<PAGE>   1
                                            The deleted portions of this Exhibit
                                                contain confidential information
                                         and have been filed separately with the
                                              Securities and Exchange Commission

                                                                    Exhibit 10.4
AMGEN(R)
                                    PRODUCT PURCHASE AGREEMENT
                                    NATIONAL MEDICAL CARE, INC.
--------------------------------------------------------------------------------
This Agreement ("Agreement"), between Amgen Inc. ("Amgen") and National Medical
Care, Inc., including its subsidiaries and affiliates that are at least fifty
and one-tenth percent (50.10%) owned by National Medical CARE, Inc. listed on
Appendix C, (collectively, "NMC"), sets forth the terms and conditions for the
purchase of EPOGEN(R) (Epoetin alfa) by NMC.

1. TERM OF AGREEMENT. The "Term" of this Agreement shall be defined as
January 1, 1999 ("Commencement Date") through December 31, 1999 ("Termination 
Date").

2. QUALIFYING PURCHASES. All terms contained herein apply only to purchases made
hereunder, at the prices set forth herein, as confirmed by Amgen ("Qualified
Purchases"), by NMC and, subject to the terms of Section 13, to all affiliates
opened, acquired, or managed by NMC during the Term, for so long as such
affiliates remain at least fifty and one-tenth percent (50.10%) owned or managed
by National Medical Care, Inc. ("Affiliates"), through wholesalers authorized by
Amgen to participate in the program ("Authorized Wholesalers"). In addition, and
also subject to the terms of Section 13, Affiliates of Renaissance Health Care,
Clinic, Inc., Optimal Renal Care, L.L.C., and/or any joint venture of NMC in
which NMC holds at least a fifty and one-tenth percent (50.10%) ownership
interest, will also be eligible to participate. Amgen agrees to approve
Authorized Wholesalers adequate to meet NMC's needs.

3.  PRICING.  See Appendix A.

4. PAYMENT TERMS. During the Term, Amgen grants NMC and Affiliates payment terms
of **DELETED** for EPOGEN(R) purchased through any Authorized Wholesaler.

5. PRODUCT ACQUISITION COSTS. As long as NMC and Affiliates are the **DELETED**,
non-governmental, freestanding dialysis center (including home dialysis
affiliates) purchaser of EPOGEN(R) in the United States and Puerto Rico during
the Term, on an annual, calendar year basis, Amgen commits that this Agreement
provides NMC and Affiliates with the **DELETED** for EPOGEN(R) available to any
freestanding dialysis center purchaser in the United States and Puerto Rico with
comparable growth and percentage of patients with hematocrit levels greater than
or equal to **DELETED**. If NMC and Affiliates are not the **DELETED**,
non-governmental, freestanding dialysis center purchaser of EPOGEN(R) in the
United States and Puerto Rico during the Term, on a calendar year basis, Amgen
may provide **DELETED** for EPOGEN(R) to **DELETED**, non-governmental,
freestanding dialysis center purchaser(s). Qualification as a freestanding
dialysis center shall be determined by Amgen in its reasonable discretion, in
accordance with Amgen's customer classification policies, which generally
classify freestanding dialysis centers as independent, exclusive providers of
dialysis services, which (a) may not obtain EPOGEN(R) from or through a
hospital, or (b) are not otherwise affiliated with a hospital, nursing home, or
integrated health care system. This commitment excludes **DELETED** available to
any governmental entities, or prices/costs mandated by Title 38 (Veterans'
Benefits) or Title 42 (The Public Health and Welfare) of the United States Code,
or any other federal or state health care program. Amgen's agreement to make
this commitment is contingent upon its ability to comply with all federal,
state, local and military laws, statutes and regulations.

6. MINIMUM PRODUCT PURCHASE AGREEMENT. NMC and Affiliates agrees to purchase a
minimum of **DELETED** of EPOGEN(R) from Amgen hereunder during each consecutive
calendar quarter during the Term.

7. DISCOUNT. Amgen will pay discounts and incentives in accordance with the
schedule and terms set forth in Appendix A and Appendix B attached hereto.

