properties makes such qualification or
licensing necessary, except for those jurisdictions where the failure to be so
qualified or licensed or to be in good standing individually or in the aggregate
would not have a material adverse effect on McKesson.

               (i)  McKesson has delivered to HBO prior to the execution of this
Agreement complete and correct copies of any amendments to its certificate of
incorporation (the "McKesson Certificate") and by-laws not filed as of the date
hereof with the McKesson Filed SEC Documents (as defined in Section 3.1(e)).

               (ii) In all material respects, the minute books of McKesson and
its 
<PAGE>
 
subsidiaries contain accurate records of all meetings and accurately reflect all
other actions taken by the stockholders, the Board of Directors and all
committees of the Board of Directors of McKesson (or, as the case may be, each
of its subsidiaries) since December 31, 1996.

          (b)  Subsidiaries.  Exhibit 21 to McKesson's Annual Report on Form 10-
               ------------   ----------                                        
K for the fiscal year ended March 31, 1998 includes all the subsidiaries of
McKesson which as of the date of this Agreement are Significant Subsidiaries (as
defined in Rule 1-02 of Regulation S-X of the SEC). All the outstanding shares
of capital stock of, or other equity interests in, each such Significant
Subsidiary have been validly issued and are fully paid and nonassessable and are
owned directly or indirectly by McKesson, free and clear of all pledges, claims,
liens, charges, encumbrances and security interests of any kind or nature
whatsoever (collectively, "Liens") and free of any other restriction (including
any restriction on the right to vote, sell or otherwise dispose of such capital
stock or other ownership interests).

          (c)  Capital Structure.  The authorized capital stock of McKesson
               -----------------                                           
consists of 400,000,000 shares of McKesson Common Stock, par value $.01 per
share and 100,000,000 shares of series preferred stock, par value $.01 per share
("McKesson Preferred Stock").  At the close of business on October 15, 1998  (i)
99,295,063 shares of McKesson Common Stock were issued and outstanding; (ii)
242,095 shares of McKesson Common Stock were held by McKesson in its treasury;
(iii) no shares of McKesson Preferred Stock were issued and outstanding; (iv)
25,151,920 shares of McKesson Common Stock were reserved for issuance pursuant
to all stock option, restricted stock or other stock-based compensation,
benefits or savings plans, agreements or arrangements in which current or former
employees or directors of McKesson or its subsidiaries participate as of the
date hereof (including, without limitation, the plans set forth on Exhibit E
                                                                   ---------
attached hereto), complete and correct copies of which, in each case as amended
as of the date hereof, have been filed as exhibits to the McKesson Filed SEC
Documents or delivered to HBO (such plans, collectively, the "McKesson Stock
Plans"); (v) 10,000,000 shares of McKesson Preferred Stock have been designated
as Series A Junior Participating Preferred Stock, of which 600,000 shares were
reserved for issuance upon exercise of preferred stock purchase rights (the
"McKesson Rights") issuable pursuant to the Rights Agreement, dated as of
October 21, 1994, by and between McKesson and First Chicago Trust Company, as
rights agent (the "McKesson Rights Agreement"); and (vi) 5,533,208 shares of
McKesson Common Stock were reserved for issuance upon conversion of the 5% Trust
Convertible Securities (the "Convertible Preferred Securities") of the McKesson
Financing Trust (the "Financing Trust"). Section 3.1(c) of the McKesson
Disclosure Schedule sets forth a complete and correct list, as of October 15,
1998, of the number of shares of McKesson Common Stock subject to employee stock
options or other rights to purchase or receive McKesson Common Stock granted
under the McKesson Stock Plans (collectively, "McKesson Employee Stock
Options"). All outstanding shares of capital stock of McKesson are, and all
shares which may be issued pursuant to the Stock Plans or the Financing Trust
will be, when issued, duly authorized, validly issued, fully paid and
nonassessable and not subject to preemptive rights. Except as set forth in this
Section 3.1(c) and except for changes since October 15, 1998 resulting from the
issuance of shares of McKesson Common Stock pursuant to the McKesson Employee
Stock Options, the Convertible Preferred Securities or as expressly permitted by
this Agreement, (x) there are not issued, reserved for issuance or outstanding
(A) any shares of capital stock or other voting securities of McKesson, (B) any
securities of McKesson or any McKesson
<PAGE>
 
subsidiary convertible into or exchangeable or exercisable for shares of capital
stock or voting securities of McKesson, (C) any warrants, calls, options or
other rights to acquire from McKesson or any McKesson subsidiary (including any
subsidiary trust), or obligations of McKesson or any McKesson subsidiary to
issue, any capital stock, voting securities or securities convertible into or
exchangeable or exercisable for capital stock or voting securities of McKesson,
and (y) there are no outstanding obligations of McKesson or any McKesson
subsidiary to repurchase, redeem or otherwise acquire any such securities or to
issue, deliver or sell, or cause to be issued, delivered or sold, any such
securities. There are no outstanding (A) securities of McKesson or any McKesson
subsidiary convertible into or exchangeable or exercisable for shares of capital
stock or other voting securities or ownership interests in any McKesson
subsidiary, (B) warrants, calls, options or other rights to acquire from
McKesson or any McKesson subsidiary, and any obligation of McKesson or any
McKesson subsidiary to issue, any capital stock, voting securities or other
ownership interests in, or any securities convertible into or exchangeable or
exercisable for any capital stock, voting securities or ownership interests in,
any McKesson subsidiary or (C) obligations of McKesson or any McKesson
subsidiary to repurchase, redeem or otherwise acquire any such outstanding
securities of McKesson subsidiaries or to issue, deliver or sell, or cause to be
issued, delivered or sold, any such securities, except with respect to the
Convertible Preferred Securities. Other than with respect to the Convertible
Preferred Securities, neither McKesson nor any McKesson subsidiary is a party to
any agreement restricting the purchase or transfer of, relating to the voting
of, requiring registration of, or granting any preemptive or, except as provided
by the terms of the McKesson Employee Stock Options, antidilutive rights with
respect to, any securities of the type referred to in the two preceding
sentences. Other than the McKesson subsidiaries, McKesson does not directly or
indirectly beneficially own any securities or other beneficial ownership
interests in any other entity except for non-controlling investments made in the
ordinary course of business in entities which are not individually or in the
aggregate material to McKesson and its subsidiaries as a whole.

          (d)  Authority; Noncontravention.  Each of McKesson and Merger Sub has
               ---------------------------                                      
all requisite corporate power and authority to enter into this Agreement, and
McKesson has all requisite corporate power and authority to enter into the
Option Agreements and, subject, to the McKesson Stockholder Approval (as defined
in Section 3.1(l)), to consummate the transactions contemplated hereby and
thereby.  The execution and delivery of this Agreement by each of McKesson and
Merger Sub, and the execution and delivery of the Option Agreements by McKesson
and the consummation by McKesson and Merger Sub of the transactions contemplated
hereby and thereby have been duly authorized by all necessary corporate action
on the part of McKesson and Merger Sub, subject, in the case of the Merger, and
the issuance of McKesson Common Stock in connection with the Merger and the
conversion of the Assumed Options, to the McKesson Stockholder Approval.  This
Agreement has been, and the Option Agreements will be, duly executed and
delivered by McKesson (and, in the case of this Agreement, by Merger Sub) and,
assuming the due authorization, execution and delivery thereof by HBO,
constitutes (or will constitute, as the case may be) the legal, valid and
binding obligation of McKesson (and, in the case of this Agreement, by Merger
Sub), enforceable against 
<PAGE>
 
McKesson (and, in the case of this Agreement, by Merger Sub) in accordance with
their terms. The execution and delivery of this Agreement does not, and the
execution and delivery of the Option Agreements and the consummation of the
transactions contemplated hereby and thereby and compliance with the provisions
of this Agreement and the Option Agreements will not, conflict with, or result
in any violation of, or default (with or without notice or lapse of time, or
both) under, or give rise to a right of termination, cancellation or
acceleration of any obligation or loss of a benefit under, or result in the
creation of any Lien upon any of the properties or assets of McKesson or any of
its subsidiaries or in any restriction on the conduct of McKesson's business or
operations under, (i) the McKesson Certificate or the by-laws of McKesson or the
comparable organizational documents of any of its subsidiaries, (ii) any loan or
credit agreement, note, bond, mortgage, indenture, trust document, lease or
other agreement, instrument, permit, concession, franchise, license or similar
authorization applicable to McKesson or any of its subsidiaries or their
respective properties or assets or (iii) subject to the governmental filings and
other matters referred to in the following sentence, any judgment, order,
decree, statute, law, ordinance, rule or regulation applicable to McKesson or
any of its subsidiaries or their respective properties or assets, other than, in
the case of clauses (ii) and (iii), any such conflicts, violations, defaults,
rights, losses, restrictions or Liens that individually or in the aggregate
would not (x) have a material adverse effect on McKesson or HBO or (y)
reasonably be expected to impair the ability of McKesson or Merger Sub to
perform its obligations under this Agreement (and, in the case of McKesson
individually, under the Option Agreements). No consent, approval, order or
authorization of, action by or in respect of, or registration, declaration or
filing with, any federal, state, local or foreign government, any court,
administrative, regulatory or other governmental agency, commission or authority
or any nongovernmental self-regulatory agency, commission or authority (a
"Governmental Entity") is required by or with respect to McKesson or any of its
subsidiaries in connection with the execution and delivery of this Agreement by
McKesson, or the execution and delivery by McKesson of the Option Agreements or
the consummation by McKesson or Merger Sub of the transactions contemplated
hereby and thereby, except for (1) the filing of a pre-merger notification and
report form by McKesson under the Hart-Scott-Rodino Antitrust Improvements Act
of 1976, as amended (the "HSR Act") or filings or notifications under the
antitrust, competition or similar laws of any foreign jurisdiction; (2) the
filing with the SEC of (A) a proxy statement relating to the McKesson
Stockholders' Meeting (as defined in Section 5.1(b)) (such proxy statement,
together with the proxy statement relating to the HBO Stockholders' Meeting (as
defined in Section 5.1(c)), in each case as amended or supplemented from time to
time, the "Joint Proxy Statement"), (B) the Form S-4 and (C) such reports under
Section 13(a), 13(d), 15(d) or 16(a) of the Securities Exchange Act of 1934, as
amended (the "Exchange Act"), as may be required in connection with this
Agreement, the Option Agreements and the transactions contemplated hereby and
thereby; (3) the filing of the Certificate of Merger with the Secretary of State
of Delaware and appropriate documents with the relevant authorities of other
states in which McKesson is qualified to do business and such filings with
Governmental Entities to satisfy the applicable requirements of state securities
or "blue sky" laws; (4) such filings and approvals of NYSE and the Pacific
Exchange, Inc. ("PSE") to permit the shares of McKesson Common Stock that are to
be issued in the Merger and under the HBO Stock Plans to be approved for
listing, subject to notice of issuance, by NYSE and the PSE; and (5) such
consents, approvals, orders or authorizations the failure of which to be made or
obtained individually or in the aggregate would not (x) have a material adverse
effect on McKesson or (y) reasonably be expected to impair the ability of
McKesson or Merger Sub to perform its 
<PAGE>
 
obligations under this Agreement.

          (e) SEC Documents; Undisclosed Liabilities.  McKesson has filed all
              --------------------------------------                         
required registration statements, prospectuses, reports, schedules, forms,
statements and other documents (including exhibits and all other information
incorporated therein) with the SEC since December 31, 1996 (the "McKesson SEC
Documents").  As of their respective dates, the McKesson SEC Documents complied
in all material respects with the requirements of the Securities Act of 1933, as
amended (the "Securities Act"), or the Exchange Act, as the case may be, and the
rules and regulations of the SEC promulgated thereunder applicable to such
McKesson SEC Documents, and none of the McKesson SEC Documents when filed
contained any untrue statement of a material fact or omitted to state a material
fact required to be stated therein or necessary in order to make the statements
therein, in light of the circumstances under which they were made, not
misleading. The financial statements of McKesson included in the McKesson SEC
Documents comply as to form, as of their respective dates of filing with the
SEC, in all material respects with applicable accounting requirements and the
published rules and regulations of the SEC with respect thereto, have been
prepared in accordance with GAAP (except, in the case of unaudited statements,
as permitted by Form 10-Q of the SEC) applied on a consistent basis during the
periods involved (except as may be indicated in the notes thereto) and fairly
present the consolidated financial position of McKesson and its consolidated
subsidiaries as of the dates thereof and the consolidated results of their
operations and cash flows for the periods then ended (subject, in the case of
unaudited statements, to normal year-end audit adjustments which are not
material). Except (i) as reflected in such financial statements or in the notes
thereto or (ii) for liabilities incurred in connection with this Agreement, the
Option Agreements or the transactions contemplated hereby or thereby, neither
McKesson nor any of its subsidiaries has any liabilities or obligations of any
nature which, individually or in the aggregate, would have a material adverse
effect on McKesson.

          (f) Information Supplied.  None of the information supplied or to be
              --------------------                                            
supplied by McKesson specifically for inclusion or incorporation by reference in
(i) the registration statement on Form S-4 to be filed with the SEC by McKesson
in connection with the issuance of McKesson Common Stock in the Merger (the
"Form S-4") will, at the time the Form S-4 becomes effective under the
Securities Act, contain any untrue statement of a material fact or omit to state
any material fact required to be stated therein or necessary to make the
statements therein not misleading or (ii) the Joint Proxy Statement will, at the
date it is first mailed to McKesson's stockholders or at the time of the
McKesson Stockholders' Meeting, contain any untrue statement of a material fact
or omit to state any material fact required to be stated therein or necessary in
order to make the statements therein, in light of the circumstances under which
they are made, not misleading.  The Form S-4 and the Joint Proxy Statement will
comply as to form in all material respects with the requirements of the Exchange
Act and the rules and regulations thereunder, except that no representation or
warranty is made by McKesson with respect to statements made or incorporated by
reference therein based on information supplied by HBO specifically for
inclusion or incorporation by reference in the Form S-4 or the Joint Proxy
Statement.
<PAGE>
 
          (g) Absence of Certain Changes or Events.  Except for liabilities
              ------------------------------------                         
incurred in connection with this Agreement, the Option Agreements or the
transactions contemplated hereby and thereby, and except as permitted by Section
4.1(a), since March 31, 1998, McKesson and its subsidiaries have conducted their
business only in the ordinary course consistent with past practice or as
disclosed in any McKesson SEC Document filed since such date and prior to the
date hereof, and there has not been (i) any material adverse change (as defined
in Section 8.3) in McKesson, (ii) any declaration, setting aside or payment of
any dividend or other distribution (whether in cash, stock or property) with
respect to any of McKesson's capital stock, (iii) any split, combination or
reclassification of any of McKesson's capital stock or any issuance or the
authorization of any issuance of any other securities in respect of, in lieu of,
or in substitution for shares of McKesson's capital stock, except for issuances
of McKesson Common Stock upon exercise or conversion of McKesson Employee Stock
Options, in each case awarded prior to the date hereof in accordance with their
present terms or issued pursuant to Section 4.1(a), (iv)(A) any granting by
McKesson or any of its subsidiaries to any current or former director, officer
or other key employee of McKesson or its subsidiaries of any increase in
compensation, bonus or other benefits, except for normal increases as a result
of promotions, normal increases of base pay or target bonuses in the ordinary
course of business or as was required under any employment agreements in effect
as of March 31, 1998, (B) any granting by McKesson or any of its subsidiaries
to any such current or former director, officer or key employee of any increase
in severance or termination pay, or (C) any entry by McKesson or any of its
subsidiaries into, or any amendment of, any employment, deferred compensation,
consulting, severance, termination or indemnification agreement with any such
current or former director or officer, or any material amendment of any of the
foregoing with any key employee, (v) except insofar as may have been disclosed
in McKesson SEC Documents filed and publicly available prior to the date of this
Agreement (as amended to the date hereof, the "McKesson Filed SEC Documents") or
required by a change in GAAP, any change in accounting methods, principles or
practices by McKesson materially affecting its assets, liabilities or business,
(vi) except insofar as may have been disclosed in the McKesson Filed SEC
Documents, any tax election that individually or in the aggregate would have a
material adverse effect on McKesson or any of its tax attributes or any
settlement or compromise of any material income tax liability, or (vii) any
action taken by McKesson or any of the McKesson subsidiaries during the period
from April 1, 1998 through the date of this Agreement that, if taken during the
period from the date of this Agreement through the Effective Time, would
constitute a breach of Section 4.1(a).

          (h) Compliance with Applicable Laws; Litigation.
              ------------------------------------------- 

              (i)    McKesson, its subsidiaries and employees hold all permits,
licenses, variances, exemptions, orders, registrations and approvals of all
Governmental Entities which are required for the operation of the businesses of
McKesson and its subsidiaries (the "McKesson Permits"), except where the failure
to have any such McKesson Permits individually or in the aggregate would not
have a material adverse effect on McKesson. Except as specifically disclosed in
the McKesson SEC Documents filed with the Commission prior to the date hereof,
McKesson and its subsidiaries are in compliance with the terms of the McKesson
Permits and all applicable laws, statutes, orders, rules, regulations, policies
or guidelines promulgated, or judgments, decisions or orders entered by any
Governmental Entity, including,
<PAGE>
 
without limitation, the Federal Prescription Drug Marketing Act and comparable
or related state law provisions, the Federal Controlled Substances Act of 1970,
the rules and regulations promulgated thereunder or otherwise by the Drug
Enforcement Administration and comparable or related state law provisions, the
Food, Drug and Cosmetic Act, the Good Manufacturing Practices and other
standards of the Food and Drug Administration, federal Medicare and Medicaid
statutes, including, without limitation, 42 U.S.C. Section 1320a-7b and 42
U.S.C. Section 1395nn or related state or local statutes or regulations,
applicable state laws regulating pharmacy or wholesaling practices, statutes and
regulations relating to billing or sale practices, the Foreign Corrupt Practices
Act of 1977 and the Occupational Safety and Health Act and the regulations
promulgated thereunder (all such laws, statutes, orders, rules, regulations,
policies, guidelines, judgments, decisions and orders, collectively, "Applicable
Laws"), relating to McKesson or its business or properties, except where the
failure to be in compliance with such Applicable Laws individually or in the
aggregate would not have a material adverse effect on McKesson. As of the date
of this Agreement, except as disclosed in the McKesson Filed SEC Documents, no
action, demand, requirement or investigation by any Governmental Entity and no
suit, action or proceeding by any person, in each case with respect to McKesson
or any of its subsidiaries or any of their respective properties, is pending or,
to the knowledge (as defined in Section 8.3) of McKesson, threatened, other
than, in each case, those the outcome of which individually or in the aggregate
would not (A) have a material adverse effect on McKesson or (B) reasonably be
expected to impair the ability of McKesson or Merger Sub to perform its
obligations under this Agreement or the Option Agreements or prevent or
materially delay the consummation of any of the transactions contemplated hereby
or thereby.

              (ii)   Neither McKesson nor any McKesson subsidiary is subject to
any outstanding order, injunction or decree which has had or, insofar as can be
reasonably foreseen, individually or in the aggregate will have, a material
adverse effect on McKesson.

          (i) Absence of Changes in Benefit Plans.  McKesson has delivered to
              -----------------------------------                            
HBO or provided to HBO for review true and complete copies of (i) all severance
and employment agreements of McKesson with directors, executive officers or key
employees, (ii) all written and material unwritten severance programs and
policies of each of McKesson and each McKesson subsidiary, and (iii) all plans
or arrangements of McKesson and each McKesson subsidiary relating to its
employees which contain change in control provisions, in each case which has not
been filed as an exhibit to a McKesson Filed SEC Document.  Since March 31,
1998, there has not been any adoption or amendment in any material respect by
McKesson or any of its subsidiaries of any (A) collective bargaining agreement
with respect to any employees of, (B) any material bonus, pension, profit
sharing, deferred compensation, incentive compensation, stock ownership, stock
purchase, stock option, phantom stock, retirement, vacation, severance,
disability, death benefit, hospitalization, medical or other plan, arrangement
or understanding providing benefits to any current or former officers, directors
or employees of, (C) any employment agreement, consulting agreement or severance
agreement with, any current or former officer or director of, or (D) any
material employment agreement, consulting agreement or severance agreement with
any employee of, McKesson or any of its wholly owned subsidiaries (collectively,
the "McKesson Benefit Plans"), or any material change in any actuarial or other
assumption used to calculate funding obligations with respect to any McKesson
pension plans, or any material change in the manner in which contributions to
any McKesson pension 
<PAGE>
 
plans are made or the basis on which such contributions are determined. Since
March 31, 1998, neither McKesson nor any McKesson subsidiary has amended any
McKesson Employee Stock Options or any McKesson Stock Plans to accelerate the
vesting of, or release restrictions on, awards thereunder, or to provide for
such acceleration in the event of a change in control.

          (j) Benefit Plans.
              ------------- 

              (i)    With respect to the McKesson Benefit Plans, to the
knowledge of McKesson, no event has occurred and there exists no condition or
set of circumstances, in connection with which McKesson or any of its
subsidiaries would be subject to any liability that individually or in the
aggregate would have a material adverse effect on McKesson under the Employee
Retirement Income Security Act of 1974, as amended ("ERISA"), the Code or any
other applicable law.

              (ii)   Each McKesson Benefit Plan has been administered in
accordance with its terms, except for any failures so to administer any McKesson
Benefit Plan that individually or in the aggregate would not have a material
adverse effect on McKesson. To the knowledge of McKesson, the McKesson Benefit
Plans have been operated, and are, in compliance with the applicable provisions
of ERISA, the Code and all other applicable laws and the terms of all applicable
collective bargaining agreements, except for any failures to be in such
compliance that individually or in the aggregate would not have a material
adverse effect on McKesson. Each McKesson Benefit Plan that is intended to be
qualified under Section 401(a) or 401(k) of the Code has received a favorable
determination letter from the Internal Revenue Service ("IRS") that it is so
qualified and each trust established in connection with any McKesson Benefit
Plan that is intended to be exempt from federal income taxation under Section
501(a) of the Code has received a determination letter from the IRS that such
trust is so exempt. To the knowledge of McKesson, no fact or event has occurred
since the date of any determination letter from the IRS which is reasonably
likely to affect adversely the qualified status of any such McKesson Benefit
Plan or the exempt status of any such trust.

              (iii)  With respect to the McKesson Benefit Plans which are
defined benefit plans and subject to Title IV of ERISA (other than any
"multiemployer plans" as defined in Section 3(37) of ERISA), the aggregate fair
market value of the assets of such plans as of January 1, 1998 was approximately
$294 million. Since January 1, 1998, there has been no material adverse change
in the funded status of any such plans.

              (iv)   Each McKesson Benefit Plan that is a "multiemployer plan"
is set forth on Section 3.1(j)(iii) of the McKesson Disclosure Schedule. With
respect to any McKesson Benefit Plan that is a multiemployer plan, (A) neither
McKesson nor any of its subsidiaries has any material contingent liability under
Section 4204 of ERISA, and, to the knowledge of McKesson, no circumstances exist
that would result in any such plan entering into a reorganization, and (B) the
aggregate withdrawal liability of McKesson and its subsidiaries, computed as if
a complete withdrawal by McKesson and any of its subsidiaries had occurred
<PAGE>
 
under each such McKesson Benefit Plan on the date hereof, would not exceed $15
million.

              (v)    No McKesson Benefit Plan provides medical benefits (whether
or not insured), with respect to current or former employees after retirement or
other termination of service (other than coverage mandated by applicable law or
benefits, the full cost of which is borne by the current or former employee)
other than individual arrangements the amounts of which are not material.

              (vi)   McKesson has previously provided to HBO a copy of each
collective bargaining or other labor union contract applicable to persons
employed by McKesson or any of its subsidiaries to which McKesson or any of its
subsidiaries is a party. No collective bargaining agreement is being negotiated
or renegotiated by McKesson or any of its subsidiaries. As of the date of this
Agreement, there is no labor dispute, strike or work stoppage against McKesson
or any of its subsidiaries pending or, to the knowledge of McKesson, threatened
which may interfere with the respective business activities of McKesson or any
of its subsidiaries, except where such dispute, strike or work stoppage
individually or in the aggregate would not have a material adverse effect on
McKesson. As of the date of this Agreement, to the knowledge of McKesson, none
of McKesson, any of its subsidiaries or any of their respective representatives
or employees has committed any material unfair labor practice in connection with
the operation of the respective businesses of McKesson or any of its
subsidiaries, and there is no material charge or complaint against McKesson or
any of its subsidiaries by the National Labor Relations Board or any comparable
governmental agency pending or threatened in writing.

              (vii)  No employee of McKesson will be entitled to any material
payment, additional benefits or any acceleration of the time of payment or
vesting of any benefits under any McKesson Benefit Plan as a result of the
transactions contemplated by this Agreement (either alone or in conjunction with
any other event such as a termination of employment), except that substantially
all McKesson Employee Stock Options will vest as of the date on which McKesson
Stockholder Approval is obtained.

              (viii) No material oral or written representation or commitment
with respect to any aspect of any McKesson Benefit Plan has been or will be made
to employees of McKesson or any McKesson subsidiaries by an authorized McKesson
employee prior to the Closing Date that is not materially in accordance with the
written or otherwise preexisting terms and provisions of such McKesson Benefit
Plans in effect immediately prior to the Closing Date.

              (ix)   Except such as would not have a material adverse effect,
there are no material unresolved claims or disputes under the terms of, or in
connection with, any McKesson Benefit Plan (other than routine undisputed claims
for benefits), and no action, legal or otherwise, has been commenced with
respect to any material claim.

              (x)    To the knowledge of McKesson, no non-exempt "prohibited
transaction" (within the meaning of Section 4975(c) of the Tax Code) involving
any McKesson Benefit Plan has occurred that could subject McKesson to any
material tax penalty or other cost or liability (by indemnification or
otherwise).
<PAGE>
 
          (k) Taxes.
              ----- 

              (i)    Each of McKesson and its subsidiaries has filed all
material tax returns and reports required to be filed by it (taking into account
all applicable extensions) and all such returns and reports are complete and
correct in all material respects, or requests for extensions to file such
returns or reports have been timely filed, granted and have not expired, except
to the extent that such failures to file, to be complete or correct or to have
extensions granted that remain in effect individually or in the aggregate would
not have a material adverse effect on McKesson. McKesson and each of its
subsidiaries has paid (or McKesson has paid or caused to be paid on its behalf)
all taxes (as defined herein) shown as due on such returns, and the most recent
financial statements contained in the McKesson Filed SEC Documents reflect an
adequate reserve in accordance with GAAP for all taxes payable by McKesson and
its subsidiaries for all taxable periods and portions thereof accrued through
the date of such financial statements.

