<PAGE>   1

EXHIBIT 10.1

                            FIRST AMENDMENT AGREEMENT

         This First Amendment Agreement is made as of the 25th day of January,
  2000, among STERIS CORPORATION, an Ohio corporation, ("Borrower"), the banking
  institutions listed on SCHEDULE 1 to the Credit Agreement, as hereinafter
  defined ("Banks"), and KEYBANK NATIONAL ASSOCIATION, as administrative agent
  for the Banks ("Agent").

         WHEREAS, Borrower, Agent and the Banks are parties to a Credit
  Agreement dated as of January 26, 1999, as the same may from time to time be
  amended, restated or otherwise modified, which provides, among other things,
  for loans aggregating Four Hundred Million Dollars ($400,000,000), all upon
  certain terms and conditions ("Credit Agreement");

         WHEREAS, Borrower, Agent and the Banks desire to amend the Credit
  Agreement to modify certain provisions thereof; and

         WHEREAS, each term used herein shall be defined in accordance with the
  Credit Agreement;

         NOW, THEREFORE, in consideration of the premises and of the mutual
  covenants herein and for other valuable considerations, Borrower, Agent and
  the Banks agree as follows:

         1. Article I of the Credit Agreement is hereby amended to delete the
  definitions of "Advantage", "Commitment Percentage" and "Commitment Period"
  therefrom and to insert in place thereof the following:

                  "Advantage" shall mean any payment (whether made voluntarily
         or involuntarily, by offset of any deposit or other indebtedness or
         otherwise) received by any Bank in respect of the Debt, if such payment
         results in that Bank having less than its Pro Rata Share of the
         Applicable Debt then outstanding, than was the case immediately before
         such payment.

                  "Commitment Percentage" shall mean Applicable Commitment
         Percentage.

                  "Commitment Period" shall mean the period from the Closing
         Date to (a) January 26, 2002, with respect to the Tranche A Commitment,
         and (b) January 25, 2000, as extended as of January 25, 2000, for an
         additional three hundred sixty-four (364) day period ending January 22,
         2001, with respect to the Tranche B Commitment; or such earlier date on
         which the Commitment shall have been terminated pursuant to Article
         VIII hereof.

         2. Article I of the Credit Agreement is hereby amended to add the
  following new definitions thereto:

                  "Applicable Commitment Percentage" shall mean, for each Bank,
         (a) with respect to the Tranche A Commitment, the percentage set forth
         opposite such Bank's name under

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         the column headed "Tranche A Commitment Percentage" as described in
         SCHEDULE 1 hereto, and (b) with respect to the Tranche B Commitment,
         the percentage set forth opposite such Bank's name under the column
         headed "Tranche B Commitment Percentage" as described in SCHEDULE 1
         hereto.

                  "Applicable Debt" shall mean (a) with respect to the Tranche A
         Commitment, collectively, (i) all Indebtedness incurred by Borrower to
         Agent or the Banks pursuant to this Agreement (other than pursuant to
         the Tranche B Commitment) and includes the principal of and interest on
         all Notes (other than the Tranche B Notes), (ii) each extension,
         renewal or refinancing thereof in whole or in part, and (iii) the
         facility fee, other fees, and any prepayment fees payable under this
         Agreement (other than the facility fees and prepayment fees payable in
         connection with the Tranche B Commitment); and (b) with respect to the
         Tranche B Commitment, collectively, (i) all Indebtedness incurred by
         Borrower to Agent or the Banks pursuant to the Tranche B Commitment and
         includes the principal of and interest on the Tranche B Notes, (ii)
         each extension, renewal or refinancing thereof in whole or in part, and
         (iii) the facility fee, other fees, and any prepayment fees payable in
         connection with the Tranche B Commitment.

                  "Equalization Event" shall mean the earlier of (a) the
         occurrence of an Event of Default specified in Section 7.10 hereof, or
         (b) the acceleration of the Debt pursuant to Section 8.1 or 8.2 hereof.