8. PAYMENT OF DISCOUNT. Any discount earned hereunder shall be calculated in
accordance with this Agreement, based on Qualified Purchases, using **DELETED**
as the calculation price, and shall be paid in the form of 

                                       1
<PAGE>   2


a wire transfer to NMC's corporate headquarters, except as otherwise provided.
Amgen will base its calculations of any discounts on **DELETED**, adjusted
for any differences and subject to Amgen's verification of all figures submitted
by NMC and Affiliates. NMC and Affiliates shall make available to Amgen any
records concerning NMC's and Affiliates' purchase amounts that Amgen or its
auditors may reasonably request. Amgen shall make available to NMC any records
concerning NMC's and Affiliates' purchase amounts that NMC or its auditors may
reasonably request. Upon vesting, Amgen will use its best efforts to make such
discount available within **DELETED** business days following receipt by Amgen
of data, in a format indicated on Appendix B, detailing NMC's and Affiliates'
EPOGEN(R) purchases for the relevant period, along with any other data required
by the terms of Appendix A, contingent upon Amgen's verification and
reconciliation of such data as set forth in Appendix B attached hereto. In no
event shall Amgen pay any discount on EPOGEN(R) distributed by NMC or Affiliates
to non-Affiliates of NMC (see Section 2 for definition of Affiliates). Subject
to the section entitled "Breach of Agreement", in the event that Amgen is
notified in writing that National Medical Care, Inc. and/or any of its
subsidiaries or Affiliates (the "Acquiree") is acquired by another entity or a
change of control otherwise occurs with respect to the Acquiree, any discount
which may have been earned hereunder prior to the effective date of the
acquisition shall vest, and shall be paid in the form of a wire transfer to
NMC's corporate headquarters subject to the conditions described herein.

9. TREATMENT OF DISCOUNT. The parties agree that they will account for any
discount earned hereunder in a way that complies with all applicable federal,
state, and local laws and regulations, including without limitation, Section
1128B(b) of the Social Security Act and its implementing regulations, and if
required by such statutes or regulations (a) claim the benefit of such discount
received, in whatever form, in the fiscal year in which such discount was earned
or the year after, (b) fully and accurately report the value of such discount in
any cost reports filed under Title XVIII or Title XIX of the Social Security
Act, or a state health care program, and c) provide, upon request by the U.S.
Department of Health and Human Services or a state agency or any other federally
funded state health care program, the information furnished by Amgen concerning
the amount or value of such discount. NMC agrees that it will advise all
Affiliates, in writing, of any discount received by NMC's corporate headquarters
hereunder with respect to purchases made by such Affiliates and that NMC will
advise said Affiliates as to their requirement to account for any such discount
in accordance with the above stated requirements.

10. COMMITMENT TO PURCHASE. NMC and Affiliates agrees to purchase EPOGEN(R) for
all of its dialysis use requirements in the United States and Puerto Rico for
recombinant human erythropoietin. Amgen agrees to make such EPOGEN(R) available
to NMC and Affiliates through its Authorized Wholesalers. In addition to other
remedies available to NMC and Affiliates, NMC and Affiliates may purchase
another brand of recombinant human erythropoietin for its dialysis use
requirements in the United States and Puerto Rico if, and only if, NMC and
Affiliates have informed Amgen, in writing, that NMC and Affiliates are unable
to acquire sufficient amounts of EPOGEN(R) to meet NMC's and Affiliates'
reasonable dialysis use requirements, and Amgen by itself, or through its
Authorized Wholesalers, is actually unable to supply NMC and Affiliates with
their reasonable dialysis use requirements of EPOGEN(R) within the time period
reasonably required by NMC and Affiliates, which, in no event shall be less than
five (5) business days after Amgen's receipt of NMC's and Affiliates' written
notice. If the preceding requirements are met, NMC and Affiliates shall only be
allowed to purchase another brand of recombinant human erythropoietin for the
time period, and to the extent, that Amgen is unable to provide NMC and
Affiliates with EPOGEN(R) to meet NMC's and Affiliates' reasonable dialysis use
requirements.

11. OWN USE. NMC hereby certifies that EPOGEN(R) purchased hereunder shall be
for the "own use" by NMC and the Affiliates of NMC.

12. AUTHORIZED WHOLESALERS. A complete list of NMC's and Affiliates' current
Authorized Wholesalers, through whom NMC and Affiliates may purchase EPOGEN(R)
hereunder is attached as Appendix D. NMC and Affiliates agrees to promptly
provide Amgen with any additions, deletions, or changes to the initial list of
Authorized Wholesalers. Amgen requires no less than thirty (30) days notice
before the effective date of change for any addition 

                                       2

<PAGE>   3

or deletion of Authorized Wholesalers hereunder. Any proposed changes to the
initial list of Authorized Wholesalers must be in writing and are subject to
reasonable approval by Amgen.