              (ii)   No deficiencies for any taxes have, to the knowledge of
McKesson, been proposed, asserted or assessed against McKesson or any of its
subsidiaries that are not adequately reserved for, except for deficiencies that
individually or in the aggregate would not have a material adverse effect on
McKesson. Except as provided in Section 3.1(k) of the Disclosure Schedule, all
of the federal income tax returns of the affiliated group of which McKesson is
the common parent have closed by virtue of the applicable statute of
limitations.

              (iii)  Neither McKesson nor any of its subsidiaries has taken any
action or knows of any fact, agreement, plan or other circumstance that is
reasonably likely to prevent the Merger from qualifying as a reorganization
within the meaning of Section 368(a) of the Code.

              (iv)   As used in this Agreement, "taxes" shall include all (x)
federal, state, local or foreign income, property, sales, excise and other taxes
or similar governmental charges, including any interest, penalties or additions
with respect thereto, (y) liability for the payment of any amounts of the type
described in (x) as a result of being a member of an affiliated, consolidated,
combined or unitary group, and (z) liability for the payment of any amounts
described in (x) or (y) as a result of being party to any tax sharing agreement
or as a result of any express or implied obligation to indemnify any other
person with respect to the payment of any amounts of the type described in
clause (x) or (y).

              (v)    Neither McKesson nor any McKesson subsidiary has made any
parachute payments as such term is defined in Section 280G of the Tax Code,
neither is obligated to make any parachute payments, and neither is a party to
any agreement that under certain circumstances could obligate it, or any
successor in interest, to make any parachute payments that will not be
deductible under Section 280G of the Tax Code. Neither McKesson nor any McKesson
subsidiary is obligated to make reimbursement or gross-up payments to any person
in respect to excess parachute payments.

          (l) Voting Requirements.  The affirmative vote at the McKesson
              -------------------                                       
Stockholders' Meeting (the "McKesson Stockholder Approval") of the holders of a
majority of 
<PAGE>
 
all outstanding shares of McKesson Common Stock present in person or by proxy
and entitled to vote at a duly convened and held meeting of McKesson
Stockholders is the only vote of the holders of any class or series of
McKesson's capital stock necessary to approve and adopt this Agreement and the
transactions contemplated hereby, including the Merger.

          (m) State Takeover Statutes; Certificate of Incorporation.  The Board
              -----------------------------------------------------            
of Directors of McKesson has adopted a resolution or resolutions approving this
Agreement and the Option Agreements and the transactions contemplated hereby and
thereby and, assuming the accuracy of HBO's representation and warranty
contained in Section 3.2(q), (a) such approval constitutes approval of the
Merger and the other transactions contemplated hereby and by the Option
Agreements by the McKesson Board of Directors under the provisions of Section
203 of the DGCL and Article VII of McKesson's Certificate of Incorporation such
that Section 203 of the DGCL and Article VII of McKesson's Certificate of
Incorporation do not apply to this Agreement, the Option Agreements and the
transactions contemplated hereby and thereby.  To the knowledge of McKesson, no
state takeover statute other than Section 203 of the DGCL (which has been
rendered inapplicable) is applicable to the Merger or the other transactions
contemplated hereby.

          (n) Accounting Matters.  To its knowledge, neither McKesson nor any of
              ------------------                                                
its affiliates (as such term is used in Section 5.9) has taken or agreed to take
any action (including, without limitation, in connection with any McKesson Stock
Plan or any agreement thereunder) that would prevent the business combination to
be effected by the Merger from being accounted for as a "pooling of interests"
and McKesson has no reason to believe that the Merger will not qualify for
"pooling of interests" accounting.

          (o) Brokers.  No broker, investment banker, financial advisor or other
              -------                                                           
person is entitled to any broker's, finder's, financial advisor's or other
similar fee or commission in connection with the transactions contemplated by
this Agreement based upon arrangements made by or on behalf of McKesson.

          (p) Opinion of Financial Advisors.  McKesson has received the opinion
              -----------------------------                                    
of Bear, Stearns & Co., Inc., dated the date of this Agreement, to the effect
that, as of such date, the Exchange Ratio for the conversion of HBO Common Stock
into McKesson Common Stock is fair from a financial point of view to the
stockholders of McKesson, a signed copy of which opinion has been delivered to
HBO.

          (q) Ownership of HBO Common Stock.  To the knowledge of McKesson, as
              -----------------------------                                   
of the date hereof or at any time within twelve months prior to the date of this
Agreement (and before giving effect to the HBO Option Agreement, which will be
entered into immediately after the execution of this Agreement), neither
McKesson nor, to its knowledge without independent investigation, any of its
affiliates, (i) beneficially owns (as defined in Rule 13d-3 under the Exchange
Act), directly or indirectly, or (ii) is party to any agreement, arrangement or
understanding for the purpose of acquiring, holding, voting or disposing of, in
each case, shares of capital stock of HBO.

          (r)  Intellectual Property.
               --------------------- 
<PAGE>
 
              (i)    McKesson or its subsidiaries own or have a valid right to
use all trademarks, service marks, trade names, Internet domain names, designs,
slogans, and general intangibles of like nature, together with all applications,
registrations and goodwill related to the foregoing (collectively,
"Trademarks"); patents (including any registration, continuations, 
continuations-in-part, renewals and applications for any of the foregoing);
copyrights (including any registrations, renewals and applications for any of
the foregoing); Software (as defined below); technology, trade secrets and other
confidential information, know-how, proprietary processes, formulae, algorithms,
models, and methodologies (collectively, "Trade Secrets") used in or necessary
for the conduct of McKesson's and each of its subsidiary's business as currently
conducted, except where the failure to possess such right would not have a
material adverse effect (all such intellectual property being referred to herein
as the "Intellectual Property"). For purposes of this Section 3.1(r), "Software"
means any and all (a) computer programs, including any and all software
implementations of algorithms, models and methodologies, whether in source code
or object code, (b) databases and compilations, including any and all data and
collections of data, whether machine readable or otherwise, (c) descriptions,
flow-charts and other work product used to design, plan, organize and develop
any of the foregoing, (d) the technology supporting any Internet site(s)
operated by or on behalf of McKesson or any of its subsidiaries and (e) all
documentation, including user manuals and training materials, relating to any of
the foregoing.

              (ii)   The Intellectual Property owned by McKesson or any of its
subsidiaries is free and clear of all Liens.

              (iii)  The Intellectual Property owned by McKesson or any of its
subsidiaries and, to McKesson's knowledge, any Intellectual Property used by
McKesson, is valid and subsisting, in full force and effect, and has not been
canceled, expired, or abandoned. There is no pending or, to McKesson's
knowledge, threatened opposition, interference or cancellation proceeding before
any court or registration authority in any jurisdiction against any
registrations in respect of the Intellectual Property owned by McKesson or any
of its subsidiaries, or, to McKesson's knowledge, against any Intellectual
Property licensed to McKesson or any of its subsidiaries.

              (iv)   To the actual knowledge of McKesson or any of its
subsidiaries, the conduct of the business of McKesson and its subsidiaries as
currently conducted does not infringe upon (either directly or indirectly such
as through contributory infringement or inducement to infringe) any intellectual
property rights owned or controlled by any third party. There are no claims or
suits pending or, to the knowledge of McKesson, threatened, and neither McKesson
nor any of its subsidiaries has received any notice of a third-party claim or
suit, (a) alleging that its activities or the conduct of its business infringes
upon, violates, or constitutes the unauthorized use of the intellectual property
rights of any third party or (b) challenging the ownership, use, validity or
enforceability of any Intellectual Property, which in any case would have a
material adverse effect.
<PAGE>
 
              (v)    There are no settlements, forebearances to sue, consents,
judgments, or orders or similar obligations which in any material respect (a)
restrict the right of McKesson or its subsidiaries to use any Intellectual
Property, or (b) restrict the business of McKesson or its subsidiaries in order
to accommodate a third party's intellectual property rights or (c) except for
licenses with customers for McKesson's Software, there are no agreements that
permit third parties to use any Intellectual Property owned or controlled by
McKesson or any of its subsidiaries.

              (vi)   McKesson and each of its subsidiaries take reasonable
measures to protect the confidentiality of Trade Secrets, including (i)
requiring its employees and independent contractors having access thereto to
execute written nondisclosure agreements and (ii) requiring all licensees to
maintain the confidentiality of its Trade Secrets. To the actual knowledge of
McKesson or its subsidiaries, no Trade Secret has been knowingly disclosed or
authorized to be disclosed to any third party other than pursuant to a
nondisclosure agreement or other appropriate instrument that adequately protects
McKesson and the applicable subsidiary's proprietary interests in and to such
Trade Secrets. To the knowledge of McKesson, no party to any nondisclosure
agreement or nondisclosure obligation relating to its Trade Secrets is in breach
or default thereof.

              (vii)  To the knowledge of McKesson, no third party is
misappropriating, infringing, diluting, or violating any Intellectual Property
owned by McKesson or any of its subsidiaries other than immaterial disputes
concerning use by a third party of Trademarks of McKesson or a subsidiary.

              (viii) The consummation of the transaction contemplated hereby
shall not result in the loss or impairment of McKesson's or of any subsidiary's
right to own or use any of the Intellectual Property, and will not require the
consent of any governmental authority, except where such loss or impairment or
the failure to obtain consent would not result in a material adverse effect.

              (ix)   Neither McKesson nor any of its subsidiaries has entered
into any software license agreement in which it (a) failed to limit its
liability to the amount of licensing fees paid pursuant to the agreement; or (b)
warranted as to the performance or functionality of the Software other than
stating that the Software would perform in accordance with its documentation
and/or specifications; except in any case in which the contrary would not have a
material adverse effect.
 
              (x)    McKesson has implemented and is currently implementing
revisions and related testing of its material Software that it licenses and
maintains pursuant to contracts with third parties ("Licensed Software") in
order to enable such Software to process accurately (including calculating,
comparing and sequencing) in all material respects date data from, into and
between the twentieth and twenty-first centuries, including leap year
calculations ("Millennial Date Data"). By December 31, 1999, all such Licensed
Software will so process Millennial Date Data without material errors or
omissions and without materially affecting functionality when used in accordance
with the product documentation provided by McKesson
<PAGE>
 
therefor and provided that all other software and all hardware and firmware used
in combination with such Licensed Software properly exchanges date data with it.
Neither McKesson nor any of its subsidiaries has made any representation or
warranty to any third party that varies in any material respect from the
preceding warranty.

               (xi)   McKesson and its subsidiaries are in the process of, and
have substantially completed obtaining, written representations or assurances
from each third party that (A) provides Millennial Date Data to McKesson or its
subsidiaries, (B) processes Millennial Date Data for McKesson or its
subsidiaries or (C) otherwise provides any material product or service to
McKesson or its subsidiaries that is dependent upon any Software, microcode,
chip or hardware system or component, including any electronic or electronically
controlled system or component (a "System") that processes any Millennial Date
Data, stating that all of such Systems that are used for, or on behalf of,
McKesson or its subsidiaries will process Millennial Date Data without
materially affecting the supply of such product or service to McKesson or its
subsidiaries after December 31, 1999.

          (s)  Certain Contracts.  Except as set forth in the McKesson Filed SEC
               -----------------                                                
Documents, neither McKesson nor any of its subsidiaries is a party to or bound
by (i) any "material contract" (as such term is defined in Item 601(b)(10) of
Regulation S-K of the SEC), (ii) any non-competition agreement or any other
agreement or obligation which purports to limit in any material respect the
manner in which, or the localities in which, all or any material portion of the
business of McKesson and its subsidiaries (including, for purposes of this
Section 3.1(s), HBO and its subsidiaries, assuming the Merger has taken place),
taken as a whole, is or would be conducted, (iii) any exclusive supply or
purchase contracts or any exclusive requirements contracts or (iv) any contract
or other agreement which would prohibit or materially delay the consummation of
the Merger or any of the transactions contemplated by this Agreement (all 
contracts of the type described in clauses (i) and (ii) being referred to herein
as "McKesson Material Contracts"). McKesson has delivered to HBO or provided to
HBO for review, prior to the execution of this Agreement, complete and correct
copies of all McKesson Material Contracts not filed as exhibits to the McKesson
Filed SEC Documents. Each McKesson Material Contract is valid and binding on
McKesson (or, to the extent a McKesson subsidiary is a party, such subsidiary)
and is in full force and effect, and McKesson and each McKesson subsidiary have
in all material respects performed all obligations required to be performed by
them to date under each McKesson Material Contract, except where such
noncompliance, individually or in the aggregate, would not have a material
adverse effect on McKesson. Neither McKesson nor any McKesson subsidiary knows
of, or has received notice of, any violation or default under (nor, to the
knowledge of McKesson, does there exist any condition which with the passage of
time or the giving of notice or both would result in such a violation or default
under) any McKesson Material Contract.

          (t)  McKesson Rights Agreement.  McKesson has taken all action
               -------------------------                                
(including, if required, redeeming all of the outstanding preferred stock
purchase rights issued pursuant to the McKesson Rights Agreement or amending the
McKesson Rights Agreement) so that the entering into of this Agreement, the
McKesson Option Agreement, the Merger, the acquisition of shares pursuant to the
McKesson Option Agreement and the other transactions contemplated hereby and
thereby do not and will not result in the grant of any rights to any person
under the 
<PAGE>
 
McKesson Rights Agreement or enable or require the McKesson Rights to be
exercised, distributed or triggered.

          (u)  Environmental Liability.  Except as set forth in the McKesson
               -----------------------                                      
Filed SEC Documents, there are no legal, administrative, arbitral or other
proceedings, claims, actions, causes of action, private environmental
investigations or remediation activities or governmental investigations of any
nature pending or threatened against McKesson or any of its subsidiaries seeking
to impose, or that could reasonably be expected to result in the imposition of,
on McKesson or any of its subsidiaries, any liability or obligation arising
under common law or under any local, state or federal environmental statute,
regulation or ordinance, including, without limitation, the Comprehensive
Environmental Response, Compensation and Liability Act of 1980, as amended
("CERCLA"), which liability or obligation could reasonably be expected to have a
material adverse effect on McKesson.  To the knowledge of McKesson, there is no
reasonable basis for any such proceeding, claim, action or governmental
investigation that would impose any liability or obligation that could
reasonably be expected to have a material adverse effect on McKesson.

          (v)  Insurance. McKesson and each of its subsidiaries have policies of
               ---------                                             
insurance and bonds of the type and in amounts customarily carried by persons
conducting businesses or owning assets similar to those of McKesson and its
subsidiaries.  There is no claim pending under any of such policies or bonds as
to which coverage has been questioned, denied or disputed by the underwriters of
such policies or bonds, except questioned, denied or disputed claims the failure
to provide coverage for which would not, individually or in the aggregate, have
a material adverse effect on McKesson. All premiums due and payable under all
such policies and bonds have been paid and McKesson and its subsidiaries are
otherwise in compliance in all material respects with the terms of such policies
and bonds. McKesson has no knowledge of any threatened termination of, or
material premium increase with respect to, any of such policies.

          (w)  Transactions with Affiliates. Except as disclosed in the McKesson
               ----------------------------                             
SEC Documents filed prior to the date of this Agreement or as disclosed in the
McKesson Disclosure Schedule, since March 31, 1998, there have been no
transactions, agreements, arrangements or understandings between McKesson and
its affiliates that would be required to be disclosed under the Item 404 of
Regulation S-K under the Securities Act.

          (x)  Full Disclosure.  None of the representations or warranties made
               ---------------                                                 
by McKesson herein or in any schedule hereto, including the McKesson Disclosure
Schedule, or any certificate furnished by McKesson pursuant to this Agreement,
contains or will contain at the Effective Time any untrue statement of a
material fact, or omits or will omit at the Effective Time, to state any
material fact necessary in order to make the statements contained herein or
therein, in the light of the circumstances under which made, not misleading.

     SECTION 3.2    Representations and Warranties of HBO.  Except as disclosed
                    -------------------------------------                      
in the Disclosure Schedule delivered by HBO to McKesson prior to the execution
of this Agreement 
<PAGE>
 
(the "HBO Disclosure Schedule") and making reference to the particular
subsection of this Agreement to which exception is being taken, HBO represents
and warrants to McKesson as follows:

          (a)  Organization, Standing and Corporate Power.
               ------------------------------------------ 

               (i)    Each of HBO and its subsidiaries (as defined in Section
8.3) is a corporation or other legal entity duly organized, validly existing and
in good standing (with respect to jurisdictions which recognize such concept)
under the laws of the jurisdiction in which it is organized and has the
requisite corporate or other power, as the case may be, and authority to carry
on its business as now being conducted, except, as to subsidiaries, for those
jurisdictions where the failure to be so organized, existing or in good standing
individually or in the aggregate would not have a material adverse effect (as
defined in Section 8.3) on HBO. Each of HBO and its subsidiaries is duly
qualified or licensed to do business and is in good standing (with respect to
jurisdictions which recognize such concept) in each jurisdiction in which the
nature of its business or the ownership, leasing or operation of its properties
makes such qualification or licensing necessary, except for those jurisdictions
where the failure to be so qualified or licensed or to be in good standing
individually or in the aggregate would not have a material adverse effect on
HBO.

               (ii)   HBO has delivered to McKesson prior to the execution of
this Agreement complete and correct copies of any amendments to its certificate
of incorporation (the "HBO Certificate") and by-laws not filed as of the date
hereof with the HBO SEC Documents (as defined in Section 3.2(e)).

               (iii)  In all material respects, the minute books of HBO and its
subsidiaries contain accurate records of all meetings and accurately reflect all
other actions taken by the stockholders, the Board of Directors and all
committees of the Board of Directors of HBO (or, as the case may be, each of its
subsidiaries) since December 31, 1996.

          (b)  Subsidiaries.  Exhibit 21 to HBO's Annual Report on Form 10-K for
               ------------   ----------                                        
the fiscal year ended December 31, 1997 includes all the subsidiaries of HBO
which as of the date of this Agreement are Significant Subsidiaries (as defined
in Rule 1-02 of Regulation S-X of the SEC).  All the outstanding shares of
capital stock of, or other equity interests in, each such Significant Subsidiary
have been validly issued and are fully paid and nonassessable and are owned
directly or indirectly by HBO, free and clear of all Liens and free of any other
restriction (including any restriction on the right to vote, sell or otherwise
dispose of such capital stock or other ownership interests).

          (c)  Capital Structure.  The authorized capital stock of HBO consists
               -----------------                                               
of 1,000,000,000 shares of HBO Common Stock, and 1,000,000 shares of preferred
stock, without par value ("HBO Preferred Stock").  At the close of business on
September 30, 1998:  (i) 431,485,014 shares of HBO Common Stock were issued and
outstanding; (ii) no shares of HBO Common Stock were held by HBO in its
treasury; (iii) no shares of HBO Preferred Stock were issued and outstanding;
(iv) 3,303,188 shares of HBO Common Stock were reserved for issuance pursuant to
all stock option, restricted stock or other stock-based compensation, benefits
or 
<PAGE>
 
savings plans, agreements or arrangements in which current or former employees
or directors of HBO or its subsidiaries participate as of the date hereof
(including, without limitation, the plans set forth on Exhibit F attached
                                                       ---------         
hereto), complete and correct copies of which, in each case as amended as of the
date hereof, have been filed as exhibits to the HBO Filed SEC Documents or
delivered to McKesson (such plans, collectively, the "HBO Stock Plans"); and (v)
20,000 shares of HBO Preferred Stock designated as Series A Junior Participating
Preferred Stock were reserved for issuance upon the exercise of preferred stock
purchase rights (the "HBO Rights") issued pursuant to the HBO Rights Agreement.
Section 3.2(c) of the HBO Disclosure Schedule sets forth a complete and correct
list, as of September 30, 1998, of the number of shares of HBO Common Stock
subject to employee stock options or other rights to purchase or receive HBO
Common Stock granted under the HBO Stock Plans (collectively, "HBO Employee
Stock Options"). All outstanding shares of capital stock of HBO are, and all
shares which may be issued pursuant to this Agreement or otherwise will be, when
issued, duly authorized, validly issued, fully paid and nonassessable and not
subject to preemptive rights. Except as set forth in this Section 3.2(c), and
except for changes since September 30, 1998 resulting from the issuance of
shares of HBO Common Stock pursuant to the HBO Employee Stock Options or as
expressly permitted by this Agreement, (x) there are not issued, reserved for
issuance or outstanding (A) any shares of capital stock or other voting
securities of HBO, (B) any securities of HBO or any HBO subsidiary convertible
into or exchangeable or exercisable for shares of capital stock or voting
securities of HBO, (C) any warrants, calls, options or other rights to acquire
from HBO or any HBO subsidiary, and any obligation of HBO or any HBO subsidiary
to issue, any capital stock, voting securities or securities convertible into or
exchangeable or exercisable for capital stock or voting securities of HBO, and
(y) there are no outstanding obligations of HBO or any HBO subsidiary to
repurchase, redeem or otherwise acquire any such securities or to issue, deliver
or sell, or cause to be issued, delivered or sold, any such securities. There
are no outstanding (A) securities of HBO or any HBO subsidiary convertible into
or exchangeable or exercisable for shares of capital stock or other voting
securities or ownership interests in any HBO subsidiary, (B) warrants, calls,
options or other rights to acquire from HBO or any HBO subsidiary, or
obligations of HBO or any HBO subsidiary to issue, any capital stock, voting
securities or other ownership interests in, or any securities convertible into
or exchangeable or exercisable for any capital stock, voting securities or
ownership interests in, any HBO subsidiary or (C) obligations of HBO or any HBO
subsidiary to repurchase, redeem or otherwise acquire any such outstanding
securities of HBO subsidiaries or to issue, deliver or sell, or cause to be
issued, delivered or sold, any such securities. Neither HBO nor any HBO
subsidiary is a party to any agreement restricting the purchase or transfer of,
relating to the voting of, requiring registration of, or granting any preemptive
or, except as provided by the terms of the HBO Employee Stock Options,
antidilutive rights with respect to, any securities of the type referred to in
the two preceding sentences. Other than the HBO subsidiaries, HBO does not
directly or indirectly beneficially own any securities or other beneficial
ownership interests in any other entity except for non-controlling investments
made in the ordinary course of business in entities which are not individually
or in the aggregate material to HBO and its subsidiaries as a whole.

          (d)  Authority; Noncontravention.  HBO has all requisite corporate
               ---------------------------                                  
power and authority to enter into this Agreement, and HBO has all requisite
corporate power and authority to enter into the Option Agreements and, subject
to the HBO Stockholder Approval (as defined in Section 3.2(l)), to consummate
the transactions contemplated hereby and thereby.  The 
<PAGE>
 
execution and delivery of this Agreement by HBO and the execution and delivery
of the Option Agreements by HBO and the consummation by HBO of the transactions
contemplated hereby and thereby have been duly authorized by all necessary
corporate action on the part of HBO, subject, in the case of the Merger, to the
HBO Stockholder Approval. This Agreement has been, and the Option Agreements
will be, duly executed and delivered by HBO and, assuming the due authorization,
execution and delivery thereof by McKesson, constitute (or will constitute, as
the case may be) the legal, valid and binding obligation of HBO, enforceable
against HBO in accordance with their terms. The execution and delivery of this
Agreement does not, and the execution and delivery of the Option Agreements and
the consummation of the transactions contemplated hereby and thereby and
compliance with the provisions of this Agreement and the Option Agreements will
not, conflict with, or result in any violation of, or default (with or without
notice or lapse of time, or both) under, or give rise to a right of termination,
cancellation or acceleration of any obligation or loss of a benefit under, or
result in the creation of any Lien upon any of the properties or assets of HBO
or any of its subsidiaries or any restriction on the conduct of HBO's business
or operations under, (i) the HBO Certificate or the by-laws of HBO or the
comparable organizational documents of any of its subsidiaries, (ii) any loan or
credit agreement, note, bond, mortgage, indenture, trust document, lease or
other agreement, instrument, permit, concession, franchise, license or similar
authorization applicable to HBO or any of its subsidiaries or their respective
properties or assets or (iii) subject to the governmental filings and other
matters referred to in the following sentence, any judgment, order, decree,
statute, law, ordinance, rule or regulation applicable to HBO or any of its
subsidiaries or their respective properties or assets, other than, in the case
of clauses (ii) and (iii), any such conflicts, violations, defaults, rights,
losses, restrictions or Liens that individually or in the aggregate would not
(x) have a material adverse effect on HBO or (y) reasonably be expected to
impair the ability of HBO to perform its obligations under this Agreement and
the Option Agreements). No consent, approval, order or authorization of, action
by, or in respect of, or registration, declaration or filing with, any
Governmental Entity is required by or with respect to HBO or any of its
subsidiaries in connection with the execution and delivery of this Agreement or
the Option Agreements by HBO or the consummation by HBO of the transactions
contemplated hereby or thereby, except for (1) the filing of a pre-merger
notification and report form by HBO under the HSR Act or filings or
notifications under the antitrust, competition or similar laws of any foreign
jurisdiction; (2) the filing with the SEC of (A) the Joint Proxy Statement
relating to the HBO Stockholders' Meeting, and (B) such reports under Section
13(a), 13(d), 15(d) or 16(a) of the Exchange Act as may be required in
connection with this Agreement and the Option Agreements and the transactions
contemplated hereby and thereby; (3) the filing of the Certificate of Merger
with the Secretary of State of Delaware and appropriate documents with the
relevant authorities of other states in which HBO is qualified to do business
and such filings with Governmental Entities to satisfy the applicable
requirements of state securities or "blue sky" laws; and (4) such consents,
approvals, orders or authorizations the failure of which to be made or obtained
individually or in the aggregate would not (x) have a material adverse effect on
HBO or (y) reasonably be expected to impair the ability of HBO to perform its
obligations under this Agreement.