                  "Payment Conditions" shall mean the following: (a) any
         regularly scheduled payment of interest on, or facility fee with
         respect to, the Tranche A Loans shall be applied by Agent and the Banks
         to the Tranche A Loans and the Tranche A Commitment, respectively; (b)
         any regularly scheduled payment of interest on, or facility fee with
         respect to, the Tranche B Loans shall be applied by Agent and the Banks
         to the Tranche B Loans and the Tranche B Commitment, respectively; (c)
         any payment of principal prior to an Equalization Event (or any other
         payment prior to an Equalization Event for which there is no apparent
         schedule of payment, as determined by Agent) shall be applied to the
         principal of the Tranche B Loans, or if there are no Tranche B Loans
         outstanding, then to the Tranche A Loans outstanding; and (d) after an
         Equalization Event, all payments shall be applied by Agent and the
         Banks first to the payment of any fees or other expenses owing to Agent
         (acting in its capacity as agent under this Agreement) and then, pro
         rata, to each Bank, based upon the aggregate amount of principal
         outstanding on the Notes of such Bank (other than the Swing Line Note)
         on the date that such Equalization Event occurred over the aggregate
         amount of principal then outstanding on all Notes of all of the Banks
         (other than the Swing Line Note) on the date that such Equalization
         Event occurred.

                  "Pro Rata Basis" or "pro rata basis" shall mean distribution
         to the Banks by Agent in accordance with the Applicable Commitment
         Percentages.

                  "Pro Rata Share" or "pro rata share" shall mean, with respect
         to the Applicable Debt, such Bank's share in accordance with such
         Bank's Applicable Commitment Percentage.

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                  "Ratable Account" or "ratable account" shall mean each Bank's
         share of the Applicable Debt in accordance with such Bank's Applicable
         Commitment Percentage.

                  "Ratable Share" or "ratable share" shall mean each Bank's
         share of the Applicable Debt in accordance with such Bank's Applicable
         Commitment Percentage.

                  "Ratably" or "ratably" shall mean, other than in Section 9.9
         hereof, in accordance with each Bank's Ratable Share.

         3. Article I of the Credit Agreement is hereby amended to add the
  following new sentence at the end thereof:

                  Whenever payments are made to Agent, "for the benefit of the
         Banks", "for the benefit of the Banks" shall mean for the benefit of
         the Banks on a Pro Rata Basis.

         4. Section 2.1 of the Credit Agreement is hereby amended to delete the
  second and third paragraphs therefrom and to insert in place thereof the
  following:

                  Each Bank, for itself and not one for any other, agrees to
         participate in Loans made hereunder during the applicable Commitment
         Period on such basis that (a) immediately after the completion of any
         borrowing by Borrower hereunder, the aggregate principal amount then
         outstanding on the Notes (other than the Swing Line Note) issued to
         such Bank shall not be in excess of the Maximum Amount for such Bank,
         (b) such aggregate principal amount outstanding on the Tranche A Note
         issued to such Bank shall represent that percentage of the aggregate
         principal amount then outstanding on all Tranche A Notes (including the
         Tranche A Note held by such Bank) which is such Bank's Applicable
         Commitment Percentage; and (c) such aggregate principal amount
         outstanding on the Tranche B Note issued to such Bank shall represent
         that percentage of the aggregate principal amount then outstanding on
         all Tranche B Notes (including the Tranche B Note held by such Bank)
         which is such Bank's Applicable Commitment Percentage.

                  Each borrowing (other than the Swing Loans) from the Banks
         hereunder shall be made pro rata according to the Banks' respective
         Applicable Commitment Percentages. The Loans may be made as Tranche A
         Loans, Tranche B Loans and Swing Loans as follows:

         5. The Credit Agreement is hereby amended to delete Section 2.3
  therefrom in its entirety and to insert in place thereof the following:

                  SECTION 2.3. PAYMENT ON NOTES, ETC. Unless otherwise provided,
         all payments of principal, interest and facility and other fees shall
         be made to Agent in immediately available funds for the account of the
         Banks on a Pro Rata Basis (except as to payments made exclusively for
         the benefit of Agent pursuant to the Agent Fee Letter). Agent, on the
         same Business Day, shall distribute to each Bank its Ratable Share of
         the amount of principal, interest, and facility and other fees received
         by it for the account of