13. SUBSIDIARIES AND AFFILIATES. Within thirty (30) days of execution of this
Agreement, NMC shall provide a current listing of all Affiliates, and other
entities, that will be participating in this Agreement, designating which
Affiliates are owned and/or managed by NMC. Such listing shall be incorporated
into this Agreement as Appendix C. Only those entities listed on Appendix C
shall be eligible to participate in this Agreement. Any NMC managed Affiliate,
or other entity with an existing contract, may participate in either their
existing agreement with Amgen, or this Agreement, but not both. Each managed
Affiliate or entity must declare under which single Amgen contract it will
participate. Only Qualified Purchases under this Agreement will be used in the
calculation of pricing, discounts or other incentives under this Agreement. NMC
will notify Amgen of changes to Appendix C, and the effective date of change.
Such effective date of change may not be earlier than the date the notice is
received by Amgen. Any proposed change to Appendix C shall be subject to the
reasonable approval of Amgen based upon Amgen's then current legal and
contractual requirements, and such proposed affiliate's classification as a
freestanding dialysis center or a home dialysis support facility.

14. BREACH OF AGREEMENT. Either party may terminate this Agreement for a
material breach upon thirty (30) days advance written notice provided such
breach remains uncured at the end of the thirty (30) day period.

15. CONFIDENTIALITY. Both Amgen and NMC agree that this Agreement represents and
contains confidential information which shall not be disclosed to any third
party, or otherwise made public, without prior written authorization of the
other party, except where such disclosure is contemplated hereunder or required
by law or court order.

16. WARRANTIES. Each party represents and warrants to the other that this
Agreement (a) has been duly authorized, executed, and delivered by it, (b)
constitutes a valid, legal, and binding agreement enforceable against it in
accordance with the terms contained herein, and (c) does not conflict with or
violate any of its other contractual obligations, expressed or implied, to which
it is a party or by which it may be bound. NMC represents and warrants that it
has the power to bind National Medical Care, Inc. and the subsidiaries and owned
Affiliates listed on Appendix C to the terms contained herein. NMC shall cause
each managed Affiliate to be bound by the terms and conditions of this Agreement
through the execution of a joinder agreement executed between NMC and each such
managed Affiliate.

17. GOVERNING LAW. This Agreement shall be governed by the laws of the State of
Delaware and the parties submit to the jurisdiction of Delaware courts, both
state and federal.

18. NOTICES. Any notice or other communication required or permitted hereunder
shall be in writing and shall be deemed given or made when delivered in person
or when received by the other party sent by U.S. Mail, return receipt requested,
at the respective party's address set forth below or at such other address as
the party shall have furnished to the other in accordance with this provision.

19. COMPLIANCE WITH HEALTH CARE PRICING LEGISLATION AND STATUTES.
Notwithstanding anything contained herein to the contrary, at any time following
the enactment of any federal, state, or local law or regulation that materially
reforms, modifies, alters, restricts, or otherwise affects the pricing of or
reimbursement available for EPOGEN(R), either party may initiate good faith
negotiations to modify this Agreement. If after forty-five (45) days the parties
are unable to in good faith mutually agree to modifications to this Agreement,
(a) either party may terminate this Agreement immediately, or (b) Amgen may
exclude any owned or managed Affiliates from participating in this Agreement
unless such owned or managed Affiliate(s) certifies in writing that they are, or
will be, exempt from the provisions of such enacted law or regulation.
Additionally, in order to assure compliance with any existing federal, state or
local statute, regulation or ordinance, Amgen reserves the right, in its
reasonable discretion, to exclude any owned or managed Affiliates from the
pricing, discount, and incentive provisions of this Agreement. In the event

                                       3
<PAGE>   4
                                

there is a future change in Medicare, Medicaid, or other federal or state
statutes or regulations or in the interpretation thereof, which renders any of
the material terms of this Agreement unlawful or unenforceable, this Agreement
shall continue and shall be amended by the parties as a result of good faith
negotiations as necessary to bring the Agreement into compliance with such
statute and regulation.

20. MISCELLANEOUS. No modification of this Agreement shall be effective unless
made in writing and signed by a duly authorized representative of each party.
This Agreement constitutes the entire agreement of the parties pertaining to the
subject matter hereof and supersedes all prior written and oral agreements and
understandings pertaining hereto including without limitation, the Terminated
Agreement. Neither party shall have the right to assign this Agreement to a
third party without the prior written consent of the other party. Neither party
shall be liable for delays in performance and nonperformance of this Agreement
or any covenant contained herein caused by fire, flood, storm, earthquake or
other act of God, war, rebellion, riot, failure of carriers to furnish
transportation, strike, lockout or other labor disturbances, act of government
authority, inability to obtain material or equipment, or any other cause of like
or different nature beyond the control of such party. However, during any time
of nonperformance by Amgen which involves NMC's and Affiliates' inability to
obtain sufficient EPOGEN(R) to meet NMC's and Affiliates' reasonable dialysis
use requirements, **DELETED** for such nonperformance **DELETED** and NMC and
Affiliates may purchase EPOGEN(R) from another supplier. The parties shall
execute and deliver all documents, provide all information, and take or refrain
from taking action as may be necessary or appropriate to achieve the purposes of
this Agreement. This Agreement may be executed in one or more counterparts, each
of which is deemed to be an original but all of which taken together constitutes
one and the same agreement. Amgen reserves the right to rescind this offer if
the parties fail to execute this Agreement within thirty (30) days from the date
of its offering.