          (e) SEC Documents; Undisclosed Liabilities.  HBO has filed all
              --------------------------------------                    
required registration statements, prospectuses, reports, schedules, forms,
statements and other documents (including exhibits and all other information
incorporated therein) with the SEC since 
<PAGE>
 
December 31, 1996 (the "HBO SEC Documents"). As of their respective dates, the
HBO SEC Documents complied in all material respects with the requirements of the
Securities Act or the Exchange Act, as the case may be, and the rules and
regulations of the SEC promulgated thereunder applicable to such HBO SEC
Documents, and none of the HBO SEC Documents when filed contained any untrue
statement of a material fact or omitted to state a material fact required to be
stated therein or necessary in order to make the statements therein, in light of
the circumstances under which they were made, not misleading. The financial
statements of HBO included in the HBO SEC Documents comply as to form, as of
their respective dates of filing with the SEC, in all material respects with
applicable accounting requirements and the published rules and regulations of
the SEC with respect thereto, have been prepared in accordance with GAAP
(except, in the case of unaudited statements, as permitted by Form 10-Q of the
SEC) applied on a consistent basis during the periods involved (except as may be
indicated in the notes thereto) and fairly present the consolidated financial
position of HBO and its consolidated subsidiaries as of the dates thereof and
the consolidated results of their operations and cash flows for the periods then
ended (subject, in the case of unaudited statements, to normal year-end audit
adjustments which are not material). Except (i) as reflected in such financial
statements or in the notes thereto or (ii) for liabilities incurred in
connection with this Agreement, the Option Agreements or the transactions
contemplated hereby or thereby, neither HBO nor any of its subsidiaries has any
liabilities or obligations of any nature which, individually or in the
aggregate, would have a material adverse effect on HBO.

          (f)  Information Supplied.  None of the information supplied or to be
               --------------------                                            
supplied by HBO specifically for inclusion or incorporation by reference in (i)
the Form S-4 will, at the time the Form S-4 becomes effective under the
Securities Act, contain any untrue statement of a material fact or omit to state
any material fact required to be stated therein or necessary to make the
statements therein not misleading or (ii) the Joint Proxy Statement will, at the
date it is first mailed to HBO's stockholders or at the time of the HBO
Stockholders' Meeting, contain any untrue statement of a material fact or omit
to state any material fact required to be stated therein or necessary in order
to make the statements therein, in light of the circumstances under which they
are made, not misleading. The Joint Proxy Statement will comply as to form in
all material respects with the requirements of the Securities Act and the
Exchange Act and the rules and regulations thereunder, except that no
representation or warranty is made by HBO with respect to statements made or
incorporated by reference therein based on information supplied by McKesson
specifically for inclusion or incorporation by reference in the Joint Proxy
Statement.

          (g)  Absence of Certain Changes or Events.  Except for liabilities
               ------------------------------------                         
incurred in connection with this Agreement, the Option Agreements or the
transactions contemplated hereby or thereby, and except as permitted by Section
4.1(b), since March 31, 1998, HBO and its subsidiaries have conducted their
business only in the ordinary course consistent with past practice or as
disclosed in any HBO SEC Document filed since such date and prior to the date
hereof, and there has not been (i) any material adverse change (as defined in
Section 8.3) in HBO, (ii) any declaration, setting aside or payment of any
dividend or other distribution (whether in cash, stock or property) with respect
to any of HBO's capital stock, (iii) any split, combination or reclassification
of any of HBO's capital stock or any issuance or the authorization of any
issuance of any other securities in respect of, in lieu of or in substitution
for shares of HBO's capital stock, except for issuances of HBO Common Stock upon
exercise or conversion of HBO
<PAGE>
 
Employee Stock Options, in each case awarded prior to the date hereof in
accordance with their present terms or issued pursuant to Section 4.1(b),
(iv)(A) any granting by HBO or any of its subsidiaries to any current or former
director, officer or other key employee of HBO or its subsidiaries of any
increase in compensation, bonus or other benefits, except for normal increases
as a result of promotions, normal increases of base pay or target bonuses in the
ordinary course of business or as was required under any employment agreements
in effect as of March 31, 1998, (B) any granting by HBO or any of its
subsidiaries to any such current or former director, officer or key employee of
any increase in severance or termination pay, or (C) any entry by HBO or any of
its subsidiaries into, or any amendment of, any employment, deferred
compensation, consulting, severance, termination or indemnification agreement
with any such current or former director, officer, or any material amendment of
any of the foregoing with any key employee, (v) except insofar as may have been
disclosed in HBO SEC Documents filed and publicly available prior to the date of
this Agreement (as amended to the date hereof, the "HBO Filed SEC Documents") or
required by a change in GAAP, any change in accounting methods, principles or
practices by HBO materially affecting its assets, liabilities or business, (vi)
except insofar as may have been disclosed in the HBO Filed SEC Documents, any
tax election that individually or in the aggregate would have a material adverse
effect on HBO or any of its tax attributes or any settlement or compromise of
any material income tax liability or (vii) any action taken by HBO or any of the
HBO subsidiaries during the period from April 1, 1998 through the date of this
Agreement that, if taken during the period from the date of this Agreement
through the Effective Time, would constitute a breach of Section 4.1(b).

          (h)  Compliance with Applicable Laws; Litigation.
               ------------------------------------------- 

               (i)    HBO, its subsidiaries and employees hold all permits,
licenses, variances, exemptions, orders, registrations and approvals of all
Governmental Entities which are required for the operation of the businesses of
HBO and its subsidiaries (the "HBO Permits") except where the failure to have
any such HBO Permits individually or in the aggregate would not have a material
adverse effect on HBO. Except as specifically disclosed in the HBO SEC Documents
filed with the Commission prior to the date hereof, HBO and its subsidiaries are
in compliance with the terms of the HBO Permits and all Applicable Laws relating
to HBO or its business or properties, except where the failure to be in
compliance with such Applicable Laws individually or in the aggregate would not
have a material adverse effect on HBO. As of the date of this Agreement, except
as disclosed in the HBO Filed SEC Documents, no action, demand, requirement or
investigation by any Governmental Entity and no suit, action or proceeding by
any person, in each case with respect to HBO or any of its subsidiaries or any
of their respective properties, is pending or, to the knowledge of HBO,
threatened, other than, in each case, those the outcome of which individually or
in the aggregate would not (A) have a material adverse effect on HBO or (B)
reasonably be expected to impair the ability of HBO to perform its obligations
under this Agreement or the Option Agreements or prevent or materially delay the
consummation of any of the transactions contemplated hereby or thereby.

               (ii)   Neither HBO nor any HBO subsidiary is subject to any
outstanding 
<PAGE>
 
order, injunction or decree which has had or, insofar as can be reasonably
foreseen, individually or in the aggregate will have, a material adverse effect
on HBO.

          (i)  Absence of Changes in Benefit Plans.  HBO has delivered to
               -----------------------------------                       
McKesson or provided to McKesson for review true and complete copies of (i) all
severance and employment agreements of HBO with directors, executive officers or
key employees, (ii) all written and material unwritten severance programs and
policies of each of HBO and each HBO subsidiary, and (iii) all plans or
arrangements of HBO and each HBO subsidiary relating to its employees which
contain change in control provisions, in each case which has not been filed as
an exhibit to an HBO Filed SEC Document.  Since March 31, 1998, there has not
been any adoption or amendment in any material respect by HBO or any of its
subsidiaries of any (A) collective bargaining agreement with respect to any
employees of, (B) any material bonus, pension, profit sharing, deferred
compensation, incentive compensation, stock ownership, stock purchase, stock
option, phantom stock, retirement, vacation, severance, disability, death
benefit, hospitalization, medical or other plan, arrangement or understanding
providing benefits to, any current or former officers, directors or employees
of, (C) any employment agreement, consulting agreement or severance agreement
with, any current or former officer or director of, or (D) any material
employment agreement, consulting agreement or severance agreement with any
employee of, HBO or any of its wholly owned subsidiaries (collectively, the "HBO
Benefit Plans"), or any material change in any actuarial or other assumption
used to calculate funding obligations with respect to any HBO pension plans, or
any material change in the manner in which contributions to any HBO pension
plans are made or the basis on which such contributions are determined. Since
March 31, 1998, neither HBO nor any HBO subsidiary has amended any HBO Employee
Stock Options or any HBO Stock Plans to accelerate the vesting of, or release
restrictions on, awards thereunder, or to provide for such acceleration in the
event of a change in control.

          (j)  Benefit Plans.
               ------------- 

               (i)    With respect to the HBO Benefit Plans, to the knowledge of
HBO, no event has occurred and there exists no condition or set of
circumstances, in connection with which HBO or any of its subsidiaries would be
subject to any liability that individually or in the aggregate could have a
material adverse effect on HBO under ERISA, the Code or any other applicable
law.

               (ii)   Each HBO Benefit Plan has been administered in accordance
with its terms, except for any failures so to administer any HBO Benefit Plan
that individually or in the aggregate would not have a material adverse effect
on HBO. To the knowledge of HBO, the HBO Benefit Plans have been operated, and
are, in compliance with the applicable provisions of ERISA, the Code and all
other applicable laws and the terms of all applicable collective bargaining
agreements, except for any failures to be in such compliance that individually
or in the aggregate would not have a material adverse effect on HBO. Each HBO
Benefit Plan that is intended to be qualified under Section 401(a) or 401(k) of
the Code has received a favorable determination letter from the IRS that it is
so qualified and each trust established in connection with any HBO Benefit Plan
that is intended to be exempt from federal income taxation under Section 501(a)
of the Code has received a determination letter from the IRS that such trust is
so exempt. To the knowledge of HBO, no fact or event has occurred since the date
of any
<PAGE>
 
determination letter from the IRS which is reasonably likely to affect adversely
the qualified status of any such HBO Benefit Plan or the exempt status of any
such trust.

               (iii)  No HBO Benefit Plan is subject to Title IV of ERISA or is
a "multiemployer plan" within the meaning of Section 3(37) of ERISA.

               (iv)   No HBO Benefit Plan provides medical benefits (whether or
not insured), with respect to current or former employees after retirement or
other termination of service (other than coverage mandated by applicable law or
benefits, the full cost of which is borne by the current or former employee)
other than individual arrangements the amounts of which are not material.

               (v)    HBO has previously provided to McKesson a copy of each
collective bargaining or other labor union contract applicable to persons
employed by HBO or any of its subsidiaries to which HBO or any of its
subsidiaries is a party. No collective bargaining agreement is being negotiated
or renegotiated by HBO or any of its subsidiaries. As of the date of this
Agreement, there is no labor dispute, strike or work stoppage against HBO or any
of its subsidiaries pending or, to the knowledge of HBO, threatened which may
interfere with the respective business activities of HBO or any of its
subsidiaries, except where such dispute, strike or work stoppage individually or
in the aggregate would not have a material adverse effect on HBO. As of the date
of this Agreement, to the knowledge of HBO, none of HBO, any of its subsidiaries
or any of their respective representatives or employees has committed any
material unfair labor practice in connection with the operation of the
respective businesses of HBO or any of its subsidiaries, and there is no
material charge or complaint against HBO or any of its subsidiaries by the
National Labor Relations Board or any comparable governmental agency pending or
threatened in writing.

               (vi)   No employee of HBO will be entitled to any material
payment, additional benefits or any acceleration of the time of payment or
vesting of any benefits under any HBO Benefit Plan as a result of the
transactions contemplated by this Agreement (either alone or in conjunction with
any other event such as a termination of employment).

               (vii)  No material oral or written representation or commitment
with respect to any aspect of any HBO Benefit Plan has been or will be made to
employees of HBO or any HBO subsidiaries by an authorized HBO employee prior to
the Closing Date that is not materially in accordance with the written or
otherwise preexisting terms and provisions of such HBO Benefit Plans in effect
immediately prior to the Closing Date.

               (viii) Except as would not have a material adverse effect, there
are no material unresolved claims or disputes under the terms of, or in
connection with, any HBO Benefit Plan (other than routine undisputed claims for
benefits), and no action, legal or otherwise, has been commenced with respect to
any material claim.

               (ix)   To the knowledge of HBO, no non-exempt "prohibited
transaction" (within the meaning of Section 4975(c) of the Tax Code) involving
any HBO Benefit Plan has occurred that could subject HBO to any material tax
penalty or other cost or liability (by  
<PAGE>
 
indemnification or otherwise).

          (k)  Taxes.
               ----- 

               (i)    Each of HBO and its subsidiaries has filed all material
tax returns and reports required to be filed by it (taking into account
applicable extensions) and all such returns and reports are complete and correct
in all material respects, or requests for extensions to file such returns or
reports have been timely filed, granted and have not expired, except to the
extent that such failures to file, to be complete or correct or to have
extensions granted that remain in effect individually or in the aggregate would
not have a material adverse effect on HBO. HBO and each of its subsidiaries has
paid (or HBO has paid or caused to be paid on its behalf) all taxes shown as due
on such returns, and the most recent financial statements contained in the HBO
Filed SEC Documents reflect an adequate reserve in accordance with GAAP for all
taxes payable by HBO and its subsidiaries for all taxable periods and portions
thereof accrued through the date of such financial statements.

               (ii)   No deficiencies for any taxes have, to the knowledge of
HBO, been proposed, asserted or assessed against HBO or any of its subsidiaries
that are not adequately reserved for, except for deficiencies that individually
or in the aggregate would not have a material adverse effect on HBO. Except as
provided in Section 3.1(k) of the Disclosure Schedule, all of the federal income
tax returns of the affiliated group of which HBO is the common parent have
closed by virtue of the applicable statute of limitations.

               (iii)  Neither HBO nor any of its subsidiaries has taken any
action or knows of any fact, agreement, plan or other circumstance that is
reasonably likely to prevent the Merger from qualifying as a reorganization
within the meaning of Section 368(a) of the Code. 

               (iv)   Neither HBO nor any HBO subsidiary has made any parachute
payments as such term is defined in Section 280G of the Tax Code, neither is
obligated to make any parachute payments, and neither is a party to any
agreement that under certain circumstances could obligate it, or any successor
in interest, to make any parachute payments that will not be deductible under
Section 280G of the Tax Code.  Neither HBO nor any HBO subsidiary is obligated
to make reimbursement or gross-up payments to any person in respect to excess
parachute payments.

          (l)  Voting Requirements.  The affirmative vote at the HBO
               -------------------                                  
Stockholders' Meeting (the "HBO Stockholder Approval") of the holders of a
majority of all outstanding shares of HBO Common Stock entitled to vote at a
duly convened and held meeting of HBO stockholders is the only vote of the
holders of any class or series of HBO's capital stock necessary to approve and
adopt this Agreement and the transactions contemplated hereby, including the
Merger.

          (m)  State Takeover Statutes; Certificate of Incorporation.  The Board
               -----------------------------------------------------            
of Directors of HBO has adopted a resolution or resolutions approving this
Agreement, the Option Agreements, and the transactions contemplated hereby and
thereby, and, assuming the accuracy of McKesson's representation and warranty
contained in Section 3.1(q), such approval constitutes 
<PAGE>
 
approval of the Merger and the other transactions contemplated hereby and
thereby by the HBO Board of Directors under the provisions of Section 203 of the
DGCL and paragraphs 11 and 12 of HBO's Certificate of Incorporation such that
Section 203 of the DGCL and paragraphs 11 and 12 of HBO's Certificate of
Incorporation do not apply to this Agreement, the Option Agreements, or the
transactions contemplated hereby and thereby. To the knowledge of HBO, no state
takeover statute other than Section 203 of the DGCL (which has been rendered
inapplicable) is applicable to the Merger or the other transactions contemplated
hereby.

          (n) Accounting Matters.  To its knowledge, neither HBO nor any of its
              ------------------                                               
affiliates (as such term is used in Section 5.9) has taken or agreed to take any
action (including, without limitation, in connection with any HBO Stock Plan or
any agreement thereunder) that would prevent the business combination to be
effected by the Merger from being accounted for as a "pooling of interests," and
HBO has no reason to believe that the Merger will not qualify for "pooling of
interests" accounting.

          (o) Brokers.  No broker, investment banker, financial advisor or other
              -------                                                           
person, is entitled to any broker's, finder's, financial advisor's or other
similar fee or commission in connection with the transactions contemplated by
this Agreement based upon arrangements made by or on behalf of HBO.

          (p) Opinion of Financial Advisors.  HBO has received the opinion of
              -----------------------------                                  
Morgan Stanley Dean Witter, dated the date of this Agreement, to the effect
that, as of such date, the Exchange Ratio for the conversion of HBO Common Stock
into McKesson Common Stock is fair from a financial point of view to holders of
shares of HBO Common Stock (other than McKesson and its affiliates), a signed
copy of which opinion has been delivered to McKesson.

          (q) Ownership of McKesson Common Stock.  To the knowledge of HBO, as
              ----------------------------------                              
of the date hereof or at any time within twelve months prior to the date of this
Agreement (and before giving effect to the McKesson Option Agreement, which will
be entered into immediately after the execution of this Agreement) neither HBO
nor, to its knowledge without independent investigation, any of its
affiliates, (i) beneficially owns (as defined in either Rule 13d-3 under the
Exchange Act) or owned, directly or indirectly, or (ii) is or was party to any
agreement, arrangement or understanding for the purpose of acquiring, holding,
voting or disposing of, in each case, shares of capital stock of McKesson.

          (r)  Intellectual Property.
               --------------------- 

               (i)  HBO or its subsidiaries own or have a valid right to use all
trademarks, service marks, trade names, Internet domain names, designs, slogans,
and general intangibles of like nature, together with all applications,
registrations and goodwill related to the foregoing (collectively,
"Trademarks"); patents (including any registration, continuations, 
continuations-in-part, renewals and applications for any of the foregoing);
copyrights (including any registrations and applications for any of the
foregoing); Software (as defined below);
<PAGE>
 
technology, trade secrets and other confidential information, know-how,
proprietary processes, formulae, algorithms, models, and methodologies
(collectively, "Trade Secrets") used in or necessary for the conduct of HBO's
and each of its subsidiary's business as currently conducted, except where the
failure to possess such right would not have a material adverse effect (all such
intellectual property being referred to herein as the "Intellectual Property").
For purposes of this Section 3.2(r), "Software" means any and all (a) computer
programs, including any and all software implementations of algorithms, models
and methodologies, whether in source code or object code, (b) databases and
compilations, including any and all data and collections of data, whether
machine readable or otherwise, (c) descriptions, flow-charts and other work
product used to design, plan, organize and develop any of the foregoing, (d) the
technology supporting any Internet site(s) operated by or on behalf of HBO or
any of its subsidiaries and (e) all documentation, including user manuals and
training materials, relating to any of the foregoing.

               (ii)  The Intellectual Property owned by HBO or any subsidiary is
free and clear of all Liens.

               (iii) The Intellectual Property owned by HBO or any of its
subsidiaries and, to HBO's knowledge, any Intellectual Property used by HBO, is
valid and subsisting, in full force and effect, and has not been canceled,
expired, or abandoned.  There is no pending or, to HBO's knowledge, threatened
opposition, interference or cancellation proceeding before any court or
registration authority in any jurisdiction against any registrations in respect
of the Intellectual Property owned by HBO or any of its subsidiaries, or, to
HBO's knowledge, against any Intellectual Property licensed to HBO or any of its
subsidiaries.

               (iv)  To the actual knowledge of HBO or any of its subsidiaries,
the conduct of the business of HBO and its subsidiaries as currently conducted
does not infringe upon (either directly or indirectly such as through
contributory infringement or inducement to infringe) any intellectual property
rights owned or controlled by any third party. There are no claims or suits
pending or, to the knowledge of HBO, threatened, and neither HBO nor any of its
subsidiaries has received any notice of a third-party claim or suit, (a)
alleging that its activities or the conduct of its business infringes upon,
violates, or constitutes the unauthorized use of the intellectual property
rights of any third party or (b) challenging the ownership, use, validity or
enforceability of any Intellectual Property, which in any case would have a
material adverse effect.

               (v)   There are no settlements, forebearances to sue, consents,
judgments, or orders or similar obligations which in any material respect (a)
restrict the right of HBO or its subsidiaries to use any Intellectual Property,
or (b) restrict the business of HBO or its subsidiaries in order to accommodate
a third party's intellectual property rights or (c) except for licenses with
customers for HBO's Software, there are no agreements that permit third parties
to use any Intellectual Property owned or controlled by HBO or any of its
subsidiaries.

               (vi)  HBO and each of its subsidiaries take reasonable measures
to protect the confidentiality of Trade Secrets, including (i) requiring its
employees and independent contractors having access thereto to execute written
nondisclosure agreements and (ii) requiring all licensees to maintain the
confidentiality of its Trade Secrets. To the actual
<PAGE>
 
knowledge of HBO or its subsidiaries, no Trade Secret has been knowingly
disclosed or authorized to be disclosed to any third party other than pursuant
to a nondisclosure agreement or other appropriate instrument that adequately
protects HBO and the applicable subsidiary's proprietary interests in and to
such Trade Secrets. To the knowledge of HBO, no party to any nondisclosure
agreement or nondisclosure obligation relating to its Trade Secrets is in breach
or default thereof.

               (vii)  To the knowledge of HBO, no third party is
misappropriating, infringing, diluting, or violating any Intellectual Property
owned by HBO or any of its subsidiaries other than immaterial disputes
concerning use by a third party of Trademarks of HBO or a subsidiary.

               (viii) The consummation of the transaction contemplated hereby
shall not result in the loss or impairment of HBO's or of any subsidiary's right
to own or use any of the Intellectual Property, and will not require the consent
of any governmental authority, except where such loss or impairment or the
failure to obtain consent would not result in a material adverse effect.

               (ix)   Neither HBO nor any of its subsidiaries has entered into
any software license agreement in which it (a) failed to limit its liability to
the amount of licensing fees paid pursuant to the agreement; or (b) warranted as
to the performance or functionality of the Software other than stating that the
Software would perform in accordance with its documentation and/or
specifications; except in any case in which the contrary would not have a
material adverse effect.

               (x)   HBO has implemented and is currently implementing revisions
and related testing of its material Software that it licenses and maintains
pursuant to contracts with third parties ("Licensed Software") in order to
enable such Software to process accurately (including calculating, comparing and
sequencing) in all material respects date data from, into and between the
twentieth and twenty-first centuries, including leap year calculations
("Millennial Date Data"). By December 31, 1999, all such Licensed Software will
so process Millennial Date Data without material errors or omissions and without
materially affecting functionality when used in accordance with the product
documentation provided by HBO therefor and provided that all other software and
all hardware and firmware used in combination with such Licensed Software
properly exchanges date data with it. Neither HBO nor any of its subsidiaries
has made any representation or warranty to any third party that varies in any
material respect from the preceding warranty.

               (xi)   HBO and its subsidiaries are in the process of, and have
substantially completed obtaining, written representations or assurances from
each third party that (A) provides Millennial Date Data to HBO or its
subsidiaries, (B) processes Millennial Date Data for HBO or its subsidiaries or
(C) otherwise provides any material product or service to HBO or its
subsidiaries that is dependent upon any Software, microcode, chip or hardware
system or component, including any electronic or electronically controlled
system or component (a "System") that processes any Millennial Date Data,
stating that all of such Systems that are used for, or on behalf of, HBO or its
subsidiaries will process Millennial Date Data without
<PAGE>
 
materially affecting the supply of such product or service to HBO and its
subsidiaries after December 31, 1999.

          (s) Certain Contracts.  Except as set forth in the HBO Filed SEC
              -----------------                                           
Documents or listed in Section 3.2(s) of the HBO Disclosure Schedule, neither
HBO nor any of its subsidiaries is a party to or bound by (i) any "material
contract" (as such term is defined in Item 601(b)(10) of Regulation S-K of the
SEC), (ii) any non-competition agreement or any other agreement or obligation
which purports to limit in any material respect the manner in which, or the
localities in which, all or any material portion of the business of HBO and its
subsidiaries (including, for purposes of this Section 3.2(s), McKesson and its
subsidiaries, assuming the Merger has taken place), taken as a whole, is or
would be conducted, (iii) any exclusive supply or purchase contracts or any
exclusive requirements contracts or (iv) any contract or other agreement which
would prohibit or materially delay the consummation of the Merger or any of the
transactions contemplated by this Agreement (all contracts of the type described
in clauses (i) and (ii) being referred to herein as "HBO Material Contracts").
HBO has delivered to McKesson or made available to McKesson for review, prior to
the execution of this Agreement, complete and correct copies of all HBO Material
Contracts not filed as exhibits to the HBO Filed SEC Documents. Each HBO
Material Contract is valid and binding on HBO (or, to the extent a HBO
subsidiary is a party, such subsidiary) and is in full force and effect, and HBO
and each HBO subsidiary have in all material respects performed all obligations
required to be performed by them to date under each HBO Material Contract,
except where such noncompliance, individually or in the aggregate, would not
have a material adverse effect on HBO. Neither HBO nor any HBO subsidiary knows
of, or has received notice of, any violation or default under (nor, to the
knowledge of HBO, does there exist any condition which with the passage of time
or the giving of notice or both would result in such a violation or default
under) any HBO Material Contract.

          (t) HBO Rights Agreement.  HBO has taken all action (including, if
              --------------------                                          
required, redeeming all of the outstanding preferred stock purchase rights
issued pursuant to the HBO Rights Agreement or amending the HBO Rights
Agreement) so that the entering into of this Agreement, the HBO Option Agreement
and the Merger, the acquisition of shares pursuant to the HBO Option Agreement
and the other transactions contemplated hereby and thereby do not and will not
result in the grant of any rights to any person under the HBO Rights Agreement
or enable or require the HBO Rights to be exercised, distributed or triggered.

          (u) Environmental Liability.  Except as set forth in the HBO Filed SEC
              -----------------------                                           
Documents, there are no legal, administrative, arbitral or other proceedings,
claims, actions, causes of action, private environmental investigations or
remediation activities or governmental investigations of any nature pending or
threatened against HBO or any of its subsidiaries seeking to impose, or that
could reasonably be expected to result in the imposition, on HBO or any of its
subsidiaries, of any liability or obligation arising under common law or under
any local, state or federal environmental statute, regulation or ordinance,
including, without limitation, CERCLA, which liability or obligation could
reasonably be expected to have a material adverse effect on HBO.  To the
knowledge of HBO, there is no reasonable basis for any such proceeding, claim,
<PAGE>
 
action or governmental investigation that would impose any liability or
obligation that could reasonably be expected to have a material adverse effect
on HBO.

          (v) Insurance.  HBO and each of its subsidiaries have policies of
              ---------                                                    
insurance and bonds of the type and in amounts customarily carried by persons
conducting businesses or owning assets similar to those of HBO and its
subsidiaries. There is no material claim pending under any of such policies or
bonds as to which coverage has been questioned, denied or disputed by the
underwriters of such policies or bonds, except questioned, denied or disputed
claims the failure to provide coverage for which would not, individually or in
the aggregate, have a material adverse effect on HBO. All premiums due and
payable under all such policies and bonds have been paid and HBO and its
subsidiaries are otherwise in compliance in all material respects with the terms
of such policies and bonds. HBO has no knowledge of any threatened termination
of, or material premium increase with respect to, any of such policies.