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         such Bank. Each payment under this Agreement shall be applied by Agent
         and the Banks in accordance with the Payment Conditions. Each Bank
         shall record (a) any principal, interest or other payment, and (b) the
         principal amount of the Prime Rate Loans and the LIBOR Loans and all
         prepayments thereof and the applicable dates with respect thereto, by
         such method as such Bank may generally employ; provided, however, that
         failure to make any such entry shall in no way detract from Borrower's
         obligations under each such Note. The aggregate unpaid amount of Loans
         set forth on the records of Agent shall be rebuttably presumptive
         evidence of the principal and interest owing and unpaid on each Note.
         Whenever any payment to be made hereunder, including, without
         limitation, any payment to be made on any Note, shall be stated to be
         due on a day that is not a Business Day, such payment shall be made on
         the next succeeding Business Day and such extension of time shall in
         each case be included in the computation of the interest payable on
         such Note; provided, however, that, with respect to any LIBOR Loan, if
         the next succeeding Business Day falls in the succeeding calendar
         month, such payment shall be made on the preceding Business Day and the
         relevant Interest Period shall be adjusted accordingly.

         6. The Credit Agreement is hereby amended to delete Section 2.4(a)
  therefrom and to insert in place thereof the following:

                  (a) Borrower shall have the right at any time or from time to
         time to prepay, on a Pro Rata Basis for all of the Banks (other than
         the Swing Line Note), all or any part of the principal amount of the
         Notes then outstanding as designated by Borrower, plus interest accrued
         on the amount so prepaid to the date of such prepayment, subject,
         however, to the Payment Conditions. Borrower shall give Agent notice of
         prepayment of any Prime Rate Loan by not later than 11:00 A.M.
         (Cleveland, Ohio time) on the Business Day such prepayment is to be
         made and written notice of the prepayment of any LIBOR Loan not later
         than 1:00 P.M.(Cleveland, Ohio time) three (3) Business Days before the
         Business Day on which such prepayment is to be made.

         7. The Credit Agreement is hereby amended to delete Section 8.4
  therefrom in its entirety and to insert in place thereof the following:

                  SECTION 8.4. EQUALIZATION PROVISION. Each Bank agrees with the
         other Banks that if it, at any time, shall obtain any Advantage over
         the other Banks or any thereof in respect of the Debt (except as to
         Swing Loans as set forth in subpart 2 of Section 2.1A hereof or subpart
         2 of Section 2.1B hereof and except under Article III hereof), it shall
         purchase from the other Banks, for cash and at par, such additional
         participation in the Applicable Debt as shall be necessary to nullify
         the Advantage. If any such Advantage resulting in the purchase of an
         additional participation as aforesaid shall be recovered in whole or in
         part from the Bank receiving the Advantage, each such purchase shall be
         rescinded, and the purchase price restored (but without interest unless
         the Bank receiving the Advantage is required to pay interest on the
         Advantage to the Person recovering the Advantage from such Bank)
         ratably to the extent of the recovery. Each Bank further agrees with
         the other Banks that if it at any time shall receive any payment for or
         on behalf of Borrower on any indebtedness owing by Borrower to that

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         Bank by reason of offset of any deposit or other indebtedness, it will
         apply such payment first to any and all Debt owing by Borrower to that
         Bank (including, without limitation, any participation purchased or to
         be purchased pursuant to this Section or any other Section of this
         Agreement), subject to the Payment Conditions. Borrower agrees that any
         Bank so purchasing a participation from the other Banks or any thereof
         pursuant to this Section may exercise all its rights of payment
         (including the right of set-off) with respect to such participation as
         fully as if such Bank was a direct creditor of Borrower in the amount
         of such participation.

         8. The Credit Agreement is hereby amended to delete SCHEDULE 1 thereof
  in its entirety and to insert in place thereof a new SCHEDULE 1, in the form
  of SCHEDULE 1 attached hereto.

         9. Concurrently with the execution of this First Amendment Agreement,
  Borrower shall:

         (a) pay to Agent an amendment fee, which shall be applied on a Pro Rata
  Basis, to each Bank with a Tranche B Commitment, in an amount equal to ten
  (10) basis points times the amount of each such Bank's Tranche B Commitment;

         (b) cause each Guarantor of Payment to consent and agree to and
  acknowledge the terms of this First Amendment Agreement; and

         (c) pay all legal fees and expenses of Agent in connection with this
  First Amendment Agreement.