PLEASE RETAIN ONE FULLY EXECUTED ORIGINAL FOR YOUR RECORDS AND RETURN THE OTHER
FULLY EXECUTED ORIGINAL TO AMGEN.

The parties executed this Agreement as of the dates set forth below.


                                       4
<PAGE>   5

                                

AMGEN INC.
a Delaware corporation
One Amgen Center Drive
Thousand Oaks, CA  91320-1799

Signature:        /s/ D.Z. Mallon
                  Dorothea Z. Mallon
                  National Account Manager
Date:             3/11/99


NATIONAL MEDICAL CARE, INC.
a Delaware corporation
Two Ledgemont Center-95 Hayden Avenue
Lexington, MA  02173

Signature:        /s/ James V. Luther
                  James V. Luther
                  Assistant Treasurer
Date:             3/11/99








                                       5

<PAGE>   6

                                


                APPENDIX A: DISCOUNT PRICING, SCHEDULE, AND TERMS

1. PRICING. NMC and Affiliates may purchase EPOGEN(R) through Authorized
Wholesalers at a guaranteed **DELETED**. Resulting prices do not include any
wholesaler markup, service fees, or other charges. The following table is
illustrative of prices in effect at the time this Agreement is presented.

<TABLE>
<CAPTION>                                                                   GUARANTEED
                                                           CURRENT          DISCOUNT % Off
EPOGEN(R) ITEM NUMBER                                      **DELETED**      PREVAILING       RESULTING
(NDC)                     DESCRIPTION                      (PACK)           **DELETED**      PRICE (PACK)
                                                           A                B                A - (A x B)

<S>     <C>               <C>                              <C>              <C>              <C>            <C>
S2      55513-126-10      2,000 Unit Single Dose vial      **DELETED**      **DELETED**      **DELETED**
                  2,000 Units/mL,1 mL vial, 10 vials/pack, 10 packs/case

S3      55513-267-10      3,000 Unit Single Dose vial      **DELETED**      **DELETED**      **DELETED**
                  3,000 Units/mL,1 mL vial,10 vials/pack, 10 packs/case

S4      55513-148-10      4,000 Unit Single Dose vial      **DELETED**      **DELETED**      **DELETED**
                  4,000 Units/mL,1 mL vial,10 vials/pack, 10 packs/case

S10     55513-144-10      10,000 Unit Single Dose vial     **DELETED**      **DELETED**      **DELETED**
                  10,000 Units/mL,1 mL vial,10 vials/pack, 10 packs/case

M10     55513-283-10      20,000 Unit Multidose vial       **DELETED**      **DELETED**      **DELETED**    
                  10,000 Units/mL, 2 mL vial,10 vials/pack, 4 packs/case 
                  
M20     55513-478-10      20,000 Unit Multidose vial       **DELETED**      **DELETED**      **DELETED**
                  20,000 Units/mL,1 mL vial,10 vials/pack, 4 packs/case
</TABLE>

2. **DELETED**. NMC and Affiliates agrees to provide **DELETED** for the
purposes of calculating the **DELETED**. Amgen will accept the **DELETED**,
multiply each **DELETED** by **DELETED**, and apply the converted results to
**DELETED**. Notwithstanding the foregoing, **DELETED**. Amgen will accept
electronic data from the lab systems that service each owned or managed
Affiliate. For NMC to qualify for the **DELETED** the following requirements
must be met:

A. REQUIREMENTS: In order to participate in the **DELETED**, NMC and Affiliates
must provide the following information for each dialysis patient to Amgen or to
a data collection vendor specified by Amgen, on a **DELETED** basis, no later
than **DELETED** days after the end of **DELETED**:


                                       6
<PAGE>   7
                                

i) all **DELETED** for each dialysis patient, the date of each test, and
a consistent, unique, alpha-numeric identifier (sufficient to consistently track
an individual patient without in any way disclosing the identity of the
patient), along with the name, address and phone number of the particular owned
or managed Affiliate at which each patient received treatment (collectively the
"Data"). Amgen may utilize the Data for any purpose, and reserves the right to
audit all Data. Under no circumstances should the Data include any patient
identifiable information including, without limitation, name, complete social
security number, address or birth date. The identity of the account submitting
the Data and any association with the Data will remain confidential. The
**DELETED** results must be derived from **DELETED** taken immediately before
dialysis treatment using any automated **DELETED** testing method **DELETED**,
must be reported to the **DELETED**, and must be submitted in a format
acceptable to Amgen. Hand written reports are not acceptable; electronic
submission of the Data is preferred, and

ii) a properly executed "Certification Letter", a sample of which is attached
hereto as Exhibit #1, that will be provided to NMC's corporate headquarters,
unless otherwise requested, before each payment period.

b. CALCULATION: Assuming NMC and Affiliates have fulfilled all requirements as
described in Section 2(a) above, NMC's **DELETED** payment will be calculated as
follows:

The "Average Patient **DELETED**" for each dialysis patient will be based upon
the average of all **DELETED** test results gathered for each patient during
each **DELETED** of the Term. The **DELETED** of all dialysis patients with
Average Patient **DELETED** greater than or equal to **DELETED**, will be
determined by dividing the total number of dialysis patients with Average
Patient **DELETED** greater than or equal to **DELETED** by the total number of
dialysis patients treated by NMC and Affiliates. The **DELETED** will be
calculated based on NMC's and Affiliates' overall performance in accordance with
Amgen's discount calculation policies. **DELETED**.

c. PAYMENT: NMC's **DELETED** will be calculated on a **DELETED** basis and paid
to NMC's corporate headquarters, except as otherwise provided hereunder. Payment
is contingent upon receipt by Amgen of the Certification Letters and all
required Data for the corresponding **DELETED**. Data shall be submitted to
Amgen **DELETED**. If Data is received **DELETED**, NMC will not qualify for the
**DELETED** for that **DELETED**. **DELETED** payments will be based upon the
Data received for the **DELETED**, and will equal a percentage of NMC's and
Affiliates' total EPOGEN(R) purchases during that **DELETED** as governed by the
**DELETED** schedule listed below. Notwithstanding the foregoing, payment for
any period during the Term that is not equivalent to a complete **DELETED**,
will be based on an average of the Data that is available for that period. Amgen
reserves the right to modify the **DELETED** if the EPOGEN(R) package insert
language changes.

<TABLE>
<CAPTION>
**DELETED** OF ALL DIALYSIS PATIENTS                                    **DELETED**
WITH AVERAGE PATIENT **DELETED** GREATER THAN OR EQUAL TO **DELETED**   **DELETED** PERCENTAGE
PLEASE DIRECT YOUR ATTENTION TO THE EPOGEN(R) PACKAGE INSERT            
-----------------------------------------------------------             ----------------------
         <S>            <C>   <C>                                       <C>
         **DELETED**    -     Over                                      **DELETED**                
         **DELETED**    -     **DELETED**                               **DELETED**                
         **DELETED**    -     **DELETED**                               **DELETED**                
         **DELETED**    -     **DELETED**                               **DELETED**                
         **DELETED**    -     **DELETED**                               **DELETED**     
         **DELETED**    -     **DELETED**                               **DELETED**
</TABLE>

d. VESTING: The **DELETED** will vest on the **DELETED** of the corresponding
   **DELETED**.

3. **DELETED**. NMC may qualify for the **DELETED** as described below.

a. CALCULATION: NMC's **DELETED** will be calculated in accordance with the
   following formula.

                                       7
<PAGE>   8
                                 

                                  **DELETED**
                  where

         A =    **DELETED** during the Term by NMC and all Affiliates. 
         B =    A percent in accordance with the table listed below.
         C =    **DELETED** during the Term by NMC and all Affiliates as listed 
                on Appendix C on the Commencement Date of this Agreement.
         D =    **DELETED** by NMC and those same Affiliates as listed on 
                Appendix C on the Commencement Date of this Agreement for the 
                same time period during the previous year.
<TABLE>
<S>                                 <C>
         **DELETED**
         **DELETED**                **DELETED**
         --------------------------------------
         **DELETED**  -Over         **DELETED**
         **DELETED**                **DELETED**
         **DELETED**                **DELETED**
         **DELETED**                **DELETED**
         **DELETED**                **DELETED**
         **DELETED**                **DELETED**
</TABLE>

For the purposes of calculating **DELETED** increases, Amgen will incorporate
purchases of **DELETED**.

b. VESTING: NMC's **DELETED** shall vest **DELETED** based upon **DELETED** and
shall be paid in accordance with the time frames set forth in Appendix B.