          (w) Transactions with Affiliates.  Except as disclosed in the HBO SEC
              ----------------------------                                     
Documents filed prior to the date of this Agreement or as disclosed in the HBO
Disclosure Schedule, since December 31, 1997, there have been no transactions,
agreements, arrangements or understandings between HBO and its affiliates that
would be required to be disclosed under the Item 404 of Regulation S-K under the
Securities Act.

          (x) Full Disclosure.  None of the representations or warranties made
              ---------------                                                 
by HBO herein or in any schedule hereto, including the HBO Disclosure Schedule,
or any certificate furnished by HBO pursuant to this Agreement, contains or will
contain at the Effective Time any untrue statement of a material fact, or omits
or will omit at the Effective Time, to state any material fact necessary in
order to make the statements contained herein or therein, in the light of the
circumstances under which made, not misleading.

                                  ARTICLE IV
                   COVENANTS RELATING TO CONDUCT OF BUSINESS

     SECTION 4.1  Conduct of Business.
                  ------------------- 

          (a) Conduct of Business by McKesson.  Except as set forth in Section
              -------------------------------                                 
4.1(a) of the McKesson Disclosure Schedule, as otherwise expressly contemplated
by this Agreement or as consented to by HBO in writing, such consent not to be
unreasonably withheld or delayed, during the period from the date of this
Agreement to the Effective Time, McKesson shall, and shall cause its
subsidiaries to, carry on their respective businesses in the ordinary course
consistent with past practice and in compliance in all material respects with
all applicable laws and regulations and, to the extent consistent therewith, use
all reasonable efforts to preserve intact their current business organizations,
use reasonable efforts to keep available the services of their current officers
and other key employees and preserve their relationships with those persons
having business dealings with them to the end that their goodwill and ongoing
businesses shall be unimpaired at the Effective Time. Without limiting the
generality of the foregoing (but subject to the above exceptions), during the
period from the date of this Agreement to the Effective Time, McKesson shall
not, and shall not permit any of its subsidiaries to:
<PAGE>
 
               (i)   other than regular quarterly dividends declared and paid by
McKesson on a basis consistent with past practice, dividends on the Convertible
Preferred Securities and other than dividends and distributions by a direct or
indirect wholly owned subsidiary of McKesson to its parent, or by a subsidiary
that is partially owned by McKesson or any of its subsidiaries, provided that
McKesson or any such subsidiary receives or is to receive its proportionate
share thereof, (x) declare, set aside or pay any dividends on, make any other
distributions in respect of, or enter into any agreement with respect to the
voting of, any of its capital stock, (y) split, combine or reclassify any of its
capital stock or issue or authorize the issuance of any other securities in
respect of, in lieu of or in substitution for shares of its capital stock,
except for issuances of McKesson Common Stock upon the exercise of McKesson
Employee Stock Options, in each case, outstanding as of the date hereof in
accordance with their present terms (including cashless exercise) or issued
pursuant to Section 4.1(a)(ii) or issuances of McKesson Common Stock upon
conversion of the Convertible Preferred Securities or (z) purchase, redeem or
otherwise acquire any shares of capital stock of McKesson or any of its
subsidiaries or any other securities thereof or any rights, warrants or options
to acquire any such shares or other securities (except, in the case of clause
(z), for the deemed acceptance of shares upon cashless exercise of McKesson
Employee Stock Options outstanding on the date hereof, or in connection with
withholding obligations relating thereto);

               (ii)  except in connection with acquisitions permitted or
contemplated by Section 4.1(a) of the McKesson Disclosure Schedule, issue,
deliver, sell, pledge or otherwise encumber or subject to any Lien any shares of
its capital stock, any other voting securities or any securities convertible
into, or any rights, warrants or options to acquire, any such shares, voting
securities or convertible securities (other than the issuance of McKesson Common
Stock upon the exercise or conversion of McKesson Employee Stock Options
outstanding as of the date hereof in accordance with their present terms or the
issuance of McKesson Employee Stock Options (and shares of McKesson Common Stock
upon the exercise thereof) granted after the date hereof in the ordinary course
of business consistent with past practice for employees (so long as such 
additional amount of McKesson Common Stock subject to McKesson Employee Stock
Options issued to such employees does not exceed 500,000 shares of McKesson
Common Stock in the aggregate);

               (iii) except as contemplated hereby, amend its certificate of
incorporation, by-laws or other comparable organizational documents;

               (iv)  acquire or agree to acquire by merging or consolidating
with, or by purchasing a substantial portion of the assets of, or by any other
manner, any business or any person, or, except for transactions in the ordinary
course of business consistent with past practice pursuant to contracts or
agreements in force at the date of this Agreement or acquisitions or investments
equal to or less than 120% of McKesson's current capital and operating budgets
(in each case, as previously provided to HBO), make any material investment
either by purchase of stock or securities, contributions to capital, property
transfers, or purchase of any property or assets of any other individual,
corporation or other entity other than a subsidiary of McKesson;

               (v)   sell, lease, license, mortgage or otherwise encumber or
subject to any Lien or otherwise dispose of any of its properties or assets
(including securitizations), other
<PAGE>
 
than in the ordinary course of business consistent with past practice,
including, without limitation, in connection with consolidation of acquired
businesses or as would not have a material adverse effect on McKesson;

               (vi)   take any action that would cause the representations and
warranties set forth in Section 3.1(g) and qualified as to materiality to be no
longer true and correct or, if not so qualified, to be no longer true and
correct in all material respects;

               (vii)  incur any indebtedness for borrowed money or issue any
debt securities or assume, guarantee or endorse, or otherwise as an
accommodation become responsible for the obligations of any person for borrowed
money, other than pursuant to a revolving credit facility or receivables
facility or commercial paper facility in effect as of the date hereof (including
any replacement facilities), in the ordinary course of business consistent with
past practice;

               (viii) settle any material claim, action or proceeding involving
money damages, except in the ordinary course of business consistent with past
practice;

               (ix)   other than in the ordinary course of business or in
connection with acquisitions permitted by Section 4.1(a) of the McKesson
Disclosure Schedule, enter into or terminate any material contract or agreement,
or make any change in any of its material leases or contracts, other than
amendments or renewals of contracts and leases without material adverse changes
of terms;

               (x)    except for increases in accordance with normal past
practice, increase in any manner the compensation or fringe benefits of any of
its officers or directors, or materially increase the foregoing in respect of
employees; enter into any commitment to pay any pension, retirement or severance
benefit to any such officers or directors, or make any material commitment to
pay the foregoing to any employees; commit itself to, or enter into, any
employment agreement involving compensation of more than $300,000 per year or a
term of more than two (2) years; adopt or commit itself to any new benefit, base
salary or stock option plan or arrangement; or amend, supplement, or accelerate
the timing of payments or vesting under, or otherwise materially amend or
supplement any existing benefit, stock option or compensation plan or
arrangement (other than as may be required by applicable law);

               (xi)   change any of the accounting methods used by McKesson or
any of its subsidiaries unless required by generally accepted accounting
principles; or

               (xii)  authorize, or commit or agree to take, any of the
foregoing actions;

provided that the limitations set forth in this Section 4.1(a) (other than
clause (iii)) shall not apply to any transaction between McKesson and any wholly
owned subsidiary or between any wholly owned subsidiaries of McKesson.

          (b)  Conduct of Business by HBO. Except as set forth in Section 4.1(b)
               --------------------------
of the HBO Disclosure Schedule, as otherwise expressly contemplated by this
Agreement or as
<PAGE>
 
consented to by McKesson in writing, such consent not to be unreasonably
withheld or delayed, during the period from the date of this Agreement to the
Effective Time, HBO shall, and shall cause its subsidiaries to, carry on their
respective businesses in the ordinary course consistent with past practice and
in compliance in all material respects with all applicable laws and regulations
and, to the extent consistent therewith, use all reasonable efforts to preserve
intact their current business organizations, use reasonable efforts to keep
available the services of their current officers and other key employees and
preserve their relationships with those persons having business dealings with
them to the end that their goodwill and ongoing businesses shall be unimpaired
at the Effective Time. Without limiting the generality of the foregoing (but
subject to the above exceptions), during the period from the date of this
Agreement to the Effective Time, HBO shall not, and shall not permit any of its
subsidiaries to:

               (i)   other than regular quarterly dividends declared and paid by
HBO on a basis consistent with past practice and dividends and distributions by
a direct or indirect wholly owned subsidiary of HBO to its parent, or by a
subsidiary that is partially owned by HBO or any of its subsidiaries, provided
that HBO or any such subsidiary receives or is to receive its proportionate
share thereof, (x) declare, set aside or pay any dividends on, make any other
distributions in respect of, or enter into any agreement with respect to the
voting of, any of its capital stock, (y) split, combine or reclassify any of its
capital stock or issue or authorize the issuance of any other securities in
respect of, in lieu of or in substitution for shares of its capital stock,
except for issuances of HBO Common Stock upon the exercise of HBO Employee Stock
Options outstanding as of the date hereof in accordance with their present terms
(including cashless exercise) or issued pursuant to Section 4.1(b)(ii) or (z)
purchase, redeem or otherwise acquire any shares of capital stock of HBO or any
of its subsidiaries or any other securities thereof or any rights, warrants or
options to acquire any such shares or other securities (except, in the case of
clause (z), for the deemed acceptance of shares upon cashless exercise of HBO 
Employee Stock Options, or in connection with withholding obligations relating
thereto);

               (ii)  except in connection with acquisitions permitted or
contemplated by Section 4.1(b) of the HBO Disclosure Schedule, issue, deliver,
sell, pledge or otherwise encumber or subject to any Lien any shares of its
capital stock, any other voting securities or any securities convertible into,
or any rights, warrants or options to acquire, any such shares, voting
securities or convertible securities (other than the issuance of HBO Common
Stock upon the exercise of HBO Employee Stock Options outstanding as of the date
hereof in accordance with their present terms or the issuance of HBO Employee
Stock Options (and shares of HBO Common Stock upon the exercise thereof)
granted after the date hereof in the ordinary course of business consistent with
past practice for employees (so long as such additional amount of HBO Common
Stock subject to HBO Employee Stock Options issued to employees does not exceed
500,000 shares of HBO Common Stock in the aggregate);

               (iii) except as contemplated hereby, amend its certificate of
incorporation, by-laws or other comparable organizational documents;
<PAGE>
 
               (iv)   acquire or agree to acquire by merging or consolidating
with, or by purchasing a substantial portion of the assets of, or by any other
manner, any business or any person, or, except for transactions in the ordinary
course of business consistent with past practice pursuant to contracts or
agreements in force at the date of this Agreement or acquisitions or investments
equal to or less than 120% of HBO's current capital and operating budgets (in
each case, as previously provided to McKesson), make any material investment
either by purchase of stock or securities, contributions to capital, property
transfers, or purchase of any property or assets of any other individual,
corporation or other entity other than a subsidiary of HBO;

               (v)    sell, lease, license, mortgage or otherwise encumber or
subject to any Lien or otherwise dispose of any of its properties or assets
(including securitizations), other than in the ordinary course of business
consistent with past practice, including, without limitation, in connection with
consolidation of acquired businesses or as would not have a material adverse
effect on HBO;

               (vi)   take any action that would cause the representations and
warranties set forth in Section 3.2(g) and qualified as to materiality to be no
longer be true and correct or, if not so qualified, to be no longer true and
correct in all material respects;

               (vii)  incur any indebtedness for borrowed money or issue any
debt securities or assume, guarantee or endorse, or otherwise as an
accommodation become responsible for the obligations of any person for borrowed
money, other than pursuant to a revolving credit facility or receivables
facility or commercial paper facility in effect as of the date hereof (including
any replacement facilities), in the ordinary course of business consistent with
past practice;

               (viii) settle any claim, action or proceeding involving money
damages, except in the ordinary course of business consistent with past
practice;

               (ix)   other than in the ordinary course of business or in
connection with acquisitions permitted by Section 4.1(b) of the HBO Disclosure
Schedule, enter into or terminate any material contract or agreement, or make
any change in any of its material leases or contracts, other than amendments or
renewals of contracts and leases without material adverse changes of terms;

               (x)    except for increases in accordance with normal past
practice, increase in any manner the compensation or fringe benefits of any of
its officers or directors, or materially increase the foregoing in respect of
employees; enter into any commitment to pay any pension, retirement or severance
benefit to any such officers or directors, or make any material commitment to
pay any of the foregoing to any employees; commit itself to, or enter into, any
employment agreement involving base salary of more than $300,000 per year or a
term of more than two (2) years; adopt or commit itself to any new benefit,
compensation or stock option plan or arrangement; or amend, supplement, or
accelerate the timing of payments or vesting under, or otherwise materially
amend or supplement any existing benefit, stock option or compensation plan or
arrangement (other than as may be required by applicable law);
<PAGE>
 
               (xi)   change any of the accounting methods used by HBO or any of
its subsidiaries unless required by generally accepted accounting principles; or

               (xii)  authorize, or commit or agree to take, any of the
foregoing actions;

provided that the limitations set forth in this Section 4.1(b) (other than
clause (iii)) shall not apply to any transaction between HBO and any wholly
owned subsidiary or between any wholly owned subsidiaries of HBO.

          (c)  Other Actions.  Except as required by law, McKesson and HBO shall
               -------------                                                    
not, and shall not permit any of their respective subsidiaries to, voluntarily
take any action that would, or that could reasonably be expected to, result in
(i) any of the representations and warranties of such party set forth in this
Agreement that are qualified as to materiality becoming untrue at the Effective
Time, (ii) any of such representations and warranties that are not so qualified
becoming untrue in any material respect at the Effective Time, or (iii) any of
the conditions to the Merger set forth in Article VI not being satisfied.

          (d)  Advice of Changes.  McKesson and HBO shall promptly advise the
               -----------------                                             
other party orally and in writing to the extent it has knowledge of (i) any
representation or warranty made by it contained in this Agreement that is
qualified as to materiality becoming untrue or inaccurate in any respect or any
such representation or warranty that is not so qualified becoming untrue or
inaccurate in any material respect, (ii) the failure by it to comply in any
material respect with or satisfy in any material respect any covenant, condition
or agreement to be complied with or satisfied by it under this Agreement and
(iii) any change or event having, or which, insofar as can reasonably be
foreseen, could reasonably be expected to have a material adverse effect on such
party or on the truth of such party's representations and warranties or the
ability of the conditions set forth in Article VI to be satisfied; provided,
however, that no such notification shall affect the representations, warranties,
covenants or agreements of the parties (or remedies with respect thereto) or the
conditions to the obligations of the parties under this Agreement.

     SECTION 4.2   No Solicitation by McKesson.
                   --------------------------- 

          (a)  McKesson shall not, nor shall it permit any of its subsidiaries
to, nor shall it authorize or permit any of its officers, directors or employees
or any investment banker, financial advisor, attorney, accountant or other
representative retained by it or any of its subsidiaries to, directly or
indirectly through another person, (i) solicit, initiate or encourage (including
by way of furnishing information), or take any other action designed to
facilitate, any inquiries or the making of any proposal the consummation of
which would constitute an Alternative Transaction (as hereinafter defined) or
(ii) participate in any discussions or negotiations regarding any Alternative
Transaction; provided, however, that if, at any time prior to the adoption of
this Agreement by the holders of McKesson Common Stock, the Board of Directors
of McKesson determines in good faith, after receipt of advice from outside
counsel, that the failure to provide such information or participate in such
negotiations or discussions would result in a reasonable possibility that the
Board of Directors of McKesson breach its fiduciary duties to McKesson's
stockholders under applicable law, McKesson may, in response to any such
proposal that was not solicited by it or that did not otherwise result from a
breach of
<PAGE>
 
this Section 4.2(a), and subject to compliance with Section 4.2(c), (x) furnish
information with respect to McKesson and its subsidiaries to any person pursuant
to a customary confidentiality agreement containing terms as to confidentiality
no less restrictive than the terms of the Confidentiality Agreement dated June
30, 1998 entered into between HBO and McKesson, as amended pursuant to Section
8.6 hereof (the "Confidentiality Agreement") and (y) participate in negotiations
regarding such proposal. For purposes of this Agreement "Alternative
Transaction" means any of (i) a transaction or series of transactions pursuant
to which any person (or group of persons) other than HBO and its subsidiaries
and other than McKesson and its subsidiaries (a "Third Party") acquires or would
acquire, directly or indirectly, beneficial ownership (as defined in Rule 13d-3
under the Exchange Act) of more than 20% of the outstanding shares of HBO or
McKesson, as the case may be, whether from HBO or McKesson or pursuant to a
tender offer or exchange offer or otherwise, (ii) any acquisition or proposed
acquisition of HBO or any of its significant subsidiaries or McKesson or any of
its significant subsidiaries, as the case may be, by a merger or other business
combination (including any so-called "merger of equals" and whether or not HBO
or any of its significant subsidiaries or McKesson or any of its significant
subsidiaries, as the case may be, is the entity surviving any such merger or
business combination) or (iii) any other transaction pursuant to which any Third
Party acquires or would acquire, directly or indirectly, control of assets
(including for this purpose the outstanding equity securities of subsidiaries of
HBO or McKesson, as the case may be, and any entity surviving any merger or
combination including any of them) of HBO or any of its subsidiaries or McKesson
or any of its subsidiaries, as the case may be, for consideration equal to 20%
or more of the fair market value of all of the outstanding shares of HBO Common
Stock or all of the outstanding shares of McKesson Common Stock, as the case may
be, on the date prior to the date hereof.

          (b)  Neither the Board of Directors of McKesson nor any committee
thereof shall (i) except as expressly permitted by this Section 4.2, withdraw,
qualify or modify, or propose publicly to withdraw, qualify or modify, in a
manner adverse to HBO, the approval or recommendation by such Board of Directors
or such committee of the Merger or this Agreement, or the issuance of McKesson
Common Stock in connection with the Merger,  (ii) approve or recommend, or
propose publicly to approve or recommend, any Alternative Transaction, or (iii)
cause McKesson to enter into any letter of intent, agreement in principle,
acquisition agreement or other similar agreement (each, a "McKesson Acquisition
Agreement") related to any Alternative Transaction. Notwithstanding the
foregoing, in the event that prior to the adoption of this Agreement by the
holders of McKesson Common Stock the Board of Directors of McKesson determines
in good faith, after it has received a McKesson Superior Proposal (as defined
below) and after receipt of advice from outside counsel, that the failure to do
so would result in a reasonable possibility that the Board of Directors of
McKesson would breach its fiduciary duties to McKesson's stockholders under
applicable law, the Board of Directors of McKesson may (subject to this and the
following sentences) inform McKesson stockholders that it no longer believes
that the Merger or this Agreement is advisable and no longer recommends approval
(a "McKesson Subsequent Determination"), but only at a time that is after the
fifth business day following HBO's receipt of written notice advising HBO that
the Board of Directors of McKesson has received a McKesson Superior Proposal
specifying the material terms and
<PAGE>
 
conditions of such Superior Proposal, identifying the person making such
McKesson Superior Proposal and stating that it intends to make a McKesson
Subsequent Determination. After providing such notice, McKesson shall provide a
reasonable opportunity to HBO to make such adjustments in the terms and
conditions of this Agreement and/or of the McKesson Option Agreement as would
enable McKesson to proceed with its recommendation to stockholders without
making a McKesson Subsequent Determination; provided, however, that any such
adjustments shall be at the discretion of the parties at such time. For purposes
of this Agreement, a "McKesson Superior Proposal" means any proposal (on its
most recently amended or modified terms, if amended or modified) made by a Third
Party to enter into an Alternative Transaction which the Board of Directors of
McKesson determines in its good faith judgment (based on the advice of a
financial advisor of nationally recognized reputation) to be more favorable to
McKesson's stockholders than the Merger taking into account all relevant factors
(including whether, in the good faith judgment of the Board of Directors of
McKesson, after obtaining advice from a financial advisor of nationally
recognized reputation, the third party is reasonably able to finance the
transaction, and any proposed changes to this Agreement and/or the McKesson
Option Agreement that may be proposed by HBO in response to such Alternative
Transaction). Notwithstanding any other provision of this Agreement, McKesson
shall submit this Agreement to its stockholders whether or not the Board of
Directors of McKesson makes a McKesson Subsequent Determination.

          (c)  In addition to the obligations of McKesson set forth in
paragraphs (a) and (b) of this Section 4.2, McKesson shall promptly advise HBO
orally and in writing of any request for information or of any proposal in
connection with an Alternative Transaction, the material terms and conditions of
such request or proposal and the identity of the person making such request or
proposal. McKesson will keep HBO reasonably informed of the status and details
(including amendments or proposed amendments) of any such request or proposal on
a current basis.

          (d)  Nothing contained in this Section 4.2 shall prohibit McKesson (i)
from taking and disclosing to its stockholders a position contemplated by Rule
14d-9 or Rule 14e-2(a) promulgated under the Exchange Act or (ii) from making
any disclosure to its stockholders if, in the good faith judgment of the Board
of Directors of McKesson, after receipt of advice from outside counsel, failure
so to disclose would be inconsistent with its fiduciary duties to McKesson's
stockholders under applicable law.

     SECTION 4.3    No Solicitation by HBO.
                    ---------------------- 

          (a)  HBO shall not, nor shall it permit any of its subsidiaries to,
nor shall it authorize or permit any of its officers, directors or employees or
any investment banker, financial advisor, attorney, accountant or other
representative retained by it or any of its subsidiaries to, directly or
indirectly through another person, (i) solicit, initiate or encourage (including
by way of furnishing information), or take any other action designed to
facilitate, any inquiries or the making of any proposal the consummation of
which would constitute an Alternative Transaction or (ii) participate in any
discussions or negotiations regarding any Alternative Transaction; provided,
however, that if, at any time prior to the adoption of this Agreement by the
holders of HBO Common Stock, the Board of Directors of HBO determines in good
faith, after receipt of
<PAGE>
 
advice from outside counsel, that the failure to provide such information or
participate in such negotiations or discussions would result in a reasonable
possibility that the Board of Directors of HBO breach its fiduciary duties to
HBO's stockholders under applicable law, HBO may, in response to any such
proposal that was not solicited by it or which did not otherwise result from a
breach of this Section 4.3(a), and subject to compliance with Section 4.3(c),
(x) furnish information with respect to HBO and its subsidiaries to any person
pursuant to a customary confidentiality agreement containing terms as to
confidentiality no less restrictive than the Confidentiality Agreement, as
amended pursuant to Section 8.6 hereof, and (y) participate in negotiations
regarding such proposal.

          (b)  Neither the Board of Directors of HBO nor any committee thereof
shall (i) except as expressly permitted by this Section 4.3, withdraw, qualify
or modify, or propose publicly to withdraw, qualify or modify, in a manner
adverse to McKesson, the approval or recommendation by such Board of Directors
or such committee of the Merger, or this Agreement, (ii) approve or recommend,
or propose publicly to approve or recommend, any Alternative Transaction, or
(iii) cause HBO to enter into any letter of intent, agreement in principle,
acquisition agreement or other similar agreement (each, an "HBO Acquisition
Agreement") related to any Alternative Transaction.  Notwithstanding the
foregoing, in the event that prior to the adoption of this Agreement by the
holders of HBO Common Stock the Board of Directors of HBO determines in good
faith, after it has received an HBO Superior Proposal (as defined below) and
after receipt of advice from outside counsel, that the failure to do so would
result in a reasonable possibility that the Board of Directors of HBO would
breach its fiduciary duties to HBO's stockholders under applicable law, the
Board of Directors of HBO may (subject to this and the following sentences)
inform HBO stockholders that it no longer believes that the Merger or this
Agreement is advisable and no longer recommends approval (an "HBO Subsequent
Determination"), but only at a time that is after the fifth business day
following McKesson's receipt of written notice advising McKesson that the Board
of Directors of HBO has received an HBO Superior Proposal, specifying the
material terms and conditions of such HBO Superior Proposal, identifying the
person making such HBO Superior Proposal and stating that it intends to make an
HBO Subsequent Determination.  After providing such notice, HBO shall provide a
reasonable opportunity to McKesson to make such adjustments in the terms and
conditions of this Agreement and/or of the HBO Option Agreement as would enable
HBO to proceed with its recommendation to stockholders without making an HBO
Subsequent Determination; provided, however, that any such adjustments shall be
at the discretion of the parties at such time.  For purposes of this Agreement,
an "HBO Superior Proposal" means any proposal (on its most recently amended or
modified terms, if amended or modified) made by a Third Party enter into an
Alternative Transaction on terms which the Board of Directors of HBO determines
in its good faith judgment (based on the advice of a financial advisor of
nationally recognized reputation) to be more favorable to HBO's stockholders
than the Merger taking into account all relevant factors (including whether, in
the good faith judgment of the Board of Directors of HBO, after obtaining advice
from a financial advisor of nationally recognized reputation, the third party is
reasonably able to finance the transaction, and any proposed changes to this
Agreement and/or the HBO Option Agreement that may be proposed by McKesson in
response to such Alternative Transaction). Notwithstanding any other provision
of this Agreement, HBO shall submit this Agreement to its stockholders whether
or not the Board of Directors of HBO make an HBO Subsequent Determination.
<PAGE>
 
          (c)  In addition to the obligations of HBO set forth in paragraphs (a)
and (b) of this Section 4.3, HBO shall promptly advise McKesson orally and in
writing of any request for information or of any proposal in connection with an
Alternative Transaction, the material terms and conditions of such request or
proposal and the identity of the person making such request or proposal. HBO
will keep McKesson reasonably informed of the status and details (including
amendments or proposed amendments) of any such request or proposal on a current
basis.

          (d)  Nothing contained in this Section 4.3 shall prohibit HBO from (i)
taking and disclosing to its stockholders a position contemplated by Rule 14d-9
or Rule 14e-2(a) promulgated under the Exchange Act or (ii) from making any
disclosure to its stockholders if, in the good faith judgment of the Board of
Directors of HBO, after receipt of advice from outside counsel, failure so to
disclose would be inconsistent with its fiduciary duties to HBO's stockholders
under applicable law.

