
<PAGE>   1
 
                         NOTICE OF GUARANTEED DELIVERY
                                      for
                        TENDER OF SHARES OF COMMON STOCK
                                       of
 
                         AMERICAN MEDSERVE CORPORATION
 
    THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK
   CITY TIME, ON THURSDAY, SEPTEMBER 11, 1997, UNLESS THE OFFER IS EXTENDED.
 
     As set forth in Section 3 of the Offer to Purchase described below, this
instrument or one substantially equivalent hereto must be used to accept the
Offer (as defined below) if (i) certificates evidencing shares of Common Stock,
par value $0.01 per share (the "Shares"), of American Medserve Corporation, a
Delaware corporation, are not immediately available, (ii) the certificates
evidencing Shares and all other required documents cannot be delivered to the
Depositary prior to the Expiration Date (as defined in Section 1 of the Offer to
Purchase), or (iii) the procedure for delivery by book-entry transfer for the
Shares cannot be completed on a timely basis. This instrument may be transmitted
by facsimile transmission or delivered by hand or mail to the Depositary.
 
                        The Depositary for the Offer is:
 
                    FIRST CHICAGO TRUST COMPANY OF NEW YORK
 
<TABLE>
<S>                                    <C>                                  <C>
             By Mail:                              By Hand:                    By Overnight Delivery:
    First Chicago Trust Company           First Chicago Trust Company       First Chicago Trust Company
            of New York                           of New York                       of New York
        Tenders & Exchanges                   Tenders & Exchanges               Tenders & Exchanges
            Suite 4660                 c/o The Depository Trust Company            Suite 4680-AMC
           P.O. Box 2569                   55 Water Street, DTC TAD          14 Wall Street, 8th Floor
Jersey City, New Jersey 07303-2569      Vietnam Veterans Memorial Plaza       New York, New York 10005
                                           New York, New York 10041
</TABLE>
 
                           By Facsimile Transmission
                       (For Eligible Institutions Only):
 
                                 (201) 222-4720
                                       or
                                 (201) 222-4721
                             To Confirm Receipt of
                         Notice of Guaranteed Delivery:
 
                                 (201) 222-4707
 
DELIVERY OF THIS INSTRUMENT TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE OR
TRANSMISSION OF INSTRUCTIONS VIA FACSIMILE TRANSMISSION OTHER THAN AS SET FORTH
ABOVE WILL NOT CONSTITUTE A VALID DELIVERY TO THE DEPOSITARY.
 
     This Notice of Guaranteed Delivery is not to be used to guarantee
signatures. If a signature on a Letter of Transmittal is required to be
guaranteed by an Eligible Institution under the instructions thereto, the
signature guarantee must appear in the applicable space provided in the
signature box in the Letter of Transmittal.
 
     The Eligible Institution that completes this form must communicate the
guarantee to the Depositary and must deliver the Letter of Transmittal and
certificates for Shares to the Depositary within the time period shown herein.
Failure to do so could result in a financial loss to the Eligible Institution.
 
              THE GUARANTEE ON THE REVERSE SIDE MUST BE COMPLETED.
<PAGE>   2
 
Ladies and Gentlemen:
 
    The undersigned hereby tender(s) to Omnicare Acquisition Corp., a Delaware
corporation and a wholly owned subsidiary of Omnicare, Inc., a Delaware
corporation, upon the terms and subject to the conditions set forth in the Offer
to Purchase dated August 14, 1997 (the "Offer to Purchase") and in the related
Letter of Transmittal (which, as amended or supplemented from time to time,
together constitute the "Offer"), receipt of which is hereby acknowledged, the
number of Shares indicated below of American Medserve Corporation, a Delaware
corporation, pursuant to the guaranteed delivery procedures set forth in Section
3 of the Offer to Purchase.

Signature(s)________________________   Address(es)_________________________
____________________________________   ____________________________________

Name(s) of Record Holders___________   Area Code and Tel. No.(s)___________
____________________________________       
   PLEASE TYPE OR PRINT
____________________________________   Check one box if Shares will be
                                       tendered by book-entry transfer  
Number of Shares____________________   / /The Depository Trust Company

Certificate No.(s) (If Available)___   / /Philadelphia Depository Trust 
____________________________________   Company
Dated_______________________  , 1997   Account Number____________________
 
                THE GUARANTEE SET FORTH BELOW MUST BE COMPLETED
                                   GUARANTEE
                    (NOT TO BE USED FOR SIGNATURE GUARANTEE)
 
    The undersigned, a member of a registered national securities exchange or of
the National Association of Securities Dealers, Inc., a commercial bank or trust
company having an office or correspondent in the United States that is a member
in good standing of the Securities Transfer Agents Medallion Program, the New
York Stock Exchange Medallion Signature Guarantee Program or the Stock Exchange
Medallion Program, or an "eligible guarantor institution," as such term is
defined in Rule 17Ad-15 under the Securities Exchange Act of 1934, as amended,
(a) represents that the above named person(s) "own(s)" the Shares tendered
hereby within the meaning of Rule 14e-4 under the Securities Exchange Act of
1934, as amended ("Rule 14e-4"), (b) represents that the tender of those Shares
complies with Rule 14e-4, and (c) guarantees to deliver to the Depositary either
the certificates evidencing all tendered Shares, in proper form for transfer, or
confirmation of book-entry transfer of such shares into the Depositary's account
at The Depository Trust Company or the Philadelphia Depository Trust Company
(each a "Book-Entry Transfer Facility"), in either case together with the Letter
of Transmittal (or a facsimile thereof), properly completed and duly executed
with any required signature guarantees, or an Agent's Message (as defined in
Section 3 of the Offer to Purchase) in the case of a book-entry delivery, and
any other required documents, all within three Nasdaq National Market trading
days after the date hereof.

___________________________________   ____________________________________
            NAME OF FIRM                       AUTHORIZED SIGNATURE   
___________________________________   Name________________________________
              ADDRESS
__________________________ZIP CODE    Title_______________________________

Area Code and Tel. No.____________    Date__________________________,1997
 
NOTE: DO NOT SEND CERTIFICATES EVIDENCING SHARES WITH THIS NOTICE. CERTIFICATES
      SHOULD BE SENT WITH YOUR LETTER OF TRANSMITTAL.
                                          