4. **DELETED**.






                                       8
<PAGE>   9
                                 


                APPENDIX B: PURCHASE DATA VERIFICATION PROCEDURE

1. VERIFICATION OF PURCHASE DATA. NMC and Amgen agree to follow the verification
and reconciliation procedure as set forth below so that the parties can use
accurate EPOGEN(R) purchase data and can meet the payment obligations defined
herein.

          (a) Each organization will designate a data analyst/coordinator as
          well as a backup coordinator.

          (b) Each **DELETED** during the calendar year, **DELETED** diskettes
          of year-to-date EPOGEN(R) purchase data to **DELETED** via express
          mail. The diskettes for the periods listed below should be received by
          **DELETED** as follows: 
<TABLE>
<CAPTION>
         Data Period                                 Due Date 
         -----------                                 --------

         <S>                                         <C>
         January 1 - March 31                        April 30
         January 1 - June 30                         July 31
         January 1 - September 30                    October 31
         January 1 - November 30                     December 11
         January 1 - December 31 (Verified)          January 17
</TABLE>

          (c) Once Amgen receives the data diskette as described above, the
          following verification and reconciliation schedule will be implemented
          by both organizations:

                  Day 1:   **DELETED**.
                  Day 3:   **DELETED**.
                  Day 5:   **DELETED**.
                  Day 6:   **DELETED**.
                  Day 8:   **DELETED**. 
                  Day 9:   **DELETED**.
                  Day 10:  **DELETED**.

          (d) **DELETED**.

                                       9
<PAGE>   10
                                 

               APPENDIX C: LIST OF NMC SUBSIDIARIES AND AFFILIATES


                                  SUBSIDIARIES:
                                  -------------

       Bio-Medical Applications Management Co., Inc. and its subsidiaries

                                   Erika, Inc.

                               Infusion Care, Inc.

                                 LifeChem, Inc.

                  National Medical Care HomeCare Division, Inc.

                         Renal Research Institute, Inc.

                            Spectra Renal Management

                                   AFFILIATES:
                                   -----------

                           See Contract List Attached












                                       10
<PAGE>   11

                                

TO ENSURE YOU RECEIVE THE APPROPRIATE DISCOUNT, IT IS IMPORTANT THAT WE HAVE
YOUR CURRENT LIST OF AUTHORIZED WHOLESALERS. THE FOLLOWING LIST REPRESENTS THE
WHOLESALERS AMGEN CURRENTLY HAS ASSOCIATED WITH YOUR CONTRACT. PLEASE UPDATE THE
LIST BY ADDING OR DELETING WHOLESALERS AS NECESSARY.

Bergen Brunswig Corporation
4000 Metropolitan Drive
Orange, CA 92668

J.M. Blanco Inc.
Calle D - Lote No. 21
Guaynabo, PR 00965

Metro Medical Supply, Inc.
3332 Powell Avenue
Nashville, TN 37204

Bellco Drug Corporation
101 East Hoffman Avenue
Lindenhurst, NY 11757



                                       11
<PAGE>   12
                                

                                                                      EXHIBIT #1

                           SAMPLE CERTIFICATION LETTER

Month X, 199X

FSDC Legal Name
Street Address
City, ST Zip

RE:  Amgen Agreement No. XXXXXX

Dear ____________:

Thank you for your participation in the **DELETED** Program. In order for us to
enroll you, we require that a duly authorized representative of your
organization sign the certification below.

Upon receipt of this executed document, we will calculate the value of your
incentive. If we do not receive the executed certification, we cannot provide
you with this incentive.

If you have any questions regarding this letter please contact me at (805)
**DELETED**. Thank you for your assistance in returning this certification.

Sincerely,

**DELETED**
FSDC Market Segment Manager


CERTIFICATION:

On behalf of FSDC Legal Name and all eligible Affiliates participating in the
**DELETED** under Agreement No. XXXXXX, the undersigned hereby certifies that
the **DELETED** data submitted for each eligible Affiliate during this
**DELETED** includes the required **DELETED** results from all dialysis
patients of such Affiliate, and does not include **DELETED** results from
non-patients. The party executing this document also represents and warrants
that it (i) has no reason to believe that the submitted **DELETED** data is
incorrect, and (ii) is authorized to make this certification on behalf of all
eligible Affiliates submitting **DELETED** data.