                                   ARTICLE V
                             ADDITIONAL AGREEMENTS

     SECTION 5.1    Preparation of the Form S-4 and the Joint Proxy Statement;
                    ----------------------------------------------------------
Stockholders' Meetings.
- ---------------------- 

          (a)  As soon as practicable following the date of this Agreement,
McKesson and HBO shall prepare and file with the SEC the Joint Proxy Statement,
and McKesson shall prepare and file with the SEC the Form S-4, in which the
Joint Proxy Statement will be included as a prospectus.  Each of McKesson and
HBO shall use commercially reasonable efforts to have the Form S-4 declared
effective under the Securities Act as promptly as practicable after such filing.
McKesson will use all commercially reasonable efforts to cause the Joint Proxy
Statement to be mailed to McKesson's stockholders, and HBO will use all
commercially reasonable efforts to cause the Joint Proxy Statement to be mailed
to HBO's stockholders, in each case as promptly as practicable after the Form S-
4 is declared effective under the Securities Act.  McKesson shall also take any
action (other than qualifying to do business in any jurisdiction in which it is
not now so qualified or to file a general consent to service of process)
required to be taken under any applicable state securities laws in connection
with the issuance of McKesson Common Stock in the Merger and the conversion of
Assumed Options, and HBO shall furnish all information concerning HBO and the
holders of HBO Common Stock as may be reasonably requested in connection with
any such action.  No filing of, or amendment or supplement to, the Form S-4 or
the Joint Proxy Statement will be made by McKesson without HBO's prior consent
(which shall not be unreasonably withheld) and without providing HBO the
opportunity to review and comment thereon.  McKesson will advise HBO promptly
after it receives notice thereof, of the time when the Form S-4 has become
effective or any supplement or amendment has been filed, the issuance of any
stop order, the suspension of the qualification of the McKesson Common Stock
issuable in connection with the Merger for offering or sale in any jurisdiction,
or any request by the SEC for amendment of the Joint Proxy Statement or the Form
S-4 or comments thereon and responses thereto or requests by the SEC for
additional information. If at any time prior to the Effective Time any
information relating to McKesson or HBO, or any of their respective affiliates,
officers or directors, should be discovered by McKesson or HBO which
<PAGE>
 
should be set forth in an amendment or supplement to any of the Form S-4 or the
Joint Proxy Statement, so that any of such documents would not include any
misstatement of a material fact or omit to state any material fact necessary to
make the statements therein, in light of the circumstances under which they were
made, not misleading, the party which discovers such information shall promptly
notify the other parties hereto and an appropriate amendment or supplement
describing such information shall be promptly filed with the SEC and, to the
extent required by law, disseminated to the stockholders of McKesson and HBO.

          (b)  McKesson shall, as promptly as practicable after the Form S-4 is
declared effective under the Securities Act, duly give notice of, convene and
hold a meeting of its stockholders (the "McKesson Stockholders' Meeting") in
accordance with the DGCL for the purpose of obtaining the McKesson Stockholder
Approval and shall, subject to the provisions of Section 4.2(b) hereof, through
its Board of Directors, recommend to its stockholders the approval and adoption
of this Agreement, the Merger and the other transactions contemplated hereby.

          (c)  HBO shall, as promptly as practicable after the Form S-4 is
declared effective under the Securities Act, duly give notice of, convene and
hold a meeting of its stockholders (the "HBO Stockholders' Meeting") in
accordance with the DGCL for the purpose of obtaining the HBO Stockholder
Approval and shall, subject to the provisions of Section 4.3(b) hereof, through
its Board of Directors, recommend to its stockholders the approval and adoption
of this Agreement, the Merger and the other transactions contemplated hereby.

          (d)  HBO and McKesson will use commercially reasonable efforts to hold
the McKesson Stockholders' Meeting and the HBO Stockholders' Meeting on the same
date and as soon as reasonably practicable after the date hereof.

          (e)  Each of McKesson's and HBO's obligations under this Section 5.1
shall at all times remain subject to the provisions of Sections 4.2(b) and
4.3(b), respectively, in the event that under the circumstances described
therein, the Board of Directors of McKesson shall have made a McKesson
Subsequent Determination or the Board of Directors of HBO shall have made an HBO
Subsequent Determination, as the case may be.

     SECTION 5.2    Letters of McKesson's Accountants.
                    --------------------------------- 

          (a)  McKesson shall use commercially reasonable efforts to cause to be
delivered to HBO two letters from McKesson's independent accountants, one dated
as of the date the Form S-4 is declared effective and one dated as of the
Closing Date, each addressed to HBO, in form and substance reasonably
satisfactory to HBO and customary in scope and substance for comfort letters
delivered by independent public accountants in connection with registration
statements similar to the Form S-4.

          (b)  McKesson shall use commercially reasonable efforts to cause to be
delivered to HBO and HBO's independent accountants a letter from McKesson's
independent accountants addressed to HBO and McKesson, dated as of the date the
Form S-4 is declared effective and as of the Closing Date, stating that
accounting for the Merger as a pooling of interests under Opinion 16 of the
Accounting Principles Board and applicable SEC rules and 
<PAGE>
 
regulations is appropriate if the Merger is closed and consummated as
contemplated by this Agreement.

     SECTION 5.3    Letters of HBO's Accountants.
                    ---------------------------- 

          (a)  HBO shall use commercially reasonable efforts to cause to be
delivered to McKesson two letters from HBO's independent accountants, one dated
as of the date the Form S-4 is declared effective and one dated as of the
Closing Date, each addressed to McKesson, in form and substance reasonably
satisfactory to McKesson and customary in scope and substance for comfort
letters delivered by independent public accountants in connection with
registration statements similar to the Form S-4.

          (b)  HBO shall use commercially reasonable efforts to cause to be
delivered to McKesson and McKesson's independent accountants a letter from HBO's
independent accountants, addressed to McKesson and HBO, dated as of the date the
Form S-4 is declared effective and as of the Closing Date, stating that
accounting for the Merger as a pooling of interests under Opinion 16 of the
Accounting Principles Board and applicable SEC rules and regulations is
appropriate if the Merger is closed and consummated as contemplated by this
Agreement.

     SECTION 5.4    Access to Information; Confidentiality.  Subject to the
                    --------------------------------------                 
Confidentiality Agreement and subject to applicable law, each of McKesson and
HBO shall, and shall cause each of its respective subsidiaries to, afford to the
other party and to the officers, employees, accountants, counsel, financial
advisors and other representatives of such other party, reasonable access during
normal business hours during the period prior to the Effective Time to all their
respective properties, books, contracts, commitments, personnel and records
(provided that such access shall not interfere with the business or operations
of such party) and, during such period, each of McKesson and HBO shall, and
shall cause each of its respective subsidiaries to, furnish promptly to the
other party (a) a copy of each report, schedule, registration statement and
other document filed by it during such period pursuant to the requirements of
federal or state securities laws and (b) all other information concerning its
business, properties and personnel as such other party may reasonably request.
No review pursuant to this Section 5.4 shall affect any representation or
warranty given by the other party hereto.  Each of McKesson and HBO will hold,
and will cause its respective officers, employees, accountants, counsel,
financial advisors and other representatives and affiliates to hold, any
nonpublic information in accordance with the terms of the Confidentiality
Agreements.  McKesson shall also cooperate with HBO and use its best efforts to
obtain an estimate of withdrawal liability from each multiemployer plan with
respect to which McKesson contributes as of the date hereof.

     SECTION 5.5    Commercially Reasonable Efforts.
                    ------------------------------- 

          (a)  Upon the terms and subject to the conditions set forth in this
Agreement, each of the parties agrees to use commercially reasonable efforts to
take, or cause to be taken, all actions, and to do, or cause to be done, and to
assist and cooperate with the other parties in doing, all things necessary,
proper or advisable to consummate and make effective, in the most expeditious
manner practicable, the Merger and the other transactions contemplated by this
<PAGE>
 
Agreement, including (i) the obtaining of all necessary actions or nonactions,
waivers, consents and approvals from Governmental Entities and the making of all
necessary registrations and filings and the taking of all steps as may be
necessary to obtain an approval or waiver from, or to avoid an action or
proceeding by, any Governmental Entity, (ii) the obtaining of all necessary
consents, approvals or waivers, and any necessary or appropriate financing
arrangements, from third parties, (iii) the defending of any lawsuits or other
legal proceedings, whether judicial or administrative, challenging this
Agreement or the consummation of the transactions contemplated by this
Agreement, including seeking to have any stay or temporary restraining order
entered by any court or other Governmental Entity vacated or reversed, and (iv)
the execution and delivery of any additional instruments necessary to consummate
the transactions contemplated by, and to fully carry out the purposes of, this
Agreement. Notwithstanding anything to the contrary in this Agreement, neither
HBO nor McKesson shall be required to hold separate (including by trust or
otherwise) or divest any of their respective businesses or assets, or enter into
any consent decree or other agreement that would restrict either HBO or McKesson
in the conduct of its business as heretofore conducted.

          (b)  In connection with and without limiting the foregoing, McKesson
and HBO shall (i) take all action necessary to ensure that no state takeover
statute or similar statute or regulation is or becomes applicable to this
Agreement, the Option Agreements, or any of the transactions contemplated hereby
and thereby and (ii) if any state takeover statute or similar statute or
regulation becomes applicable to such agreements or transactions, take all
action necessary to ensure that such transactions may be consummated as promptly
as practicable on the terms contemplated by this Agreement and otherwise to
minimize the effect of such statute or regulation on the Merger and the other
transactions contemplated by this Agreement.

     SECTION 5.6    Indemnification, Exculpation and Insurance.
                    ------------------------------------------ 

          (a)  McKesson agrees to maintain in effect in accordance with their
terms all rights to indemnification and exculpation from liabilities for acts or
omissions occurring at or prior to the Effective Time existing as of the date of
this Agreement in favor of the current or former directors or officers of HBO
and its subsidiaries as provided in their respective certificates of
incorporation or by-laws (or comparable organizational documents) and any
indemnification agreements of HBO.  In addition, from and after the Effective
Time, directors and officers of HBO who become directors or officers of McKesson
will be entitled to the same indemnity rights and protections, and directors'
and officers' liability insurance, as are afforded from time to time to other
directors and officers of McKesson.

          (b)  In the event that McKesson or any of its successors or assigns
(i) consolidates with or merges into any other person and is not the continuing
or surviving corporation or entity of such consolidation or merger or (ii)
transfers or conveys all or substantially all of its properties and assets to
any person, then, and in each such case, proper provision will be made so that
the successors and assigns of McKesson assume the obligations set forth in this
Section 5.6.

          (c)  McKesson shall use commercially reasonable efforts to provide to
HBO's current directors and officers, for six years after the Effective Time,
liability insurance covering 
<PAGE>
 
acts or omissions occurring prior to the Effective Time with respect to those
persons who are currently covered by HBO's directors' and officers' liability
insurance policy on terms with respect to such coverage and amount no less
favorable than those of such policy in effect on the date hereof, provided that
in no event shall McKesson be required to expend more than 150% of the current
amount expended by HBO to maintain such coverage.

          (d)  The provisions of this Section 5.6 (i) are intended to be for the
benefit of, and will be enforceable by, each indemnified party, his or her heirs
and his or her representatives and (ii) are in addition to, and not in
substitution for, any other rights to indemnification or contribution that any
such person may have by contract or otherwise.

     SECTION 5.7    Fees and Expenses.
                    ----------------- 

          (a)  Except as set forth in this Section 5.7 and in Section 7.2, all
fees and expenses incurred in connection with the Merger, this Agreement, the
Option Agreements and the transactions contemplated by this Agreement and the
Option Agreements shall be paid by the party incurring such fees or expenses,
whether or not the Merger is consummated, except that each of HBO and McKesson
shall bear and pay one-half of the costs and expenses incurred in connection
with (i) the filing, printing and mailing of the Form S-4 and the Joint Proxy
Statement (including SEC filing fees) and (ii) the filings of the premerger
notification and report forms under the HSR Act (including filing fees).   In
addition, all transfer taxes incurred in connection with the Merger arising on
or after the Effective Time shall be borne by McKesson.

     SECTION 5.8    Public Announcements.  HBO and McKesson will consult with
                    --------------------                                     
each other before issuing, and provide each other the opportunity to review,
comment upon and concur with, and use reasonable efforts to agree on, any press
release or other public statements with respect to the transactions contemplated
by this Agreement, the Option Agreements, including the Merger, and shall not
issue any such press release or make any such public statement prior to such
consultation, except as either party may determine is required by applicable
law, court process or by obligations pursuant to any listing agreement with any
national securities exchange or stock market.  The parties agree that the
initial press release to be issued with respect to the transactions contemplated
by this Agreement shall be in the form heretofore agreed to by the parties.

     SECTION 5.9    Affiliates.
                    ---------- 

          (a)  As soon as practicable after the date hereof, HBO shall deliver
to McKesson a letter identifying all persons who may be deemed to be, at the
time this Agreement is submitted for adoption by the stockholders of HBO,
"affiliates" of HBO for purposes of Rule 145 under the Securities Act or for
purposes of qualifying the Merger for pooling of interests accounting treatment
under Opinion 16 of the Accounting Principles Board and applicable SEC rules and
regulations, and such list shall be updated as necessary to reflect changes from
the date hereof. HBO shall use commercially reasonable efforts to cause each
person identified on such
<PAGE>
 
list to deliver to McKesson on or before the date immediately preceding the date
of filing the Form S-4, written agreements substantially in the forms attached
as Exhibit G and Exhibit H hereto, and in the event any other person becomes an
   ---------     ---------
affiliate of HBO thereafter to cause such person to deliver such an agreement to
McKesson as soon as practicable but in any event at Closing. McKesson shall use
commercially reasonable efforts to cause all persons who are "affiliates" of
McKesson for purposes of qualifying the Merger for pooling of interests
accounting treatment under Opinion 16 of the Accounting Principles Board and
applicable SEC rules and regulations to deliver to HBO on or before the date
immediately preceding the date of filing of the Form S-4, a written agreement
substantially in the form attached as Exhibit H hereto, and in the event any
                                      ---------
other person becomes an affiliate of McKesson thereafter to cause such person to
deliver such an agreement to HBO as soon as practicable but in any event at
Closing.

          (b)  McKesson shall use commercially reasonable efforts to publish no
later than 45 days after the end of the first full month after the Effective
Time in which there are at least 30 days of post Merger combined operations,
combined sales and net income figures as contemplated by and in accordance with
the terms of SEC Accounting Series Release No. 135.

     SECTION 5.10    NYSE and PSE Listings.  McKesson shall use commercially
                     ---------------------                                  
reasonable efforts to cause the McKesson Common Stock issuable under Article II
to be approved for listing on NYSE and PSE, subject to official notice of
issuance, as promptly as practicable after the date hereof, and in any event
prior to the Closing Date.

     SECTION 5.11    Tax Treatment.  Each of HBO and McKesson shall use
                     -------------                                     
commercially reasonable efforts to cause the Merger to qualify as a
reorganization under the provisions of Section 368 of the Code and to obtain the
opinions of counsel referred to in Section 6.1(g).  The parties will
characterize the Merger as such a reorganization for purposes of all tax returns
and other filings.

     SECTION 5.12    Pooling of Interests.  Each of McKesson and HBO shall use
                     --------------------                                     
commercially reasonable efforts to cause the transactions contemplated by this
Agreement, including the Merger, to be accounted for as a pooling of interests
under Opinion 16 of the Accounting Principles Board and applicable SEC rules and
regulations, and such accounting treatment to be accepted by the SEC, and each
of McKesson and HBO agrees that it shall take no action that would cause such
accounting treatment not to be obtained.

     SECTION 5.13    Post-Merger Operations.  Following the Effective Time, the
                     ----------------------                                    
principal corporate offices of McKesson HBOC, Inc. shall be San Francisco and
the headquarters for HBO and its subsidiaries shall be Atlanta, until such time
as the Board of Directors of McKesson otherwise determines.

     SECTION 5.14    Conveyance Taxes.  HBO and McKesson shall cooperate in the
                     ----------------                                          
preparation, execution and filing of all returns, questionnaires, applications
or other documents regarding any real property transfer or gains, sales, use,
transfer, value added, stock transfer and stamp taxes, any transfer, recording,
registration and other fees or any similar taxes which become payable in
connection with the transactions contemplated by this Agreement that are
<PAGE>
 
required or permitted to be filed on or before the Effective Time.

     SECTION 5.15   Employee Benefits.  Each of HBO and McKesson agrees to
                    -----------------                                     
provide immediately following the Effective Time employee benefits and
compensation arrangements for all their respective employees, at a level no less
favorable in the aggregate than those provided for such employees immediately
prior to the Effective Time, subject to later amendment or other alteration as
may be directed by the Board of Directors of McKesson subsequent to the
Effective Time.

                                  ARTICLE VI
                              CONDITIONS PRECEDENT

     SECTION 6.1    Conditions to Each Party's Obligation to Effect the Merger.
                    ----------------------------------------------------------  
The respective obligation of each party to effect the Merger is subject to the
satisfaction or waiver on or prior to the Closing Date of the following
conditions:

          (a)  Stockholder Approvals.  Each of the McKesson Stockholder Approval
               ---------------------                                            
and the HBO Stockholder Approval shall have been obtained.

          (b)  HSR Act.  The waiting period (and any extension thereof)
               -------                                                 
applicable to the Merger under the HSR Act shall have been terminated or shall
have expired.

          (c)  Governmental, Regulatory and Other Approvals.  (i) Other than the
               --------------------------------------------                     
filing of the Certificate of Merger provided for under Section 1.3 and filings
pursuant to the HSR Act (which are addressed in Section 6.1(b)), all consents,
approvals and actions of, filings with and notices to any Governmental Entity
required of McKesson, HBO or any of their subsidiaries to consummate the Merger
and the other transactions contemplated hereby (together with the matters
contemplated by Section 6.1(b), the "Requisite Regulatory Approvals") shall have
been obtained and (ii) except as would not have a material adverse effect on
any of McKesson, HBO or the Surviving Corporation, the consents and approvals
set forth on Section 3.1(d) of the McKesson Disclosure Schedule and Section
3.2(d) of the HBO Disclosure Schedule shall have been obtained or shall no
longer be required.

          (d)  No Injunctions or Restraints.  No judgment, order, decree,
               ----------------------------                              
statute, law, ordinance, rule or regulation, entered, enacted, promulgated,
enforced or issued by any court or other Governmental Entity of competent
jurisdiction or other legal restraint or prohibition (collectively,
"Restraints") shall be in effect (i) preventing the consummation of the Merger,
or (ii) which otherwise is reasonably likely to have a material adverse effect
on McKesson or HBO, as applicable; provided, however, that each of the parties
shall have used commercially reasonable efforts to prevent the entry of any such
Restraints and to appeal as promptly as possible any such Restraints that may be
entered.

          (e)  Form S-4.  The Form S-4 shall have become effective under the
               --------                                                     
Securities Act prior to the mailing of the Joint Proxy Statement by each of
McKesson and HBO to their respective stockholders and no stop order or
proceedings seeking a stop order shall be threatened by the SEC or shall have
been initiated by the SEC.
<PAGE>
 
          (f) NYSE and PSE Listings.  The shares of McKesson Common Stock
              ---------------------                                      
issuable to HBO's stockholders as contemplated by Article II shall have been
approved for listing on NYSE and PSE, subject to official notice of issuance.

          (g) Tax Opinions.  HBO shall have received from Jones, Day, Reavis &
              ------------                                                    
Pogue, counsel to HBO, and McKesson shall have received from Skadden, Arps,
Slate, Meagher & Flom LLP, counsel to McKesson, an opinion, dated as of the date
that the Registration Statement is declared effective, to the effect that the
Merger will constitute a "reorganization" within the meaning of Section 368(a)
of the Code, and such opinions shall not have been withdrawn or materially
modified as of the Closing Date.

          In rendering such opinions, each of counsel for HBO and McKesson shall
be entitled to receive and rely upon representations of fact contained in
certificates of officers of HBO and McKesson, which representations shall be in
form and substance satisfactory to such counsel.

          (h) Pooling Letters.  HBO and McKesson shall have received letters
              ---------------                                               
from each of McKesson's independent accountants and HBO's independent
accountants, dated as of the Closing Date, in each case addressed to HBO and
McKesson, stating that the Merger qualifies for accounting as a pooling of
interests under Opinion 16 of the Accounting Principles Board and applicable SEC
rules and regulations.

     SECTION 6.2    Conditions to Obligations of HBO.  The obligation of HBO to
                    --------------------------------                           
effect the Merger is further subject to satisfaction or waiver of the following
conditions:

          (a) Representations and Warranties.  The representations and
              ------------------------------                          
warranties of McKesson set forth herein shall be true and correct both when made
and at and as of the Closing Date, as if made at and as of such time (except to
the extent expressly made as of an earlier date, in which case as of such date),
except where the failure of such representations and warranties to be so true
and correct (without giving effect to any limitation as to "materiality" or
"material adverse effect" set forth therein) does not have, and is not likely to
have, individually or in the aggregate, a material adverse effect on McKesson.

          (b) Performance of Obligations of McKesson.  McKesson shall have
              --------------------------------------                      
performed in all material respects all obligations required to be performed by
it under this Agreement at or prior to the Closing Date.

          (c) No Material Adverse Change.  At any time after the date of this
              --------------------------                                     
Agreement there shall not have occurred any material adverse change relating to
McKesson.

          (d) McKesson Rights Agreement.  The McKesson Rights issued pursuant to
              -------------------------                                         
the McKesson Rights Agreement shall not have become nonredeemable, exercisable,
distributed or triggered pursuant to the terms of such agreement.

     SECTION 6.3    Conditions to Obligations of McKesson.  The obligation of
                    -------------------------------------                    
McKesson to 
<PAGE>
 
effect the Merger is further subject to satisfaction or waiver of the following
conditions:

          (a) Representations and Warranties.  The representations and
              ------------------------------                          
warranties of HBO set forth herein shall be true and correct both when made and
at and as of the Closing Date, as if made at and as of such time (except to the
extent expressly made as of an earlier date, in which case as of such date),
except where the failure of such representations and warranties to be so true
and correct (without giving effect to any limitation as to "materiality," or
"material adverse effect" set forth therein) does not have, and is not likely to
have, individually or in the aggregate, a material adverse effect on HBO.

          (b) Performance of Obligations of HBO.  HBO shall have performed in
              ---------------------------------                              
all material respects all obligations required to be performed by it under this
Agreement at or prior to the Closing Date.

          (c) No Material Adverse Change.  At any time after the date of this
              --------------------------                                     
Agreement there shall not have occurred any material adverse change relating to
HBO.

          (d) HBO Rights Agreement.  The HBO Rights issued pursuant to the HBO
              --------------------                                            
Rights Agreement shall not have become nonredeemable, exercisable, distributed
or triggered pursuant to the terms of such agreement.


                                  ARTICLE VII
                       TERMINATION, AMENDMENT AND WAIVER

     SECTION 7.1    Termination.  This Agreement may be terminated at any time
                    -----------                                               
prior to the Effective Time, and (except in the case of 7.1(e) or 7.1(f))
whether before or after the McKesson Stockholder Approval or the HBO Stockholder
Approval:

          (a) by mutual written consent of HBO and McKesson, if the Board of
Directors of each so determines by a vote of a majority of its entire Board;

          (b) by either the Board of Directors of HBO or the Board of Directors
of McKesson:

              (i)    if the Merger shall not have been consummated by March 31,
1999, unless such termination right has been expressly restricted in writing by
the Board of Directors of HBO or McKesson, as the case may be; provided,
however, that the right to terminate this Agreement pursuant to this Section
7.1(b)(i) shall not be available to any party whose failure to perform any of
its obligations under this Agreement results in the failure of the Merger to be
consummated by such time;

              (ii)   if the McKesson Stockholder Approval shall not have been
<PAGE>
 
obtained at a McKesson Stockholders' Meeting duly convened therefor or at any
adjournment or postponement thereof;

              (iii)  if the HBO Stockholder Approval shall not have been
obtained at an HBO Stockholders' Meeting duly convened therefor or at any
adjournment or postponement thereof;

              (iv)   if any Restraint having any of the effects set forth in
Section 6.1(d) shall be in effect and shall have become final and nonappealable,
or if any Governmental Entity that must grant a Requisite Regulatory Approval
has denied approval of the Merger and such denial has become final and
nonappealable; provided, that the party seeking to terminate this Agreement
pursuant to this Section 7.1(b)(iv) shall have used commercially reasonable
efforts to prevent the entry of and to remove such Restraint or to obtain such
Requisite Regulatory Approval, as the case may be;

          (c) by the Board of Directors of HBO (provided that HBO is not then in
material breach of any representation, warranty, covenant or other agreement
contained herein), if McKesson shall have breached or failed to perform any of
its representations, warranties, covenants or other agreements contained in this
Agreement, which breach or failure to perform (A) would give rise to the failure
of a condition set forth in Section 6.2(a) or (b), and (B) is incapable of
being cured by McKesson or is not cured within 30 days of written notice
thereof;

          (d) by the Board of Directors of McKesson (provided that McKesson is
not then in material breach of any representation, warranty, covenant or other
agreement contained herein), if HBO shall have breached or failed to perform in
any material respect any of its representations, warranties, covenants or other
agreements contained in this Agreement, which breach or failure to perform (A)
would give rise to the failure of a condition set forth in Section 6.3(a) or
(b), and (B) is incapable of being cured by HBO or is not cured within 30 days
of written notice thereof;

          (e) by the Board of Directors of HBO, at any time prior to the
McKesson Stockholders' Meeting, if the McKesson Board of Directors shall have
(A) failed to include in the Joint Proxy Statement to the McKesson Stockholders,
its recommendation without modification or qualification that such stockholders
approve this Agreement and the transactions contemplated hereby, or (B)
subsequently withdrawn such recommendation or (C) modified or qualified such
recommendation in a manner adverse to the interests of HBO;

          (f) by the Board of Directors of McKesson, at any time prior to the
HBO Stockholders' Meeting, if the HBO Board of Directors shall have (A) failed
to include in the Joint Proxy Statement to the HBO Stockholders, its
recommendation without modification or qualification that such stockholders
approve this Agreement and the transaction contemplated hereby, or (B)
subsequently withdrawn such recommendation or (C) modified or qualified such
recommendation in a manner adverse to the interests of McKesson;

          (g) by HBO if the Board of Directors of McKesson shall have failed to
take any of the actions contemplated by Section 5.1 as a result of the exercise
of its rights under 
<PAGE>
 
Section 5.1(e); or

          (h) by McKesson if the Board of Directors of HBO shall have failed to
take any of the actions contemplated by Section 5.1 as a result of the exercise
of its rights under Section 5.1(e).