         10. Borrower hereby represents and warrants to Agent and the Banks that
  (a) Borrower has the legal power and authority to execute and deliver this
  First Amendment Agreement, (b) the officers executing this First Amendment
  Agreement have been duly authorized to execute and deliver the same and bind
  Borrower with respect to the provisions hereof, (c) the execution and delivery
  hereof by Borrower and the performance and observance by Borrower of the
  provisions hereof do not violate or conflict with the organizational
  agreements of Borrower or any law applicable to Borrower or result in a breach
  of any provision of or constitute a default under any other agreement,
  instrument or document binding upon or enforceable against Borrower, (d) no
  Unmatured Event of Default or Event of Default exists under the Credit
  Agreement, nor will any occur immediately after the execution and delivery of
  this First Amendment Agreement or by the performance or observance of any
  provision hereof, (e) Borrower is not aware of any claim or offset against, or
  defense or counterclaim to, any of Borrower's obligations or liabilities under
  the Credit Agreement or any Related Writing, and (f) this First Amendment
  Agreement constitutes a valid and binding obligation of Borrower in every
  respect, enforceable in accordance with its terms.

         11. Each reference that is made in the Credit Agreement or any other
  writing to the Credit Agreement shall hereafter be construed as a reference to
  the Credit Agreement as amended hereby. Except as herein otherwise
  specifically provided, all provisions of the Credit Agreement shall remain in
  full force and effect and be unaffected hereby. This First Amendment Agreement
  is a Related Writing as defined in the Credit Agreement.

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         12. Borrower and each Guarantor of Payment, by signing below, hereby
  waives and releases Agent and each of the Banks and the respective directors,
  officers, employees, attorneys, affiliates and subsidiaries of each of the
  foregoing from any and all claims, offsets, defenses and counterclaims of
  which Borrower or such Guarantor of Payment is aware, such waiver and release
  being with full knowledge and understanding of the circumstances and effect
  thereof and after having consulted legal counsel with respect thereto.

         13. This First Amendment Agreement may be executed in any number of
  counterparts, by different parties hereto in separate counterparts and by
  facsimile signature, each of which when so executed and delivered shall be
  deemed to be an original and all of which taken together shall constitute but
  one and the same agreement.

         14. The rights and obligations of all parties hereto shall be governed
  by the laws of the State of Ohio, without regard to principles of conflicts of
  laws.

                  [Remainder of page intentionally left blank.]



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         15. JURY TRIAL WAIVER. BORROWER, AGENT AND EACH OF THE BANKS WAIVE ANY
  RIGHT TO HAVE A JURY PARTICIPATE IN RESOLVING ANY DISPUTE, WHETHER SOUNDING IN
  CONTRACT, TORT OR OTHERWISE, AMONG BORROWER, AGENTS AND THE BANKS, OR ANY
  THEREOF, ARISING OUT OF, IN CONNECTION WITH, RELATED TO, OR INCIDENTAL TO THE
  RELATIONSHIP ESTABLISHED AMONG THEM IN CONNECTION WITH THIS AGREEMENT OR ANY
  NOTE OR OTHER INSTRUMENT, DOCUMENT OR AGREEMENT EXECUTED OR DELIVERED IN
  CONNECTION HEREWITH OR THE TRANSACTIONS RELATED THERETO. THIS WAIVER SHALL NOT
  IN ANY WAY AFFECT, WAIVE, LIMIT, AMEND OR MODIFY AGENT'S OR ANY BANK'S ABILITY
  TO PURSUE REMEDIES PURSUANT TO ANY CONFESSION OF JUDGMENT OR COGNOVIT
  PROVISION CONTAINED IN ANY NOTE OR OTHER INSTRUMENT, DOCUMENT OR AGREEMENT
  AMONG BORROWER, AGENTS AND THE BANKS, OR ANY THEREOF.