FSDC LEGAL NAME

Signature:        _____________________________
Print Name:       _____________________________
Print Title:      _____________________________
Date:             _____________________________


  



                                     12
<PAGE>   13

AMGEN(R)            AMENDMENT #1 DATED APRIL 23, 1999 TO AGREEMENT NO.  19984813
--------------------------------------------------------------------------------
Effective April 1, 1999 the undersigned hereby agree to amend the Product
Purchase Agreement referenced above ("Agreement") between Amgen and NMC as
follows:

SECTION 4 OF THE AGREEMENT, ENTITLED "PAYMENT TERMS", SHALL BE AMENDED AND
RESTATED IN ITS ENTIRETY AS FOLLOWS:

4. PAYMENT TERMS. The terms and conditions of this Agreement shall apply whether
NMC and/or Affiliates purchase EPOGEN(R) through an Authorized Wholesaler or
from Amgen directly.

THE FOLLOWING SECTIONS UNDER APPENDIX A OF THE AGREEMENT SHALL BE AMENDED AND
RESTATED IN THEIR ENTIRETY AS FOLLOWS:


                APPENDIX A: DISCOUNT PRICING, SCHEDULE, AND TERMS


2.    **DELETED**. NMC and Affiliates agree to provide **DELETED** for the
purposes of calculating the **DELETED**. Amgen will accept the **DELETED**,
multiply each **DELETED** by **DELETED**, and apply the converted results to
**DELETED**. Notwithstanding the foregoing, **DELETED**. Amgen will accept
electronic data from the lab systems that service each owned or managed
Affiliate. For NMC to qualify for the **DELETED** the following requirements
must be met:

a. REQUIREMENTS: In order to participate in the **DELETED**, NMC and Affiliates
must provide the following information for each dialysis patient to Amgen or to
a data collection vendor specified by Amgen, on a **DELETED** basis, no later
than **DELETED** days after the end of **DELETED**:

         i) facility ID, patient ID (sufficient to consistently track an
individual patient without in any way disclosing the identity of the patient),
**DELETED** EPOGEN(R), **DELETED** flag, treatment date, **DELETED** reading,
**DELETED** flag, **DELETED** reading, **DELETED** flag, calculated
**DELETED** value, and **DELETED** (collectively the "Data"). Amgen may utilize
the Data for any purpose, and reserves the right to audit all Data. Under no
circumstances should the Data include any patient identifiable information
including, without limitation, name, complete social security number, address or
birth date. The identity of the account submitting the Data and any association
with the Data will remain confidential. The **DELETED** must be derived from
**DELETED** taken immediately before dialysis treatment using any automated
**DELETED** testing method **DELETED**, must be reported to the **DELETED**, and
must be submitted in a format acceptable to Amgen. Hand written reports are not
acceptable; electronic submission of the Data is preferred, and

         ii) a properly executed "Certification Letter", a sample of which is
attached hereto as Exhibit #1, that will be provided to NMC's corporate
headquarters, unless otherwise requested, before each payment period.

b. CALCULATION: Assuming NMC and Affiliates have fulfilled all requirements as
described in Section 2(a) above, NMC's **DELETED** payment will be calculated as
follows:

The "Average Patient **DELETED**" for each dialysis patient will be based upon
the average of all **DELETED** test results gathered for each patient during
each **DELETED** of the Term. The **DELETED** of all dialysis patients with
Average Patient **DELETED** greater than or equal to **DELETED**, will be
determined by dividing the total number of dialysis patients with Average
Patient **DELETED** greater than or equal **DELETED**, by the total number of
dialysis patients treated by NMC and Affiliates. The **DELETED** will be
calculated based on NMC's and Affiliates' overall performance in accordance with
Amgen's discount calculation policies. **DELETED**.


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<PAGE>   14
                                 

c. PAYMENT: The **DELETED** will be calculated on a **DELETED** basis and paid
to NMC's corporate headquarters, except as otherwise provided hereunder.
Payment is contingent upon receipt by Amgen of the Certification Letters and
all required Data for the corresponding **DELETED**. Data shall be submitted to
Amgen **DELETED**. If Data is received **DELETED**, NMC will not qualify for
the OHI for that **DELETED**. **DELETED** payments will be based upon the Data
received from the previous **DELETED**, and will equal a percentage of NMC's
and Affiliates' total EPOGEN(R) Qualified Purchases during that **DELETED**
(exclusive of any Qualified Purchases of EPOGEN(R) made by NMC or any Affiliate
not meeting the Data submission requirements described above) as governed by
the **DELETED** schedule listed below. Notwithstanding the foregoing, payment
for any period during the Term that is not equivalent to a complete
**DELETED**, will be based on an average of the Data that is available for that
period. Amgen reserves the right to modify the **DELETED** if the EPOGEN(R)
package insert language changes.
        