     SECTION 7.2   Effect of Termination.
                   --------------------- 

          (a) In the event of termination of this Agreement as provided in
Section 7.1 hereof, and subject to the provisions of Section 8.1 hereof, this
Agreement shall forthwith become void and there shall be no liability on the
part of any of the parties, except (i) as set forth in this Section 7.2 and in
Sections 5.4, 5.7, 3.1(o) and 3.2(o) hereof, and (ii) nothing herein shall
relieve any party from liability for any willful breach hereof.

          (b) If this Agreement is terminated (i) by HBO pursuant to Section
7.1(e) hereof, (ii) by HBO or McKesson pursuant to Section 7.1(b)(ii) hereof
because of the failure to obtain the required approval from the McKesson
stockholders and at the time of such termination or prior to the meeting of
McKesson's stockholders there shall have been an offer or proposal for, an
announcement of any intention with respect to (including, without limitation,
the filing of a statement of beneficial ownership on Schedule 13D discussing the
possibility of or reserving the right to engage in), or any agreement with
respect to, a transaction that would constitute an Alternative Transaction (as
defined in Section 4.2(a) hereof, except that for purposes of clause (i) of such
definition, the applicable percentage shall be fifty percent (50%)) involving
McKesson or any of the McKesson Subsidiaries (whether or not such offer,
proposal, announcement or agreement shall have been rejected or shall have been
withdrawn prior to the time of such termination or of the meeting), (iii) by HBO
as a result of McKesson's material breach of Section 4.2 or 5.1 hereof which, in
the case of Section 5.1 only, is not cured within 30 days after notice thereof
to McKesson, or (iv) by HBO pursuant to Section 7.1(g) hereof, McKesson shall
pay to HBO a termination fee of Two Hundred Million Dollars ($200,000,000.00)
(the "McKesson Termination Fee").

          (c) If this Agreement is terminated (i) by McKesson pursuant to
Sections 7.1 (f) hereof, (ii) by HBO or McKesson pursuant to Section 7.1(b)(iii)
hereof because of the failure to obtain the required approval from the HBO
stockholders and at the time of such termination or prior to the meeting of
HBO's stockholders there shall have been an offer or proposal for, an
announcement of any intention with respect to (including, without limitation,
the filing of a statement of beneficial ownership on Schedule 13D discussing the
possibility of or reserving the right to engage in), any agreement with respect
to, a transaction that would constitute an Alternative Transaction (as defined
in Section 4.2(a) hereof, except that for purposes of clause (i) of such
definition, the applicable percentage shall be fifty percent (50%) involving HBO
or any of the HBO Subsidiaries (whether or not such offer, proposal,
announcement or agreement shall have been rejected or shall have been withdrawn
prior to the time of such termination or of the meeting), (iii) by McKesson as a
result of HBO's material breach of Section 4.3 or 5.1 hereof which, in the case
of Section 5.1 only, is not cured within 30 days after notice thereof to HBO or
(iv) by McKesson pursuant to Section 7.1(h) hereof, HBO shall pay to McKesson a
termination fee of Two Hundred Million Dollars ($200,000,000.00) (the "HBO
Termination Fee").
<PAGE>
 
          (d) Each Termination Fee payable under Sections 7.2(b) and (c) above
shall be payable in cash, payable no later than one business day following the
delivery of notice of termination to the other party.

          (e) HBO and McKesson agree that the agreements contained in Sections
7.2(b) and (c) above are an integral part of the transaction contemplated by
this Agreement and constitute liquidated damages and not a penalty.  If one
party fails to promptly pay to the other any fee due under such Sections 7.2(b)
and (c), the defaulting party shall pay the costs and expenses (including legal
fees and expenses) in connection with any action, including the filing of any
lawsuit or other legal action, taken to collect payment.

     SECTION 7.3    Amendment.  Subject to compliance with applicable law, this
                    ---------                                                  
Agreement may be amended by the parties  at any time before or after the
McKesson Stockholder Approval or the HBO Stockholder Approval; provided,
however, that after any such approval, there may not be, without further
approval of such the stockholders of McKesson (in the case of the McKesson
Stockholders Approval) and the stockholders of HBO (in the case of the HBO
Stockholders Approval), any amendment of this Agreement that changes the amount
or the form of the consideration to be delivered to the holders of McKesson
Common Stock hereunder, or which by law otherwise expressly requires the further
approval of such stockholders.  This Agreement may not be amended except by an
instrument in writing signed on behalf of each of the parties hereto and duly
approved by the parties' respective Boards of Directors or a duly designated
committee thereof.

     SECTION 7.4    Extension; Waiver.  At any time prior to the Effective Time,
                    -----------------                                           
a party may, subject to the proviso of Section 7.3 (and for this purpose
treating any waiver referred to below as an amendment), (a) extend the time for
the performance of any of the obligations or other acts of the other parties,
(b) waive any inaccuracies in the representations and warranties of the other
parties contained in this Agreement or in any document delivered pursuant to
this Agreement or (c) waive compliance by the other party with any of the
agreements or conditions contained in this Agreement.  Any agreement on the part
of a party to any such extension or waiver shall be valid only if set forth in
an instrument in writing signed on behalf of such party.  Any extension or
waiver given in compliance with this Section 7.4 or failure to insist on strict
compliance with an obligation, covenant, agreement or condition shall not
operate as a waiver of, or estoppel with respect to, any subsequent or other
failure.

                                 ARTICLE VIII
                              GENERAL PROVISIONS

     SECTION 8.1    Nonsurvival of Representations and Warranties.  None of the
                    ---------------------------------------------              
representations and warranties in this Agreement or in any instrument delivered
pursuant to this Agreement shall survive the Effective Time.  This Section 8.1
shall not limit any covenant or agreement of the parties which by its terms
contemplates performance after the Effective Time.
<PAGE>
 
     SECTION 8.2    Notices.  All notices, requests, claims, demands and other
                    -------                                                   
communications under this Agreement shall be in writing and shall be deemed
given if delivered personally, telecopied (which is confirmed) or sent by
overnight courier (providing proof of delivery) to the parties at the
following addresses (or at such other address for a party as shall be specified
by like notice):

          (a)  if to HBO, to

               HBO & Company
               301 Perimeter Center North
               Atlanta, Georgia  30346
               Telecopy No.:  770/393-6020
               Attention:  Charles W. McCall

               with a copy to:

               Jones, Day, Reavis & Pogue
               3500 SunTrust Plaza
               303 Peachtree Street, N.E.
               Atlanta, Georgia  30308-3242
               Telecopy No.:  (404) 581-8330
               Attention:  John E. Zamer, Esq.

          (b)  if to McKesson, to

               McKesson Corporation
               One Post Street
               San Francisco, California  94104
               Telecopy No.  415/983-8826
               Attention:  Mark A. Pulido

               with a copy to:

               Skadden, Arps, Slate, Meagher & Flom LLP
               919 Third Avenue
               New York, New York  10022
               Telecopy No.:  212/735-3691
               Attention:  Peter A. Atkins, Esq.

     SECTION 8.3    Definitions.  For purposes of this Agreement:
                    -----------                                  

          (a) except for purposes of Section 5.10, an "affiliate" of any person
means another person that directly or indirectly, through one or more
intermediaries, controls, is controlled by, or is under common control with,
such first person, where "control" means the possession, directly or indirectly,
of the power to direct or cause the direction of the management 
<PAGE>
 
policies of a person, whether through the ownership of voting securities, by
contract, as trustee or executor, or otherwise;

          (b) "material adverse change" or "material adverse effect" means, when
used in connection with McKesson or HBO, any change, effect, event, occurrence
or state of facts that is or could reasonably be expected to be materially
adverse to the business, financial condition or results of operations of such
party and its subsidiaries taken as a whole, it being understood that none of
the following shall be deemed by itself or by themselves, either alone or in
combination, to constitute a material adverse effect:  (i) a change in the
market price or trading volume of HBO or McKesson Common Stock, as the case may
be or (ii) conditions affecting the U.S. economy as a whole;

          (c) "person" means an individual, corporation, partnership, limited
liability company, joint venture, association, trust, unincorporated
organization or other entity;

          (d) a "subsidiary" of any person means another person, an amount of
the voting securities, other voting ownership or voting partnership interests of
which is sufficient to elect at least a majority of its Board of Directors or
other governing body (or, if there are no such voting interests, 50% or more of
the equity interests of which) is owned directly or indirectly by such first
person; and

          (e) "knowledge" of any person which is not an individual means the
knowledge of such person's executive officers or senior management of such
person's operating divisions and segments.

     SECTION 8.4    Interpretation.  When a reference is made in this Agreement
                    --------------                                             
to an Article, Section or Exhibit, such reference shall be to an Article or
Section of, or an Exhibit to, this Agreement unless otherwise indicated.
Whenever the words "include", "includes" or "including" are used in this
Agreement, they shall be deemed to be followed by the words "without
limitation".  The words "hereof", "herein" and "hereunder" and words of similar
import when used in this Agreement shall refer to this Agreement as a whole and
not to any particular provision of this Agreement.  All terms defined in this
Agreement shall have the defined meanings when used in any certificate or other
document made or delivered pursuant hereto unless otherwise defined therein.
The definitions contained in this Agreement are applicable to the singular as
well as the plural forms of such terms and to the masculine as well as to the
feminine and neuter genders of such term.  Any agreement, instrument or statute
defined or referred to herein or in any agreement or instrument that is referred
to herein means such agreement, instrument or statute as from time to time
amended, modified or supplemented, including (in the case of agreements or
instruments) by waiver or consent and (in the case of statutes) by succession of
comparable successor statutes and references to all attachments thereto and
instruments incorporated therein.  References to a person are also to its
permitted successors and assigns.
<PAGE>
 
     SECTION 8.5    Counterparts.  This Agreement may be executed in one or more
                    ------------                                                
counterparts, all of which shall be considered one and the same agreement and
shall become effective when one or more counterparts have been signed by each of
the parties and delivered to the other parties.

     SECTION 8.6    Entire Agreement; No Third-Party Beneficiaries.  This
                    ----------------------------------------------       
Agreement (including the documents and instruments referred to herein), the
Option Agreements, the Support Agreement and the Confidentiality Agreement (as
amended below) (a) constitute the entire agreement, and supersede all prior
agreements and understandings, both written and oral, between the parties with
respect to the subject matter of this Agreement and (b) except for the
provisions of Section 5.7, are not intended to confer upon any person other than
the parties any rights or remedies.  The Confidentiality Agreement is hereby
amended to delete therefrom Section 11.

     SECTION 8.7    Governing Law.  This Agreement shall be governed by, and
                    -------------                                           
construed in accordance with, the laws of the State of Delaware, regardless of
the laws that might otherwise govern under applicable principles of conflict of
laws thereof.

     SECTION 8.8    Assignment.  Neither this Agreement nor any of the rights,
                    ----------                                                
interests or obligations under this Agreement shall be assigned, in whole or in
part, by operation of law or otherwise by either of the parties hereto without
the prior written consent of the other party.  Any assignment in violation of
the preceding sentence shall be void.  Subject to the preceding two sentences,
this Agreement will be binding upon, inure to the benefit of, and be enforceable
by, the parties and their respective successors and assigns.

     SECTION 8.9    Consent to Jurisdiction.  Each of the parties hereto (a)
                    -----------------------                                 
consents to submit itself to the personal jurisdiction of any federal court
located in the State of Delaware or any Delaware state court in the event any
dispute arises out of this Agreement or any of the transactions contemplated by
this Agreement, (b) agrees that it will not attempt to deny or defeat such
personal jurisdiction by motion or other request for leave from any such court,
and (c) agrees that it will not bring any action relating to this Agreement or
any of the transactions contemplated by this Agreement in any court other than a
federal court sitting in the State of Delaware or a Delaware state court.

     SECTION 8.10   Headings, Etc. The headings and table of contents contained
                    -------------
in this Agreement are for reference purposes only and shall not affect in any
way the meaning or interpretation of this Agreement.

     SECTION 8.11   Severability.  If any term or other provision of this
                    ------------                                         
Agreement is invalid, illegal or incapable of being enforced by any rule of law
or public policy, all other conditions and provisions of this Agreement shall
nevertheless remain in full force and effect, insofar as the foregoing can be
accomplished without materially affecting the economic benefits anticipated by
the parties to this Agreement.  Upon such determination that any term or other
provision is invalid, illegal or incapable of being enforced, the parties hereto
shall negotiate in good faith to modify this Agreement so as to effect the
original intent of the parties as closely as possible to the fullest extent
permitted by applicable law in an acceptable manner to the end that the
transactions contemplated hereby are fulfilled to the extent possible.
<PAGE>
 
          IN WITNESS WHEREOF, HBO, McKesson and Merger Sub have caused this
Agreement to be signed by their respective officers thereunto duly authorized,
all as of the date first written above.


                                              HBO & COMPANY
 
 
 
                                              By:   /s/ Jay P. Gilbertson
                                                    ---------------------
                                                    Jay P. Gilbertson,
                                                    President, Co-Chief 
                                                    Operating Officer
                                                    and Chief Financial Officer
 

                                              MCKESSON CORPORATION
 
 
 
                                              By:   /s/ Mark A. Pulido
                                                    ----------------------
                                                    Mark A. Pulido,
                                                    President and Chief 
                                                    Executive Officer
 

                                              MCKESSON MERGER SUB, INC.
 
 
 
                                              By:   /s/ Richard H. Hawkins
                                                    ------------------------
                                                    Richard H. Hawkins,
                                                    President
 

<PAGE>
 
                            STOCK OPTION AGREEMENT
                         (MCKESSON CORPORATION SHARES)

     THIS STOCK OPTION AGREEMENT (this "AGREEMENT"), dated October 17, 1998,
between McKesson Corporation, a Delaware corporation ("Issuer"), and HBO &
Company, a Delaware corporation ("Grantee"),

                             W I T N E S S E T H:

     WHEREAS, Grantee, Issuer, and McKesson Merger Sub, Inc., a wholly-owned
subsidiary of Issuer, have entered into an Agreement and Plan of Merger, dated
as of October 17, 1998 (the "Merger Agreement"), which provides, among other
things, for the merger of Merger Sub with and into Grantee, such that Grantee
will become a wholly-owned subsidiary of Issuer, and stockholders of Grantee
will become stockholders of Issuer (capitalized terms used herein without
definition shall have the respective meanings specified in the Merger
Agreement); and

     WHEREAS, as a condition to Grantee's entering into the Merger Agreement and
in consideration therefor, Issuer has agreed to grant Grantee the Option (as
hereinafter defined);

     NOW, THEREFORE, in consideration of the foregoing and the mutual covenants
and agreements set forth herein and in the Merger Agreement, the parties hereto
agree as follows:

     1.  Grant of Option.  Issuer hereby grants to Grantee an unconditional,
         ---------------                                                    
irrevocable option (the "Option") to purchase, subject to the terms hereof,
Nineteen Million Seven Hundred Fifty Nine Thousand Seven Hundred Seventeen
(19,759,717) shares of fully paid and nonassessable common stock of the Issuer,
par value $.01 per share ("Common Stock"), equal to 19.9% of the shares of
Common Stock outstanding as of the date hereof, together with any associated
purchase rights (the "Rights") under the Rights Agreement, dated as of October
21, 1994, as amended from time to time, between Issuer and First Chicago Trust
Company (references to shares purchasable upon exercise of the Option shall be
deemed to include the associated Rights), at a purchase price  of $88.6875 per
share of Common Stock as adjusted in accordance with the provisions of Section 5
of this Agreement (such price, as adjusted if applicable, the "Option Price").

     2.  (a)  Exercise of Option.  Grantee may exercise the Option, in whole or
              ------------------                                               
part, and from time to time, if, but only if, a Triggering Event (as hereinafter
defined) shall have occurred prior to the occurrence of an Option Termination
Event (as hereinafter defined), provided that Grantee shall have sent the
written notice of such exercise (as provided in subsection (e) of this Section
2) on or prior to the last date of the 12-month period following such Triggering
Event (the "Option Expiration Date").


                                       1
<PAGE>
 
          (b)  Option Termination Event. The term "Option Termination Event"
               ------------------------   
shall mean any of the following events: (i) the Effective Time of the Merger;
(ii) termination of the Merger Agreement by either party pursuant to Section
7.1(b)(iv) of the Merger Agreement, whether or not such termination occurs prior
to the occurrence of a Triggering Event, provided that the matter giving rise to
the order, decree, ruling or other action providing the basis for termination
under Section 7.1(b)(iv) shall not have been initiated by Issuer; or (iii)
termination of the Merger Agreement by either party pursuant to any other
provision of the Merger Agreement if such termination occurs prior to the
occurrence of a Triggering Event; or (iv) 12 months after the first occurrence
of a Triggering Event.

          (c)  Triggering Event. The term "Triggering Event" shall mean any
               ----------------   
event that would entitle either party to terminate the Merger Agreement and
permit Grantee to receive any fee from Issuer pursuant to Section 7.2 of the
Merger Agreement.

          (d)  Notice of Triggering Event. Issuer shall notify Grantee promptly
               --------------------------   
in writing of the occurrence of any Triggering Event, it being understood that
the giving of such notice by Issuer shall not be a condition to the right of
Grantee to exercise the Option or for a Triggering Event to have occurred.

          (e)  Notice of Exercise; Closing. In the event Grantee is entitled to
               ---------------------------      
and wishes to exercise the Option, it shall send to Issuer a written notice (the
date of which being herein referred to as the "Notice Date") specifying (i) the
total number of shares it will purchase pursuant to such exercise and (ii) a
place and date not earlier than three business days nor later than 60 business
days from the Notice Date for the closing of such purchase (the "Closing Date");
provided, that if the closing of the purchase and sale pursuant to the Option
(the "Closing") cannot be consummated, in the reasonable opinion of Grantee, by
reason of any applicable judgment, decree, order, law or regulation, the period
of time that otherwise would run pursuant to this sentence shall run instead
from the date on which such restriction on consummation has expired or been
terminated; and provided further, without limiting the foregoing, that if, in
the reasonable opinion of Grantee, prior notification to or approval of any
regulatory agency is required in connection with such purchase, Grantee shall
promptly file the required notice or application for approval and shall
expeditiously process the same and the period of time that otherwise would run
pursuant to this sentence shall run instead from the date on which any required
notification periods have expired or been terminated or such approvals have been
obtained and any requisite waiting period or periods shall have passed.  Any
exercise of the Option shall be deemed to occur on the Closing Date relating
thereto. Notwithstanding this subsection (e), in no event shall any Closing Date
be more than 12 months after the related Notice Date, and if the Closing Date
shall not have occurred within 12 months after the related Notice Date due to
the failure to obtain any such required approval, the exercise of the Option
effected on the Notice Date shall be deemed to have been rescinded.  In the
event (x) Grantee receives official notice that an approval of any regulatory
authority required for the purchase of Option Shares (as hereinafter defined)
will not be issued or granted, (y) a Closing Date shall not have occurred within
12 months after the related Notice Date due to the failure to obtain any such
required approval or (z) Grantee shall have the right pursuant to the last
sentence of subsection (d) of Section 7 or subsection (f) of Section 7 to
exercise the Option, Grantee shall nevertheless


                                       2
<PAGE>
 
be entitled to exercise its right as set forth in Section 7.

          (f)  Purchase Price. At the Closing referred to in subsection (e) of
               --------------   
this Section 2, Grantee shall pay to Issuer the aggregate purchase price for the
shares of Common Stock purchased pursuant to the exercise of the Option in
immediately available funds by wire transfer to a bank account designated by
Issuer, provided that failure or refusal of Issuer to designate such a bank
account shall not preclude Grantee from exercising the Option.

          (g)  Issuance of Common Stock. At such Closing, simultaneously with
               ------------------------   
the delivery of immediately available funds as provided in subsection (f) of
this Section 2, Issuer shall deliver to Grantee a certificate or certificates
representing the number of shares of Common Stock purchased by the Grantee and,
if the Option should be exercised in part only, a new Option evidencing the
rights of Grantee thereof to purchase the balance of the shares purchasable
hereunder, and the Grantee shall deliver to Issuer this Agreement and a letter
agreeing that Grantee will not offer to sell or otherwise dispose of such shares
in violation of applicable law or the provisions of this Agreement.

          (h)  Legend. Certificates for Common Stock delivered at a Closing
               ------   
hereunder may be endorsed with a restrictive legend that shall read
substantially as follows:

          "THE TRANSFER AND VOTING OF THE SHARES REPRESENTED BY THIS CERTIFICATE
          IS SUBJECT TO CERTAIN PROVISIONS OF AN AGREEMENT BETWEEN THE
          REGISTERED HOLDER HEREOF AND ISSUER AND TO RESALE RESTRICTIONS ARISING
          UNDER THE SECURITIES ACT OF 1933, AS AMENDED. A COPY OF SUCH AGREEMENT
          IS ON FILE AT THE PRINCIPAL OFFICE OF ISSUER AND WILL BE PROVIDED TO
          THE HOLDER HEREOF WITHOUT CHARGE UPON RECEIPT BY ISSUER OF A WRITTEN
          REQUEST THEREFOR."

It is understood and agreed that: (i) the reference to the resale restrictions
of the Securities Act of 1933, as amended (the "Securities Act"), in the above
legend shall be removed by delivery of substitute certificate(s) without such
reference if Grantee shall have delivered to Issuer a copy of a letter from the
staff of the SEC, or an opinion of counsel, in form and substance reasonably
satisfactory to Issuer, to the effect that such legend is not required for
purposes of the Securities Act; (ii) the reference to the provisions of this
Agreement in the above legend shall be removed by delivery of substitute
certificate(s) without such reference if the shares have been sold or
transferred in compliance with the provisions of this Agreement and under
circumstances that do not require the retention of such reference; and (iii) the
legend shall be removed in its entirety if the conditions in the preceding
clauses (i) and (ii) and both satisfied.  In addition, such certificates shall
bear any other legend as may be required by law.

          (i)  Record Holder; Expenses.  Upon the Closing, Grantee shall be
               -----------------------                                     
deemed to be the holder of record of the shares of Common Stock issuable upon
such exercise, 


                                       3
<PAGE>
 
notwithstanding that the stock transfer books of Issuer shall then be closed or
that certificates representing such shares of Common Stock shall not then be
actually delivered to Grantee or the Issuer shall have failed or refused to
designate the bank account described in subsection (f) of this Section 2. Issuer
shall pay all expenses, and any and all United States federal, state and local
taxes and other charges that may be payable in connection with the preparation,
issuance and delivery of stock certificates under this Section 2 in the name of
Grantee or its assignee, transferee or designee.

     3.   Reservation of Shares.  Issuer agrees: (i) that it shall at all times
          ---------------------                                                
maintain, free from preemptive rights, sufficient authorized but unissued or
treasury shares of Common Stock (and other securities issuable pursuant to
Section 5) so that the Option may be exercised without additional authorization
of Common Stock (or such other securities) after giving effect to all other
options, warrants, convertible securities and other rights to purchase Common
Stock (or such other securities); (ii) that it will not, by charter amendment or
through reorganization, consolidation, merger, dissolution or sale of assets, or
by any other voluntary act avoid or seek to avoid the observance or performance
of any of the covenants, stipulations or conditions to be observed or performed
hereunder by Issuer; (iii) promptly to take all action as may from time to time
be required (including without limitation complying with all premerger
notification, reporting and waiting periods in 15 U.S.C. Section 18a the rules
and regulations thereunder) in order to permit Grantee to exercise the Option
and the Issuer duly and effectively to issue shares of Common Stock pursuant
hereto; and (iv) promptly to take all action provided herein to protect the
rights of Grantee against dilution.

     4.   Lost Options.  Upon receipt by Issuer of evidence reasonably
          ------------                                                
satisfactory to it of the loss, theft, destruction or mutilation of this
Agreement, and (in the case of loss, theft or destruction) of reasonably
satisfactory indemnification, and upon surrender and cancellation of this
Agreement, if mutilated, Issuer will execute and deliver a new Agreement of like
tenor and date.

     5.   Adjustment upon Changes in Capitalization.  The number of shares of
          -----------------------------------------                          
Common Stock purchasable upon the exercise of the Option shall be subject to
adjustment from time to time as provided in this Section 5.

          (a) In the event that any additional shares of Common Stock are issued
or otherwise become outstanding after the date hereof (other than by reason of
subsection (b) of this Section 5), the number of shares of Common Stock subject
to the Option shall be increased so that, after such issuance of additional
shares, such number of shares then remaining subject to the Option, together
with shares theretofore issued pursuant to the Option, equals 19.9% of the
number of such shares of Common Stock then issued and outstanding.

          (b) In the event of any change in Common Stock by reason of stock
dividends, other dividends on the Common Stock payable in securities or other
property (other than regular cash dividends), stock splits, merger,
recapitalization, combinations, subdivisions, 


                                       4
<PAGE>
 
conversions, exchanges of shares or other similar transactions, then the type
and number of shares of Common Stock purchasable upon exercise hereof shall be
appropriately adjusted so that Grantee shall receive upon exercise of the Option
and payment of the aggregate Option Price hereunder the number and class of
shares or other securities or property that Grantee would have received in
respect of Common Stock if the Option had been exercised in full immediately
prior to such event, or the record date therefor, as applicable.

          (c)  Whenever the number of shares of outstanding Common Stock changes
after the date hereof, the Option Price shall be adjusted by multiplying the
Option Price by a fraction the numerator of which shall be equal to the
aggregate number of shares of Common Stock purchasable prior to the adjustment
and the denominator of which shall be equal to the aggregate number of shares of
Common Stock purchasable immediately after the adjustment.

     6.   Registration Rights.
          ------------------- 

          (a)  In the event that the Grantee shall desire to sell any of the
shares of Common Stock issued upon total or partial exercise of this Option
("Option Shares") within two years after the purchase of such Option Shares
pursuant hereto, and such sale requires, in the opinion of counsel to the
Grantee, which opinion shall be reasonably satisfactory to Issuer and its
counsel, registration of such Option Shares under the Securities Act, Issuer
will cooperate with the Grantee and any underwriters in registering such Option
Shares for resale, including, without limitation, promptly filing a registration
statement which complies with the requirements of applicable federal and state
securities laws and entering into an underwriting agreement with such
underwriters upon such terms and conditions as are customarily contained in
underwriting agreements with respect to secondary distributions;  provided that
Issuer shall not be required to have declared effective more than two
registration statements hereunder and shall be entitled to delay the filing or
effectiveness of any registration statement for up to 180 days if the offering
would, in the judgment of the Board of Directors of Issuer, require premature
disclosure of any material corporate development or otherwise interfere with or
adversely affect any pending or proposed offering of securities of the Issuer or
any other material transaction involving the Issuer.