                                 STERIS CORPORATION

                                 By:  /s/Bill R. Sanford
                                      -----------------------------------------
                                          Bill R. Sanford, Chairman of the
                                          Board, President, and Chief
                                          Executive Officer

                                 and:  /s/Les C. Vinney
                                      -----------------------------------------
                                          Les C. Vinney, Senior Vice President
                                          Finance and Operations, and Chief
                                          Financial Officer

                                 KEYBANK NATIONAL ASSOCIATION,
                                    as a Bank and as Agent

                                 By:  /s/ J.T. Taylor
                                      -----------------------------------------
                                          J.T. Taylor, Vice President

                                 NATIONAL CITY BANK

                                 By  /s/ Robert S. Coleman
                                      -----------------------------------------
                                         Robert S. Coleman, Vice President

                                 BANK ONE, NA,

                                 By: /s/ Babette Casey Coerdt
                                     ------------------------------------------
                                 Title: Managing Director
                                        ---------------------------------------



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                                PNC BANK, NATIONAL ASSOCIATION,

                                By:    /s/ Bryon Pike
                                       ---------------------------------
                                Title: Vice President
                                       ---------------------------------

                                ABN AMRO BANK N.V.,
                                PITTSBURGH BRANCH,

                                By:    /s/ Roy D. Hasbrook
                                       ---------------------------------
                                Title: Group Vice President and Director
                                       ---------------------------------

                                and:   /s/ Gregory D. Amoroso
                                       ---------------------------------
                                Title: Senior Vice President
                                       ---------------------------------

                                THE BANK OF NEW YORK

                                By:    /s/ Jonathan Rollins
                                       ---------------------------------
                                Title: Vice President
                                       ---------------------------------

                                HARRIS TRUST AND SAVINGS BANK

                                By:    /s/ Jeffrey C. Nicholson
                                       ---------------------------------
                                Title: Managing Director
                                       ---------------------------------


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                                   SCHEDULE 1


<TABLE>
<CAPTION>
                                        TRANCHE A        TRANCHE A         TRANCHE B         TRANCHE B
                                       COMMITMENT        COMMITMENT        COMMITMENT       COMMITMENT
         BANKING INSTITUTIONS            AMOUNT          PERCENTAGE          AMOUNT         PERCENTAGE          MAXIMUM AMOUNT
         --------------------            ------          ----------          ------         ----------          --------------
<S>                                    <C>                  <C>            <C>                  <C>             <C>
   KeyBank National                    $50,000,000          20.00%         $43,125,000          28.75%          $93,125,000
    Association
   National City Bank                  $46,875,000          18.75%         $28,125,000          18.75%          $75,000,000
   Bank One, NA                        $46,875,000          18.75%         $28,125,000          18.75%          $75,000,000
   ABN AMRO Bank N.V.,                 $31,250,000          12.50%         $18,750,000          12.50%          $50,000,000
    Pittsburgh Branch
   PNC Bank, National                  $31,250,000          12.50%         $18,750,000          12.50%          $50,000,000
   Association
   The Bank of New York                $21,875,000           8.75%                  $0              0%          $21,875,000
   Harris Trust and Savings            $21,875,000           8.75%         $13,125,000           8.75%          $35,000,000
   Bank

                                      $250,000,000         100.00%        $150,000,000         100.00%         $400,000,000
   Total Commitment Amount                                                                                     $400,000,000
</TABLE>






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                            GUARANTOR ACKNOWLEDGMENT
                            ------------------------

         The undersigned consents and agrees to and acknowledges the terms of
  the foregoing First Amendment Agreement. The undersigned further agrees that
  the obligations of the undersigned pursuant to the Guaranty of Payment
  executed by the undersigned shall remain in full force and effect and be
  unaffected hereby.

                                MEDICAL & ENVIRONMENTAL DESIGNS,
                                   INC.
                                ECOMED, INC.
                                AMERICAN STERILIZER COMPANY
                                STERIS INTERNATIONAL SALES
                                   CORPORATION
                                STERIS EUROPE, INC.
                                STERIS ASIA PACIFIC, INC.
                                STERIS LATIN AMERICA, INC.
                                STERIS INC.
                                STERIS USA DISTRIBUTION
                                   CORPORATION
                                HTD HOLDING CORP.
                                HAUSTED, INC.
                                ISOMEDIX INC.
                                ISOMEDIX OPERATIONS INC.
                                ISOMEDIX (PUERTO RICO), INC.

                                By:  /s/ Bill R. Sanford
                                   -----------------------------------------
                                         Bill R. Sanford, President of each
                                         of the foregoing Companies

                                HSTD LLC

                                By:      HTD Holding Corp., its member

                                         By:  /s/ Bill R. Sanford
                                      --------------------------------------
                                                  Bill R. Sanford, President



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