<TABLE>
<CAPTION>
**DELETED** OF ALL DIALYSIS PATIENTS                         **DELETED**
WITH AVERAGE PATIENT **DELETED** GREATER THAN OR EQUAL       **DELETED** PERCENTAGE 
TO **DELETED**
PLEASE DIRECT YOUR ATTENTION TO THE EPOGEN(R) PACKAGE INSERT
------------------------------------------------------------

         <S>               <C>      <C>                       <C>
         **DELETED**       -        Over                      **DELETED**
         **DELETED**       -        **DELETED**               **DELETED**
         **DELETED**       -        **DELETED**               **DELETED**
         **DELETED**       -        **DELETED**               **DELETED**
         **DELETED**       -        **DELETED**               **DELETED**
         **DELETED**       -        **DELETED**               **DELETED**
</TABLE>

VESTING:  The **DELETED** will vest on the **DELETED** of the corresponding 
**DELETED**.


         THE FOLLOWING NEW SECTION SHALL BE ADDED TO APPENDIX A:

3. **DELETED**: NMC shall be eligible to receive an **DELETED** if
certain **DELETED** are transmitted to Amgen **DELETED**. The **DELETED** will
be calculated as a percentage of **DELETED** attributable to NMC and all
Affiliates during that **DELETED**. In order to qualify for the **DELETED** the
following **DELETED** must be submitted by NMC and all Affiliates in an
**DELETED** acceptable to Amgen: Facility ID, patient ID (sufficient to
consistently track an individual patient without in any way disclosing the
identity of the patient), **DELETED**, **DELETED** flag, treatment date,
**DELETED** reading, **DELETED** flag, **DELETED** reading, **DELETED** flag,
calculated **DELETED** value, **DELETED**, and **DELETED**. Such **DELETED**
Data must be submitted on a **DELETED** basis, and no later than **DELETED**
after the end of each **DELETED**. If such **DELETED** Data is not submitted by
NMC, any Affiliate or managed Affiliate, for any given **DELETED** of the Term,
the **DELETED** attributable to those entities (NMC and /or Affiliates)
**DELETED** The **DELETED** will vest on the **DELETED** of the corresponding
**DELETED**, and will be paid **DELETED** thereafter.

ALL SECTIONS FOLLOWING THE ADDITION OF NEW SECTION 3(ABOVE) SHALL BE RENUMBERED
SEQUENTIALLY TO ALLOW FOR THE ADDITION OF THIS NEW SECTION.

4. **DELETED**. NMC may qualify for the **DELETED** as described below.

a. CALCULATION: NMC's **DELETED** will be calculated in accordance with the 
   following formula.


                                   **DELETED**
                  where

         A = **DELETED** during the Term by NMC and all Affiliates. 


                                       14
<PAGE>   15
                                


         B =      A percent in accordance with the table listed below.
         C =      **DELETED** during the Term by NMC and all Affiliates as
                  listed on Appendix C on the Commencement Date of this
                  Agreement.
         D =      **DELETED** by NMC and those same Affiliates as listed on
                  Appendix C on the Commencement Date of this Agreement for the
                  same time period during the previous year.

         **DELETED**
         **DELETED**                **DELETED**
         --------------------------------------
         **DELETED**-Over           **DELETED**
         **DELETED**                **DELETED**
         **DELETED**                **DELETED**
         **DELETED**                **DELETED**
         **DELETED**                **DELETED**
         **DELETED**                **DELETED**

For the purposes of calculating **DELETED** increases, Amgen will incorporate
purchases of **DELETED**.

b. VESTING: NMC's **DELETED** shall vest **DELETED** based upon **DELETED** and
shall be paid in accordance with the time frames set forth in Appendix B.

Amgen reserves the right to rescind this offer if the parties fail to execute
this Amendment within 30 days from the date of its offering. This will allow
Amgen time to notify designated wholesalers, who also require lead time to
implement the change(s).

Please retain one fully executed original for your records and return the other
fully executed original to Amgen.

THE PARTIES EXECUTED THIS AMENDMENT AS OF THE DATES SET FORTH BELOW.



                                       15
<PAGE>   16
                                


AMGEN INC.

Signature:        /s/  Lauren Hirsch
                  Lauren Hirsch
                  Assoc. Director, National Accounts
Date:             5/3/99

NATIONAL MEDICAL CARE, INC.

Signature:        /s/ James V. Luther 
                  James V. Luther
                  Assistant Treasurer
Date:             4/28/99





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