          (b)  If the Common Stock is registered pursuant to the provisions of
this Section 6, the Issuer agrees (i) to furnish copies of the registration
statement and the prospectus relating to the Option Shares covered thereby in
such numbers as the Grantee may from time to time reasonably request and (ii) if
any event shall occur as a result of which it becomes necessary to amend or
supplement any registration statement or prospectus, to prepare and file under
the applicable securities laws such amendments and supplements as may be
necessary to keep effective for at least 90 days a prospectus covering the
Option Shares meeting the requirements of such securities laws, and to furnish
the Grantee such numbers of copies of the registration statement and prospectus
as amended or supplemented as may reasonably be requested.  Issuer shall bear
the cost of the registration, including, but not limited to, all registration
and filing fees, printing expenses, and fees and disbursements of counsel and
accountants for the Issuer, except 


                                       5
<PAGE>
 
that the Grantee shall pay the fees and disbursements of its counsel, the
underwriting fees and selling commissions applicable to the Option Shares sold
by the Grantee. Issuer shall indemnify and hold harmless Grantee, its affiliates
and its officers, directors and controlling persons from and against any and all
losses, claims, damages, liabilities and expenses arising out of or based upon
any alleged untrue statement contained or incorporated by reference in, and
alleged omission to state a material fact required to be contained in, each
registration statement filed pursuant to this paragraph; provided, however, that
this provision does not apply to any loss, liability, claim, damage or expense
to the extent it arises out of any such untrue statement or omission made in
reliance upon and in conformity with written information furnished to Issuer by
the Grantee, its affiliates and its officers expressly for use in any
registration statement (or any amendment thereto) or any preliminary prospectus
filed pursuant to this paragraph. Issuer shall also indemnify and hold harmless
each underwriter and each person who controls any underwriter within the meaning
of either the Securities Act or the Securities Exchange Act of 1934, as amended,
against any and all losses, claims, damages, liabilities and expenses arising
out of or based upon any such statements contained or incorporated by reference
in, and alleged omissions from, each registration statement filed pursuant to
this paragraph; provided, however, that this provision does not apply to any
loss, liability, claim, damage or expense to the extent it arises out of any
untrue statement or omission made in reliance upon and in conformity with
written information furnished to Issuer by the underwriters expressly for use in
any registration statement (or any amendment thereto) or any preliminary
prospectus filed pursuant to this paragraph.

     7.   Repurchase of Option and Option Shares.  (a)  Within ten business days
          --------------------------------------                                
following the occurrence of a Repurchase Event (as defined below), Issuer shall
(i) deliver an offer (a "Repurchase Offer") to repurchase the Option from
Grantee at a price (the "Option Repurchase Price") equal to the amount by which
(A) the Alternative Transaction Price (as defined below) exceeds (B) the Option
Price, multiplied by the number of shares for which the Option may then be
exercised, and (ii) deliver an offer (also, a "Repurchase Offer") to repurchase
the Option Shares from Grantee at a price (the "Option Share Repurchase Price")
equal to the Alternative Transaction Price multiplied by the number of Option
Shares then held by Grantee.  The term "Alternative Transaction Price" shall
mean, as of any date for the determination thereof, the price per share of
Common Stock paid pursuant to the Alternative Transaction or, in the event of
sale of assets of Issuer, the last per share sale price of Common Stock on the
fourth trading day following the announcement of such sale.  If the
consideration paid or received in the Alternative Transaction shall be other
than in cash, the value of such consideration shall be determined by a
nationally recognized investment banking firm selected by Grantee, which
determination shall be conclusive for all purposes of this Agreement.

          (b) Upon the occurrence of a Repurchase Event and whether or not
Issuer shall have made a Repurchase Offer under Section 7(a) at the request (the
date of such request being the "Option Repurchase Request Date") of Grantee
delivered prior to the Option Expiration Date, Issuer shall repurchase the
Option from Grantee at the Option Repurchase Price and (ii) at the request (the
date of such request being the "Option Share Repurchase Request Date") of


                                       6
<PAGE>
 
Grantee delivered prior to the Option Expiration Date, Issuer shall repurchase
such number of the Option Shares from Grantee as Grantee shall designate at the
Option Share Repurchase Price.

          (c) Grantee may accept Issuer's Repurchase Offer under Section 7(a) or
may exercise its right to require Issuer to repurchase the Option and/or any
Option Shares pursuant to Section 7(b) by a written notice or notices stating
that Grantee elects to accept such offer or to require Issuer to repurchase the
Option and/or the Option Shares in accordance with the provisions of this
Section 7. As promptly as practicable, and in any event within five business
days, after the last to occur of (i) the surrender to it of this Agreement
and/or Certificates for Option Shares, as applicable, (ii) receipt of a notice
of election under this Section 7(c) or (iii) the occurrence of a Repurchase
Event, Issuer shall deliver or cause to be delivered to Grantee the Option
Repurchase Price and/or the Option Share Repurchase Price and/or the portion
thereof that Issuer is not then prohibited from so delivering under applicable
Law.

          (d) Issuer hereby undertakes to use its best efforts to obtain all
required regulatory and legal approvals and to file any required notices as
promptly as practicable in order to accomplish any repurchase contemplated by
this Section 7.  Nonetheless, to the extent that Issuer is prohibited under
applicable law, from repurchasing the Option and/or any Option Shares in full,
Issuer shall immediately so notify Grantee and thereafter deliver or cause to be
delivered, from time to time, to Grantee, the portion of the Option Repurchase
Price and the Option Share Repurchase Price, respectively, that it is no longer
prohibited from delivering, within five business days after the date on which
Issuer is no longer so prohibited; provided, however, that if Issuer at any time
after receipt of a notice of election to repurchase pursuant to Section 7(c) is
prohibited under applicable Law, from delivering to Grantee, the Option
Repurchase Price or the Option Share Repurchase Price, respectively, in full,
Grantee, may revoke its notice of election for repurchase of the Option or the
Option Shares either in whole or in part whereupon, in the case of a revocation
in part, Issuer shall promptly (i) deliver to Grantee, that portion of the
Option Repurchase Price or the Option Share Repurchase Price that Issuer is not
prohibited from delivering after taking into account any such revocation and
(ii) deliver to Grantee, as appropriate, either (a) a new agreement evidencing
the right of Grantee to purchase that number of shares of Common Stock equal to
the number of shares of Common Stock purchasable immediately prior to the
delivery of the notice of repurchase less the number of shares of Common Stock
covered by the portion of the Option repurchased or (b) a certificate for the
number of Option Shares covered by the revocation.  If an Option Termination
Event shall have occurred prior to the date a notice described in the first
sentence of this subsection (d) is filed by Issuer, or shall be scheduled to
occur at any time before the expiration of a period ending on the thirtieth day
after such date, Grantee shall nonetheless have the right to exercise the Option
until the expiration of such 30-day period.

          (e) The term "Repurchase Event" shall mean a Triggering Event followed
by the consummation of any transaction included in the definition of Alternative
Transaction (as so defined in Section 7.2(b) of the Merger Agreement).


                                       7
<PAGE>
 
          (f)  Notwithstanding anything to the contrary in Sections 2(a) and
2(e), if the Grantee delivers a notice under Section 7(b) specifying that (i)
such notice relates to an anticipated Repurchase Event under Section 7(e), and
is based on the Issuer's public announcement of the execution of an agreement
providing for an Alternative Transaction, such notice shall be deemed to
constitute an election to exercise the Option, as to the number of Option Shares
not theretofore purchased pursuant to one or more prior exercises of the Option,
on the fifth business day following the public announcement of the consummation
of the transaction contemplated by such agreement, in which event a Closing
shall occur with respect to such unpurchased Option Shares in accordance with
Section 2(e) on such fifth business day (or such later date as determined
pursuant to the proviso in the first sentence of Section 2(e)).

          8.   Representations and Warranties of the Issuer.  Issuer hereby
               --------------------------------------------                
represents and warrants to Grantee as follows:

               (a) Issuer has full corporate power and authority to execute and
deliver this Agreement and to consummate the transactions contemplated hereby.
The execution and delivery of this Agreement and the consummation of the
transactions contemplated hereby have been duly and validly authorized by the
Board of Directors of Issuer and no other corporate proceedings on the part of
Issuer are necessary to authorize this Agreement or to consummate the
transactions so contemplated.  This Agreement has been duly and validly executed
and delivered by Issuer.  This Agreement is the valid and legally binding
obligation of Issuer, enforceable against Issuer in accordance with its terms.

               (b) Issuer has taken all necessary corporate action to authorize
and reserve and to permit it to issue, and at all times from the date hereof
through the termination of this Agreement in accordance with its terms will have
reserved for issuance upon the exercise of the Option, that number of shares of
Common Stock equal to the maximum number of shares of Common Stock at any time
and from time to time issuable hereunder, and all such shares, upon issuance
pursuant hereto, will be duly authorized, validly issued, fully paid,
nonassessable, and will be delivered free and clear of all claims, liens,
encumbrance and security interests and not subject to any preemptive rights.

               (c) The execution and delivery of this Agreement does not, and
the consummation of the transactions contemplated hereby will not, conflict
with, or result in any violation pursuant to any provisions of the charter or 
by-laws of Issuer or any Issuer subsidiary or subject to obtaining any approvals
or consents contemplated hereby, result in any violation of any loan or credit
agreement, note, mortgage, indenture, lease, plan or other agreement,
obligation, instrument, permit, concession, franchise, license, judgment, order,
decree, statute, law, ordinance, rule or regulation applicable to Issuer or any
Issuer subsidiary or their respective properties or assets which violation would
have, individually or in the aggregate, a material adverse effect (as defined in
the Merger Agreement).

               (d) The Issuer has taken, and will in the future take, all steps
necessary 


                                       8
<PAGE>
 
to irrevocably exempt the transactions contemplated by this Agreement from any
applicable state takeover law and from any applicable charter or contractual
provision containing change of control or anti-takeover provisions.

          9.   Assignment of Option by Grantee.  Neither of the parties hereto
               -------------------------------                                
may assign any of its rights or obligations under this Option Agreement or the
Option created hereunder to any other person, without the express written
consent of the other party.

          10.  Limitation of Grantee Profit.  (a)  Notwithstanding any other
               ----------------------------                                 
provision of this Agreement, in no event shall the Grantee's Total Profit (as
hereinafter defined) exceed $220,000,000.00 and, if it otherwise would exceed
such amount, the Grantee, at its sole election, shall either (i) reduce the
number of shares of Common Stock subject to this Option, (ii) deliver to the
Issuer for cancellation Option Shares previously purchased by Grantee (valued,
for the purposes of this Section 10(a) at the average closing sales price per
share of Common Stock (or if there is no sale on such date then the average
between the closing bid and ask prices on any such day) as reported by The New
York Stock Exchange for the twenty consecutive trading days preceding the day on
which the Grantee's Total Profit exceeds $220,000,000.00), (iii) pay cash to the
Issuer, or (iv) any combination thereof, so that Grantee's actually realized
Total Profit shall not exceed $220,000,000.00 after taking into account the
foregoing actions.

               (b) As used herein, the term "Total Profit" shall mean the amount
(before taxes) of the following: (a) the aggregate amount of (i) (x) the net
cash amounts received by Grantee pursuant to the sale of Option Shares (or any
other securities into which such Option Shares are converted or exchanged) to
any unaffiliated party or to Issuer pursuant to this Agreement, less (y) the
Grantee's purchase price of such Option Shares, (ii) any amounts received by
Grantee on the transfer of the Option (or any portion thereof) to any
unaffiliated party, if permitted hereunder or to Issuer pursuant to this
Agreement, and (iii) the amount received by Grantee pursuant to Section 7.2 of
the Merger Agreement;  minus (b) the amount of cash theretofore paid to the
Issuer pursuant to this Section 10 plus the value of the Option Shares
theretofore delivered to the Issuer for cancellation pursuant to this Section
10.

               (c) Notwithstanding any other provision of this Agreement,
nothing in this Agreement shall affect the ability of Grantee to receive nor
relieve Issuer's obligation to pay a fee pursuant to Section 7.2 of the Merger
Agreement; provided that if Total Profit received by Grantee would exceed
$220,000,000.00 following the receipt of such fee, Grantee shall be obligated to
comply with the terms of Section 10(a) within 5 days of the later of (i) the
date of receipt of such fee and (ii) the date of receipt of the net cash by
Grantee pursuant to the sale of Option Shares (or, any other securities into
which such Option Shares are converted or exchanged) to any unaffiliated party
or to Issuer pursuant to this Agreement.

               (d) Notwithstanding any other provision of this Agreement, the
Option may not be exercised for a number of Option Shares that would, as of the
Notice Date, result in a Notional Total Profit (as defined below) of more than
$220,000,000.00. "Notional Total Profit"


                                       9
<PAGE>
 
shall mean, with respect to any number of Option Shares as to which the Grantee
may propose to exercise the Option, the Total Profit determined as of the Notice
Date assuming that the Option was exercised on such date for such number of
Option Shares and assuming such Option Shares, together with all other Option
Shares held by the Grantee and its affiliates as of such date, were sold for
cash at the closing sales price for Common Stock as of the close of business on
the preceding trading day.

          11.  First Refusal.  At any time after the first occurrence of a
               -------------                                              
Triggering Event and prior to the later of (a) the expiration of 18 months
immediately following the first purchase of shares of Common Stock pursuant to
the Option and (b) the Option Termination Date, if Grantee shall desire to sell,
assign, transfer or otherwise dispose of all or any of the Option or the shares
of Common Stock or other securities acquired by it pursuant to the Option, it
shall give Issuer written notice of the proposed transaction (an "Offeror's
Notice"), identifying the proposed transferee, accompanied by a copy of a
binding offer to purchase the Option or such shares or other securities signed
by such transferee and setting forth the terms of the proposed transaction.  An
Offeror's Notice shall be deemed an offer by Grantee to Issuer, which may be
accepted within 20 business days of the receipt by Issuer of such Offeror's
Notice, on the same terms and conditions and at the same price at which Grantee
is proposing to transfer the Option or such shares or other securities to such
transferee.  The purchase of the Option or any such shares or other securities
by Issuer shall be settled within 10 business days of the date of the acceptance
of the offer and the purchase price shall be paid to Grantee in immediately
available funds; provided that, if prior notification to or approval of any
regulatory authority is required in connection with such purchase, Issuer shall
promptly file the required notice or application for approval and shall
expeditiously process the same (and Grantee shall cooperate with Issuer in the
filing of any such notice or application and the obtaining of any such approval)
and the period of time that otherwise would run pursuant to this sentence shall
run instead from the date on which, as the case may be, (a) the required
notification period has expired or been terminated or (b) such approval has been
obtained and, in either event, any requisite waiting period shall have passed.
In the event of the failure or refusal of Issuer to purchase all of the Option
or all of the shares or other securities covered by an Offeror's Notice or if
any regulatory authority disapproves Issuer's proposed purchase of any portion
of the Option or such shares or other securities, Grantee may, within 60 days
from the date of the Offeror's Notice (subject to any necessary extension for
regulatory notification, approval or waiting periods), sell all, but not less
than all, of such portion of the Option or such shares or other securities to,
the proposed transferee at no less than the price specified and on terms no more
favorable than those set forth in the Offeror's Notice.  The requirements of
this Section 11 shall not apply to (w) any disposition as a result of which the
proposed transferee would own beneficially not more than 2% of the outstanding
voting power of Issuer, (x) any disposition of Common Stock or other securities
by a person to whom Grantee has assigned its rights under the Option with the
consent of Issuer, (y) any sale by means of a public offering registered under
the Securities Act in which steps are taken to reasonably assure that no
purchaser will acquire securities representing more than 2% of the outstanding
voting power of Issuer or (z) any transfer to a wholly owned subsidiary of
Grantee which agrees in writing to be bound by the terms hereof.


                                      10
<PAGE>
 
          12.  Voting.  For a period of 18 months from the date of exercise of
               ------                                                         
the Option, so long as Grantee beneficially owns any Option Shares, Grantee
agrees to (a) be present, in person or represented by proxy, at all stockholder
meetings of Issuer, so that all Option Shares beneficially owned by Grantee may
be counted for the purpose of determining the presence of a quorum at such
meetings, and (b) vote or cause to be voted all Option Shares beneficially owned
by it, with respect to all matters submitted to shareholders for a vote, in the
same proportion as shares of Common Stock are voted by shareholders unaffiliated
with Grantee.

          13.  Application for Regulatory Approval.  Each of Grantee and Issuer
               -----------------------------------                             
will use its best efforts to make all filings with, and to obtain consents of,
all third parties and governmental authorities necessary to the consummation of
the transactions contemplated by this Agreement, including without limitation
making application to list the shares of Common Stock issuable hereunder on The
New York Stock Exchange upon official notice of issuance.

          14.  Specific Performance.  The parties hereto acknowledge that
               --------------------                                      
damages would be an inadequate remedy for a breach of this Agreement by either
party hereto and that the obligations of the parties hereto shall be enforceable
by either party hereto through injunctive or other equitable relief.

          15.  Separability of Provisions.  If any term, provision, covenant or
               --------------------------                                      
restriction contained in this Agreement is held by a court or a federal or state
regulatory agency of competent jurisdiction to be invalid, void or
unenforceable, the remainder of the terms, provisions and covenants and
restrictions contained in this Agreement shall remain in full force and effect,
and shall in no way be affected, impaired or invalidated.

          16.  Notices.  All notices, claims, demands and other communications
               -------                                                        
hereunder shall be deemed to have been duly given or made when delivered in
person, by overnight courier or by facsimile at the respective addresses of the
parties set forth in the Merger Agreement.

          17.  Governing Law.  This Agreement shall be governed by and construed
               -------------                                                    
in accordance with the laws of the State of Delaware, regardless of the laws
that might otherwise govern under applicable principles of conflicts of laws
thereof.

          18.  Counterparts.  This Agreement may be executed in two or more
               ------------                                                
counterparts, each of which will be deemed to be an original, but all of which
shall constitute one and the same agreement.

          19.  Expenses.  Except as otherwise expressly provided herein or in
               --------                                                      
the Merger Agreement, each of the parties hereto shall bear and 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.


                                      11
<PAGE>
 
          20.  Entire Agreement.  Except as otherwise expressly provided herein
               ----------------                                                
or in the Merger Agreement, this Agreement contains the entire agreement between
the parties with respect to the transactions contemplated hereunder and
supersedes all prior arrangements or understandings with respect thereof,
written or oral.  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 permitted assigns. Nothing in this Agreement, expressed or
implied, is intended to confer upon any party, other than the parties hereto,
and their respective successors except as assigns, any rights, remedies,
obligations or liabilities under or by reason of this Agreement, except as
expressly provided herein. Any provision of this Agreement may be waived only in
writing 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.

          21.  Further Assurances.  In the event of any exercise of the Option
               ------------------                                             
by Grantee, Issuer and Grantee shall execute and deliver all other documents and
instruments and take all other action that may be reasonably necessary in order
to consummate the transactions provided for by such exercise.  Nothing contained
in this Agreement shall be deemed to authorize Issuer or Grantee to breach any
provision of the Merger Agreement.


                                      12
<PAGE>
 
          IN WITNESS WHEREOF, Issuer and Grantee have caused this Agreement to
be signed by their respective officers thereunto duly authorized, all as of the
date first written above.

                              MCKESSON CORPORATION


                              By:/s/ Mark A. Pulido
                                 -------------------------------------
                                 Mark A. Pulido,
                                 President and Chief Executive Officer



                              HBO & COMPANY


                              By:/s/ Jay P. Gilbertson
                                 -------------------------------------
                                 Jay P. Gilbertson,
                                 President, Co-Chief Operating Officer
                                 and Chief Financial Officer


                                      13

<PAGE>
 


                             STOCK OPTION AGREEMENT
                             (HBO & COMPANY SHARES)

     THIS STOCK OPTION AGREEMENT (this "AGREEMENT"), dated October 17, 1998,
between McKESSON CORPORATION, a Delaware corporation ("Grantee"), and  HBO &
COMPANY, a Delaware corporation ("Issuer"),

                              W I T N E S S E T H:

     WHEREAS, Grantee, Issuer, and McKesson Merger Sub, Inc., a wholly-owned
subsidiary of Grantee, have entered into an Agreement and Plan of Merger, dated
as of October 17, 1998 (the "Merger Agreement"), which provides, among other
things, for the merger of Merger Sub with and into Issuer, such that Issuer will
become a wholly-owned subsidiary of Grantee, and stockholders of Issuer will
become stockholders of Grantee (capitalized terms used herein without definition
shall have the respective meanings specified in the Merger Agreement); and

     WHEREAS, as a condition to Grantee's entering into the Merger Agreement and
in consideration therefor, Issuer has agreed to grant Grantee the Option (as
hereinafter defined);

     NOW, THEREFORE, in consideration of the foregoing and the mutual covenants
and agreements set forth herein and in the Merger Agreement, the parties hereto
agree as follows:

     1.  Grant of Option.  Issuer hereby grants to Grantee an unconditional,
         ---------------                                                    
irrevocable option (the "Option") to purchase, subject to the terms hereof,
Eighty-Five Million Eight Hundred Sixty-Five Thousand Five Hundred Seventeen
(85,865,517) shares of fully paid and nonassessable common stock of the Issuer,
par value $.05 per share ("Common Stock"), equal to 19.9% of the shares of
Common Stock outstanding as of the date hereof, together with any associated
purchase rights (the "Rights") under the Rights Agreement, dated as of February
12, 1991, as amended from time to time, between Issuer and The Citizens and
Southern Trust Company (Georgia), N.A. (references to shares purchasable upon
exercise of the Option shall be deemed to include the associated Rights), at a
purchase price of $32.81 per share of Common Stock as adjusted in accordance
with the provisions of Section 5 of this Agreement (such price, as adjusted if
applicable, the "Option Price").

     2.  (a)  Exercise of Option.  Grantee may exercise the Option, in whole or
              ------------------                                               
part, and from time to time, if, but only if, a Triggering Event (as hereinafter
defined) shall have occurred prior to the occurrence of an Option Termination
Event (as hereinafter defined), provided that Grantee shall have sent the
written notice of such exercise (as provided in subsection (e) of this Section
2) on or prior to the last date of the 12-month period following such 


                                       1
<PAGE>
 
Triggering Event (the "Option Expiration Date").

     (b) Option Termination Event.  The term "Option Termination Event" shall
         ------------------------                                            
mean any of the following events: (i) the Effective Time of the Merger; (ii)
termination of the Merger Agreement by either party pursuant to Section
7.1(b)(iv) of the Merger Agreement, whether or not such termination occurs prior
to the occurrence of a Triggering Event, provided that the matter giving rise to
the order, decree, ruling or other action providing the basis for termination
under Section 7.1(b)(iv) shall not have been initiated by Issuer; or (iii)
termination of the Merger Agreement by either party pursuant to any other
provision of the Merger Agreement if such termination occurs prior to the
occurrence of a Triggering Event; or (iv) 12 months after the first occurrence
of a Triggering Event.

     (c) Triggering Event.  The term "Triggering Event" shall mean any event
         ----------------                                                   
that would entitle either party to terminate the Merger Agreement and permit
Grantee to receive any fee from Issuer pursuant to Section 7.2 of the Merger
Agreement.

     (d) Notice of Triggering Event.  Issuer shall notify Grantee promptly in
         --------------------------                                          
writing of the occurrence of any Triggering Event, it being understood that the
giving of such notice by Issuer shall not be a condition to the right of Grantee
to exercise the Option or for a Triggering Event to have occurred.

     (e) Notice of Exercise; Closing.  In the event Grantee is entitled to and
         ---------------------------                                          
wishes to exercise the Option, it shall send to Issuer a written notice (the
date of which being herein referred to as the "Notice Date") specifying (i) the
total number of shares it will purchase pursuant to such exercise and (ii) a
place and date not earlier than three business days nor later than 60 business
days from the Notice Date for the closing of such purchase (the "Closing Date");
provided, that if the closing of the purchase and sale pursuant to the Option
(the "Closing") cannot be consummated, in the reasonable opinion of Grantee, by
reason of any applicable judgment, decree, order, law or regulation, the period
of time that otherwise would run pursuant to this sentence shall run instead
from the date on which such restriction on consummation has expired or been
terminated; and provided further, without limiting the foregoing, that if, in
the reasonable opinion of Grantee, prior notification to or approval of any
regulatory agency is required in connection with such purchase, Grantee shall
promptly file the required notice or application for approval and shall
expeditiously process the same and the period of time that otherwise would run
pursuant to this sentence shall run instead from the date on which any required
notification periods have expired or been terminated or such approvals have been
obtained and any requisite waiting period or periods shall have passed.  Any
exercise of the Option shall be deemed to occur on the Closing Date relating
thereto.  Notwithstanding this subsection (e), in no event shall any Closing
Date be more than 12 months after the related Notice Date, and if the Closing
Date shall not have occurred within 12 months after the related Notice Date due
to the failure to obtain any such required approval, the exercise of the Option
effected on the Notice Date shall be deemed to have been rescinded.  In the
event (x) Grantee receives official notice that an approval of any regulatory
authority required for the purchase of Option Shares (as hereinafter defined)
will not be issued or granted, (y) a Closing Date shall not have occurred within
12 months after the related Notice Date due to the failure to obtain any such
required approval or (z) Grantee shall have the right pursuant to the last
sentence of subsection 


                                       2
<PAGE>
 
(d) of Section 7 or subsection (f) of Section 7 to exercise the Option, Grantee
shall nevertheless be entitled to exercise its right as set forth in Section 7.

     (f) Purchase Price.  At the Closing referred to in subsection (e) of this
         --------------                                                       
Section 2, Grantee shall pay to Issuer the aggregate purchase price for the
shares of Common Stock purchased pursuant to the exercise of the Option in
immediately available funds by wire transfer to a bank account designated by
Issuer, provided that failure or refusal of Issuer to designate such a bank
account shall not preclude Grantee from exercising the Option.

     (g) Issuance of Common Stock.  At such Closing, simultaneously with the
         ------------------------                                           
delivery of immediately available funds as provided in subsection (f) of this
Section 2, Issuer shall deliver to Grantee a certificate or certificates
representing the number of shares of Common Stock purchased by the Grantee and,
if the Option should be exercised in part only, a new Option evidencing the
rights of Grantee thereof to purchase the balance of the shares purchasable
hereunder, and the Grantee shall deliver to Issuer this Agreement and a letter
agreeing that Grantee will not offer to sell or otherwise dispose of such shares
in violation of applicable law or the provisions of this Agreement.

     (h) Legend.  Certificates for Common Stock delivered at a Closing hereunder
         ------                                                                 
may be endorsed with a restrictive legend that shall read substantially as
follows:

     "THE TRANSFER AND VOTING OF THE SHARES REPRESENTED BY THIS CERTIFICATE IS
     SUBJECT TO CERTAIN PROVISIONS OF AN AGREEMENT BETWEEN THE REGISTERED HOLDER
     HEREOF AND ISSUER AND TO RESALE RESTRICTIONS ARISING UNDER THE SECURITIES
     ACT OF 1933, AS AMENDED. A COPY OF SUCH AGREEMENT IS ON FILE AT THE
     PRINCIPAL OFFICE OF ISSUER AND WILL BE PROVIDED TO THE HOLDER HEREOF
     WITHOUT CHARGE UPON RECEIPT BY ISSUER OF A WRITTEN REQUEST THEREFOR."

It is understood and agreed that: (i) the reference to the resale restrictions
of the Securities Act of 1933, as amended (the "Securities Act"), in the above
legend shall be removed by delivery of substitute certificate(s) without such
reference if Grantee shall have delivered to Issuer a copy of a letter from the
staff of the SEC, or an opinion of counsel, in form and substance reasonably
satisfactory to Issuer, to the effect that such legend is not required for
purposes of the Securities Act; (ii) the reference to the provisions of this
Agreement in the above legend shall be removed by delivery of substitute
certificate(s) without such reference if the shares have been sold or
transferred in compliance with the provisions of this Agreement and under
circumstances that do not require the retention of such reference; and (iii) the
legend shall be removed in its entirety if the conditions in the preceding
clauses (i) and (ii) and both satisfied.  In addition, such certificates shall
bear any other legend as may be required by law.

     (i) Record Holder; Expenses. Upon the Closing, Grantee shall be deemed to
         -----------------------


                                       3
<PAGE>
 
be the holder of record of the shares of Common Stock issuable upon such
exercise, notwithstanding that the stock transfer books of Issuer shall then be
closed or that certificates representing such shares of Common Stock shall not
then be actually delivered to Grantee or the Issuer shall have failed or refused
to designate the bank account described in subsection (f) of this Section 2.
Issuer shall pay all expenses, and any and all United States federal, state and
local taxes and other charges that may be payable in connection with the
preparation, issuance and delivery of stock certificates under this Section 2 in
the name of Grantee or its assignee, transferee or designee.

     3.   Reservation of Shares.  Issuer agrees: (i) that it shall at all times
          ---------------------                                                
maintain, free from preemptive rights, sufficient authorized but unissued or
treasury shares of Common Stock (and other securities issuable pursuant to
Section 5) so that the Option may be exercised without additional authorization
of Common Stock (or such other securities) after giving effect to all other
options, warrants, convertible securities and other rights to purchase Common
Stock (or such other securities); (ii) that it will not, by charter amendment or
through reorganization, consolidation, merger, dissolution or sale of assets, or
by any other voluntary act avoid or seek to avoid the observance or performance
of any of the covenants, stipulations or conditions to be observed or performed
hereunder by Issuer; (iii) promptly to take all action as may from time to time
be required (including without limitation complying with all premerger
notification, reporting and waiting periods in 15 U.S.C. Section 18a the rules
and regulations thereunder) in order to permit Grantee to exercise the Option
and the Issuer duly and effectively to issue shares of Common Stock pursuant
hereto; and (iv) promptly to take all action provided herein to protect the
rights of Grantee against dilution.

     4.   Lost Options.  Upon receipt by Issuer of evidence reasonably
          ------------                                                
satisfactory to it of the loss, theft, destruction or mutilation of this
Agreement, and (in the case of loss, theft or destruction) of reasonably
satisfactory indemnification, and upon surrender and cancellation of this
Agreement, if mutilated, Issuer will execute and deliver a new Agreement of like
tenor and date.

     5.   Adjustment upon Changes in Capitalization.  The number of shares of
          -----------------------------------------                          
Common Stock purchasable upon the exercise of the Option shall be subject to
adjustment from time to time as provided in this Section 5.

          (a) In the event that any additional shares of Common Stock are issued
or otherwise become outstanding after the date hereof (other than by reason of
subsection (b) of this Section 5), the number of shares of Common Stock subject
to the Option shall be increased so that, after such issuance of additional
shares, such number of shares then remaining subject to the Option, together
with shares theretofore issued pursuant to the Option, equals 19.9% of the
number of such shares of Common Stock then issued and outstanding.

          (b) In the event of any change in Common Stock by reason of stock
dividends, other dividends on the Common Stock payable in securities or other
property (other than regular cash dividends), stock splits, merger,
recapitalization, combinations, subdivisions, conversions, exchanges of shares
or other similar transactions, then the type and number of shares of Common
Stock purchasable upon exercise hereof shall be appropriately adjusted so 


                                       4
<PAGE>
 
that Grantee shall receive upon exercise of the Option and payment of the
aggregate Option Price hereunder the number and class of shares or other
securities or property that Grantee would have received in respect of Common
Stock if the Option had been exercised in full immediately prior to such event,
or the record date therefor, as applicable.

          (c) Whenever the number of shares of outstanding Common Stock changes
after the date hereof, the Option Price shall be adjusted by multiplying the
Option Price by a fraction the numerator of which shall be equal to the
aggregate number of shares of Common Stock purchasable prior to the adjustment
and the denominator of which shall be equal to the aggregate number of shares of
Common Stock purchasable immediately after the adjustment.

     6.   Registration Rights.
          ------------------- 

          (a) In the event that the Grantee shall desire to sell any of the
shares of Common Stock issued upon total or partial exercise of this Option
("Option Shares") within two years after the purchase of such Option Shares
pursuant hereto, and such sale requires, in the opinion of counsel to the
Grantee, which opinion shall be reasonably satisfactory to Issuer and its
counsel, registration of such Option Shares under the Securities Act, Issuer
will cooperate with the Grantee and any underwriters in registering such Option
Shares for resale, including, without limitation, promptly filing a registration
statement which complies with the requirements of applicable federal and state
securities laws and entering into an underwriting agreement with such
underwriters upon such terms and conditions as are customarily contained in
underwriting agreements with respect to secondary distributions;  provided that
Issuer shall not be required to have declared effective more than two
registration statements hereunder and shall be entitled to delay the filing or
effectiveness of any registration statement for up to 180 days if the offering
would, in the judgment of the Board of Directors of Issuer, require premature
disclosure of any material corporate development or otherwise interfere with or
adversely affect any pending or proposed offering of securities of the Issuer or
any other material transaction involving the Issuer.

          (b) If the Common Stock is registered pursuant to the provisions of
this Section 6, the Issuer agrees (i) to furnish copies of the registration
statement and the prospectus relating to the Option Shares covered thereby in
such numbers as the Grantee may from time to time reasonably request and (ii) if
any event shall occur as a result of which it becomes necessary to amend or
supplement any registration statement or prospectus, to prepare and file under
the applicable securities laws such amendments and supplements as may be
necessary to keep effective for at least 90 days a prospectus covering the
Option Shares meeting the requirements of such securities laws, and to furnish
the Grantee such numbers of copies of the registration statement and prospectus
as amended or supplemented as may reasonably be requested.  Issuer shall bear
the cost of the registration, including, but not limited to, all registration
and filing fees, printing expenses, and fees and disbursements of counsel and
accountants for the Issuer, except that the Grantee shall pay the fees and
disbursements of its counsel, the underwriting fees and selling commissions
applicable to the Option Shares sold by the Grantee.  Issuer shall indemnify and
hold harmless Grantee, its affiliates and its officers, directors and
controlling persons from and against any and all losses, claims, damages,
liabilities and expenses arising out of or based upon any alleged untrue
statement contained or incorporated by reference in, and alleged 


                                       5
<PAGE>
 
omission to state a material fact required to be contained in, each registration
statement filed pursuant to this paragraph; provided, however, that this
provision does not apply to any loss, liability, claim, damage or expense to the
extent it arises out of any such untrue statement or omission made in reliance
upon and in conformity with written information furnished to Issuer by the
Grantee, its affiliates and its officers expressly for use in any registration
statement (or any amendment thereto) or any preliminary prospectus filed
pursuant to this paragraph. Issuer shall also indemnify and hold harmless each
underwriter and each person who controls any underwriter within the meaning of
either the Securities Act or the Securities Exchange Act of 1934, as amended,
against any and all losses, claims, damages, liabilities and expenses arising
out of or based upon any such statements contained or incorporated by reference
in, and alleged omissions from, each registration statement filed pursuant to
this paragraph; provided, however, that this provision does not apply to any
loss, liability, claim, damage or expense to the extent it arises out of any
untrue statement or omission made in reliance upon and in conformity with
written information furnished to Issuer by the underwriters expressly for use in
any registration statement (or any amendment thereto) or any preliminary
prospectus filed pursuant to this paragraph.

     7.   Repurchase of Option and Option Shares.  (a)  Within ten business days
          --------------------------------------                                
following the occurrence of a Repurchase Event (as defined below), Issuer shall
(i) deliver an offer (a "Repurchase Offer") to repurchase the Option from
Grantee at a price (the "Option Repurchase Price") equal to the amount by which
(A) the Alternative Transaction Price (as defined below) exceeds (B) the Option
Price, multiplied by the number of shares for which the Option may then be
exercised, and (ii) deliver an offer (also, a "Repurchase Offer") to repurchase
the Option Shares from Grantee at a price (the "Option Share Repurchase Price")
equal to the Alternative Transaction Price multiplied by the number of Option
Shares then held by Grantee.  The term "Alternative Transaction Price" shall
mean, as of any date for the determination thereof, the price per share of
Common Stock paid pursuant to the Alternative Transaction or, in the event of
sale of assets of Issuer, the last per share sale price of Common Stock on the
fourth trading day following the announcement of such sale.  If the
consideration paid or received in the Alternative Transaction shall be other
than in cash, the value of such consideration shall be determined by a
nationally recognized investment banking firm selected by Grantee, which
determination shall be conclusive for all purposes of this Agreement.

          (b) Upon the occurrence of a Repurchase Event and whether or not
Issuer shall have made a Repurchase Offer under Section 7(a) at the request (the
date of such request being the "Option Repurchase Request Date") of Grantee
delivered prior to the Option Expiration Date, Issuer shall repurchase the
Option from Grantee at the Option Repurchase Price and (ii) at the request (the
date of such request being the "Option Share Repurchase Request Date") of
Grantee delivered prior to the Option Expiration Date, Issuer shall repurchase
such number of the Option Shares from Grantee as Grantee shall designate at the
Option Share Repurchase Price.

          (c) Grantee may accept Issuer's Repurchase Offer under Section 7(a) or
may exercise its right to require Issuer to repurchase the Option and/or any
Option Shares pursuant to 


                                       6
<PAGE>
 
Section 7(b) by a written notice or notices stating that Grantee elects to
accept such offer or to require Issuer to repurchase the Option and/or the
Option Shares in accordance with the provisions of this Section 7. As promptly
as practicable, and in any event within five business days, after the last to
occur of (i) the surrender to it of this Agreement and/or Certificates for
Option Shares, as applicable, (ii) receipt of a notice of election under this
Section 7(c) or (iii) the occurrence of a Repurchase Event, Issuer shall deliver
or cause to be delivered to Grantee the Option Repurchase Price and/or the
Option Share Repurchase Price and/or the portion thereof that Issuer is not then
prohibited from so delivering under applicable Law.

          (d) Issuer hereby undertakes to use its best efforts to obtain all
required regulatory and legal approvals and to file any required notices as
promptly as practicable in order to accomplish any repurchase contemplated by
this Section 7.  Nonetheless, to the extent that Issuer is prohibited under
applicable law, from repurchasing the Option and/or any Option Shares in full,
Issuer shall immediately so notify Grantee and thereafter deliver or cause to be
delivered, from time to time, to Grantee, the portion of the Option Repurchase
Price and the Option Share Repurchase Price, respectively, that it is no longer
prohibited from delivering, within five business days after the date on which
Issuer is no longer so prohibited; provided, however, that if Issuer at any time
after receipt of a notice of election to repurchase pursuant to Section 7(c) is
prohibited under applicable Law, from delivering to Grantee, the Option
Repurchase Price or the Option Share Repurchase Price, respectively, in full,
Grantee, may revoke its notice of election for repurchase of the Option or the
Option Shares either in whole or in part whereupon, in the case of a revocation
in part, Issuer shall promptly (i) deliver to Grantee, that portion of the
Option Repurchase Price or the Option Share Repurchase Price that Issuer is not
prohibited from delivering after taking into account any such revocation and
(ii) deliver to Grantee, as appropriate, either (a) a new agreement evidencing
the right of Grantee to purchase that number of shares of Common Stock equal to
the number of shares of Common Stock purchasable immediately prior to the
delivery of the notice of repurchase less the number of shares of Common Stock
covered by the portion of the Option repurchased or (b) a certificate for the
number of Option Shares covered by the revocation. If an Option Termination
Event shall have occurred prior to the date a notice described in the first
sentence of this subsection (d) is filed by Issuer, or shall be scheduled to
occur at any time before the expiration of a period ending on the thirtieth day
after such date, Grantee shall nonetheless have the right to exercise the Option
until the expiration of such 30-day period.

          (e) The term "Repurchase Event" shall mean a Triggering Event followed
by the consummation of any transaction included in the definition of Alternative
Transaction (as so defined in Section 7.2(b) of the Merger Agreement).

          (f) Notwithstanding anything to the contrary in Sections 2(a) and
2(e), if the Grantee delivers a notice under Section 7(b) specifying that (i)
such notice relates to an anticipated Repurchase Event under Section 7(e), and
is based on the Issuer's public announcement of the execution of an agreement
providing for an Alternative Transaction, such notice shall be deemed to
constitute an election to exercise the Option, as to the number of Option Shares
not theretofore purchased pursuant to one or more prior exercises of the Option,
on the fifth business day following the public announcement of the consummation
of the transaction contemplated by such agreement, in which event a Closing
shall occur with respect to 


                                       7
<PAGE>
 
such unpurchased Option Shares in accordance with Section 2(e) on such fifth
business day (or such later date as determined pursuant to the proviso in the
first sentence of Section 2(e)).

      8.  Representations and Warranties of the Issuer.  Issuer hereby
          --------------------------------------------                
represents and warrants to Grantee as follows:

          (a) Issuer has full corporate power and authority to execute and
deliver this Agreement and to consummate the transactions contemplated hereby.
The execution and delivery of this Agreement and the consummation of the
transactions contemplated hereby have been duly and validly authorized by the
Board of Directors of Issuer and no other corporate proceedings on the part of
Issuer are necessary to authorize this Agreement or to consummate the
transactions so contemplated.  This Agreement has been duly and validly executed
and delivered by Issuer.  This Agreement is the valid and legally binding
obligation of Issuer, enforceable against Issuer in accordance with its terms.

          (b) Issuer has taken all necessary corporate action to authorize and
reserve and to permit it to issue, and at all times from the date hereof through
the termination of this Agreement in accordance with its terms will have
reserved for issuance upon the exercise of the Option, that number of shares of
Common Stock equal to the maximum number of shares of Common Stock at any time
and from time to time issuable hereunder, and all such shares, upon issuance
pursuant hereto, will be duly authorized, validly issued, fully paid,
nonassessable, and will be delivered free and clear of all claims, liens,
encumbrance and security interests and not subject to any preemptive rights.

          (c) The execution and delivery of this Agreement does not, and the
consummation of the transactions contemplated hereby will not, conflict with, or
result in any violation pursuant to any provisions of the charter or by-laws of
Issuer or any Issuer subsidiary or subject to obtaining any approvals or
consents contemplated hereby, result in any violation of any loan or credit
agreement, note, mortgage, indenture, lease, plan or other agreement,
obligation, instrument, permit, concession, franchise, license, judgment, order,
decree, statute, law, ordinance, rule or regulation applicable to Issuer or any
Issuer subsidiary or their respective properties or assets which violation would
have, individually or in the aggregate, a material adverse effect (as defined in
the Merger Agreement).

          (d) The Issuer has taken, and will in the future take, all steps
necessary to irrevocably exempt the transactions contemplated by this Agreement
from any applicable state takeover law and from any applicable charter or
contractual provision containing change of control or anti-takeover provisions.

      9.  Assignment of Option by Grantee. Neither of the parties hereto may
          -------------------------------
assign any of its rights or obligations under this Option Agreement or the
Option created hereunder to any other person, without the express written
consent of the other party.

      10. Limitation of Grantee Profit.  (a)  Notwithstanding any other
          ----------------------------                                 
provision of this Agreement, in no event shall the Grantee's Total Profit (as
hereinafter defined) exceed $220,000,000.00 and, if it otherwise would exceed
such amount, the Grantee, at its sole election, 

                                       8
<PAGE>
 
shall either (i) reduce the number of shares of Common Stock subject to this
Option, (ii) deliver to the Issuer for cancellation Option Shares previously
purchased by Grantee (valued, for the purposes of this Section 10(a) at the
average closing sales price per share of Common Stock (or if there is no sale on
such date then the average between the closing bid and ask prices on any such
day) as reported by The Nasdaq National Market for the twenty consecutive
trading days preceding the day on which the Grantee's Total Profit exceeds
$220,000,000.00), (iii) pay cash to the Issuer, or (iv) any combination thereof,
so that Grantee's actually realized Total Profit shall not exceed
$220,000,000.00 after taking into account the foregoing actions.

          (b) As used herein, the term "Total Profit" shall mean the amount
(before taxes) of the following: (a) the aggregate amount of (i) (x) the net
cash amounts received by Grantee pursuant to the sale of Option Shares (or any
other securities into which such Option Shares are converted or exchanged) to
any unaffiliated party or to Issuer pursuant to this Agreement, less (y) the
Grantee's purchase price of such Option Shares, (ii) any amounts received by
Grantee on the transfer of the Option (or any portion thereof) to any
unaffiliated party, if permitted hereunder or to Issuer pursuant to this
Agreement, and (iii) the amount received by Grantee pursuant to Section 7.2 of
the Merger Agreement;  minus (b) the amount of cash theretofore paid to the
Issuer pursuant to this Section 10 plus the value of the Option Shares
theretofore delivered to the Issuer for cancellation pursuant to this Section
10.

          (c) Notwithstanding any other provision of this Agreement, nothing in
this Agreement shall affect the ability of Grantee to receive nor relieve
Issuer's obligation to pay a fee pursuant to Section 7.2 of the Merger
Agreement; provided that if Total Profit received by Grantee would exceed
$220,000,000.00 following the receipt of such fee, Grantee shall be obligated to
comply with the terms of Section 10(a) within 5 days of the later of (i) the
date of receipt of such fee and (ii) the date of receipt of the net cash by
Grantee pursuant to the sale of Option Shares (or, any other securities into
which such Option Shares are converted or exchanged) to any unaffiliated party
or to Issuer pursuant to this Agreement.

          (d) Notwithstanding any other provision of this Agreement, the Option
may not be exercised for a number of Option Shares that would, as of the Notice
Date, result in a Notional Total Profit  (as defined below) of more than
$220,000,000.00.  "Notional Total Profit" shall mean, with respect to any number
of Option Shares as to which the Grantee may propose to exercise the Option, the
Total Profit determined as of the Notice Date assuming that the Option was
exercised on such date for such number of Option Shares and assuming such Option
Shares, together with all other Option Shares held by the Grantee and its
affiliates as of such date, were sold for cash at the closing sales price for
Common Stock as of the close of business on the preceding trading day.

     11.  First Refusal.  At any time after the first occurrence of a
          -------------                                              
Triggering Event and prior to the later of (a) the expiration of 18 months
immediately following the first purchase of shares of Common Stock pursuant to
the Option and (b) the Option Termination Date, if Grantee shall desire to sell,
assign, transfer or otherwise dispose of all or any of the Option or the shares
of  Common Stock or other securities acquired by it pursuant to the Option, it
shall give 


                                       9
<PAGE>
 
Issuer written notice of the proposed transaction (an "Offeror's Notice"),
identifying the proposed transferee, accompanied by a copy of a binding offer to
purchase the Option or such shares or other securities signed by such transferee
and setting forth the terms of the proposed transaction. An Offeror's Notice
shall be deemed an offer by Grantee to Issuer, which may be accepted within 20
business days of the receipt by Issuer of such Offeror's Notice, on the same
terms and conditions and at the same price at which Grantee is proposing to
transfer the Option or such shares or other securities to such transferee. The
purchase of the Option or any such shares or other securities by Issuer shall be
settled within 10 business days of the date of the acceptance of the offer and
the purchase price shall be paid to Grantee in immediately available funds;
provided that, if prior notification to or approval of any regulatory authority
is required in connection with such purchase, Issuer shall promptly file the
required notice or application for approval and shall expeditiously process the
same (and Grantee shall cooperate with Issuer in the filing of any such notice
or application and the obtaining of any such approval) and the period of time
that otherwise would run pursuant to this sentence shall run instead from the
date on which, as the case may be, (a) the required notification period has
expired or been terminated or (b) such approval has been obtained and, in either
event, any requisite waiting period shall have passed. In the event of the
failure or refusal of Issuer to purchase all of the Option or all of the shares
or other securities covered by an Offeror's Notice or if any regulatory
authority disapproves Issuer's proposed purchase of any portion of the Option or
such shares or other securities, Grantee may, within 60 days from the date of
the Offeror's Notice (subject to any necessary extension for regulatory
notification, approval or waiting periods), sell all, but not less than all, of
such portion of the Option or such shares or other securities to, the proposed
transferee at no less than the price specified and on terms no more favorable
than those set forth in the Offeror's Notice. The requirements of this Section
11 shall not apply to (w) any disposition as a result of which the proposed
transferee would own beneficially not more than 2% of the outstanding voting
power of Issuer, (x) any disposition of Common Stock or other securities by a
person to whom Grantee has assigned its rights under the Option with the consent
of Issuer, (y) any sale by means of a public offering registered under the
Securities Act in which steps are taken to reasonably assure that no purchaser
will acquire securities representing more than 2% of the outstanding voting
power of Issuer or (z) any transfer to a wholly owned subsidiary of Grantee
which agrees in writing to be bound by the terms hereof.

          12.  Voting.  For a period of 18 months from the date of exercise of
               ------                                                         
the Option, so long as Grantee beneficially owns any Option Shares, Grantee
agrees to (a) be present, in person or represented by proxy, at all stockholder
meetings of Issuer, so that all Option Shares beneficially owned by Grantee may
be counted for the purpose of determining the presence of a quorum at such
meetings, and (b) vote or cause to be voted all Option Shares beneficially owned
by it, with respect to all matters submitted to shareholders for a vote, in the
same proportion as shares of Common Stock are voted by shareholders unaffiliated
with Grantee.

          13.  Application for Regulatory Approval.  Each of Grantee and Issuer
               -----------------------------------                             
will use its best efforts to make all filings with, and to obtain consents of,
all third parties and governmental authorities necessary to the consummation of
the transactions contemplated by this Agreement, including without limitation
making application to list the shares of Common Stock issuable hereunder on The
Nasdaq National Market upon official notice of issuance.


                                      10
<PAGE>
 
          14.  Specific Performance.  The parties hereto acknowledge that
               --------------------                                      
damages would be an inadequate remedy for a breach of this Agreement by either
party hereto and that the obligations of the parties hereto shall be enforceable
by either party hereto through injunctive or other equitable relief.

          15.  Separability of Provisions.  If any term, provision, covenant or
               --------------------------                                      
restriction contained in this Agreement is held by a court or a federal or state
regulatory agency of competent jurisdiction to be invalid, void or
unenforceable, the remainder of the terms, provisions and covenants and
restrictions contained in this Agreement shall remain in full force and effect,
and shall in no way be affected, impaired or invalidated.

          16.  Notices.  All notices, claims, demands and other communications
               -------                                                        
hereunder shall be deemed to have been duly given or made when delivered in
person, by overnight courier or by facsimile at the respective addresses of the
parties set forth in the Merger Agreement.

          17.  Governing Law.  This Agreement shall be governed by and construed
               -------------                                                    
in accordance with the laws of the State of Delaware, regardless of the laws
that might otherwise govern under applicable principles of conflicts of laws
thereof.

          18.  Counterparts.  This Agreement may be executed in two or more
               ------------                                                
counterparts, each of which will be deemed to be an original, but all of which
shall constitute one and the same agreement.

          19.  Expenses.  Except as otherwise expressly provided herein or in
               --------                                                      
the Merger Agreement, each of the parties hereto shall bear and 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.

          20.  Entire Agreement.  Except as otherwise expressly provided herein
               ----------------                                                
or in the Merger Agreement, this Agreement contains the entire agreement between
the parties with respect to the transactions contemplated hereunder and
supersedes all prior arrangements or understandings with respect thereof,
written or oral.  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 permitted assigns.  Nothing in this Agreement, expressed or
implied, is intended to confer upon any party, other than the parties hereto,
and their respective successors except as assigns, any rights, remedies,
obligations or liabilities under or by reason of this Agreement, except as
expressly provided herein.  Any provision of this Agreement may be waived only
in writing 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.

          21.  Further Assurances.  In the event of any exercise of the Option
               ------------------                                             
by Grantee, Issuer and Grantee shall execute and deliver all other documents and
instruments and take all other action that may be reasonably necessary in order
to consummate the transactions provided for by such exercise.  Nothing contained
in this Agreement shall be deemed to authorize Issuer or Grantee to breach any
provision of the Merger Agreement.


                                      11
<PAGE>
 
          IN WITNESS WHEREOF, Issuer and Grantee have caused this Agreement to
be signed by their respective officers thereunto duly authorized, all as of the
date first written above.

                                    MCKESSON CORPORATION


                                    By: /s/ Mark A. Pulido
                                        --------------------------------------
                                         Mark A. Pulido,    
                                         President and Chief Executive Officer
                                         

                                    HBO & COMPANY


                                    By: /s/ Jay P. Gilbertson 
                                        --------------------------------------
                                         Jay P. Gilbertson,  
                                         President, Co-Chief Operating Officer 
                                         and Chief Financial Officer        


                                      12


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