<PAGE>   1
                                                                     EXHIBIT 4.1




                 FIRST AMENDMENT, dated as of July 1, 1998 (this "Amendment"),
to the Second Amended and Restated Credit Agreement, dated as of April 25, 1997
(the "Credit Agreement"), among Sybron International Corporation, a Wisconsin
corporation (the "Parent"), Ormco Corporation, a Delaware corporation
("Ormco"), Kerr Corporation, a Delaware corporation ("Kerr"), Nalge Nunc
International Corporation, a Delaware corporation ("NNI"), Erie Scientific
Company, a Delaware corporation ("Erie"), Barnstead Thermolyne Corporation, a
Delaware corporation ("Barnstead"; Ormco, Kerr, NNI, Erie and Barnstead are
collectively referred to herein as the "Subsidiary Borrowers"), the several
banks and other financial institutions from time to time parties thereto (the
"Lenders"), Chase Securities Inc., as Arranger, and The Chase Manhattan Bank, a
New York banking corporation, as administrative agent for the Lenders
thereunder (in such capacity, the "Administrative Agent").


                              W I T N E S S E T H:


                 WHEREAS, pursuant to the Credit Agreement, the Lenders have
agreed to make, and have made, certain loans and other extensions of credit to
the Parent and the Subsidiary Borrowers; and

                 WHEREAS, the Parent and the Subsidiary Borrowers have
requested, and, upon this Amendment becoming effective, the Lenders have
agreed, that (i) additional term loan commitments of the Lenders shall be
extended by $100,000,000 in the aggregate and (ii) certain provisions of the
Credit Agreement be amended in the manner provided for in this Amendment.

                 NOW, THEREFORE, the parties hereto hereby agree as follows:

         I.      Defined Terms.  Terms defined in the Credit Agreement and used
herein shall have the meanings given to them in the Credit Agreement.

                    II.     Amendments to Credit Agreement.

                 1.       Amendments to Section 1.  (a)  Subsection 1.1 of the
Credit Agreement is hereby amended by deleting the definitions of "Aggregate
Commitment", "Amendment Effective Date", "Commitment Percentage",
"Commitments", "Excepted Indebtedness," "Notes" contained therein in their
entirety and adding the following definitions in alphabetical order:

                 "`Additional Term Loans':  as defined in subsection 2.28.

                 `Additional Term Loan Commitment':  with respect to each
         Lender, the amount set forth under the heading "Additional Term Loan
         Commitment" opposite such Lender's name on Schedule I, as such amount
         may be reduced from time to time pursuant to this Agreement.
<PAGE>   2
                                                                               2



                 `Additional Term Loan Commitment Percentage':  as to any
         Lender at any time, the percentage which such Lender's Additional Term
         Loan Commitment then constitutes of the Aggregate Additional Term Loan
         Commitment.

                 `Additional Term Notes':  as defined in subsection 2.29.

                 `Aggregate Additional Term Loan Commitment': shall mean
         $100,000,000, as     such amount may be reduced from time to time
         pursuant to the terms of this    Agreement.

                 `Aggregate Commitment':  shall mean the amount equal to (a)
         prior to the Amendment Effective Date, the sum of the aggregate
         principal amount of the Term Loans outstanding, the Aggregate
         Additional  Term Loan Commitment and the Aggregate Revolving Credit
         Commitment, and (b) following the Amendment Effective Date, the sum of
         the aggregate principal amount of the TA Loans outstanding and the
         Aggregate Revolving Credit Commitment, or if the Aggregate Revolving
         Credit Commitment has been terminated, the sum of the Aggregate
         Outstanding Extensions of Credit of each Lender.

                 `Amendment Effective Date':  the date on which the First
         Amendment dated as of July 1, 1998 to the Credit Agreement became
         effective pursuant to the terms thereof.

                 `Commitment Percentage':  as to any Lender at any time, such
         Lender's Term Loan Commitment Percentage, Additional Term Loan
         Commitment Percentage or Revolving Credit Commitment Percentage, as
         the context may require.

                 `Commitments':  the collective reference to the Term Loan
         Commitments, the Additional Term Loan Commitments and the Revolving
         Credit Commitments; each as the context may require, a "Commitment".

                 `Consolidated  Senior Debt':  as of the date of determination,
         the sum of (a) all Indebtedness of the Parent and its Subsidiaries,
         determined on a consolidated basis in accordance with GAAP (including
         the Loans) minus (b) the aggregate principal amount of all outstanding
         Subordinated Indebtedness and all outstanding Permitted Indebtedness.

                 `Excepted Indebtedness':  the Indebtedness of the Parent or
         any of its Subsidiaries permitted to exist pursuant to subsection 7.2
         (other than subsection 7.2(h) thereof).

                 `Notes':  the collective reference to the Revolving Credit
         Notes, the CAF Advance Notes, the Swing Line Notes, the Term Notes and
         the Additional Term Notes.

                 `Permitted Indebtedness':  any unsecured Indebtedness of the
         Parent or any of its Subsidiaries (a) no part of the principal of
         which is stated to be payable or is required to be paid (whether by
         way of mandatory sinking fund, mandatory redemption, mandatory
         prepayment or otherwise) prior to the Termination Date, and the
         payment of the principal of and any interest on which and other
         obligations of the Parent and the Subsidiary Borrowers in respect
         thereof are subordinated to the prior payment in full of the principal
<PAGE>   3
                                                                               3





         of and interest (including post-petition interest) on the Notes and
         all other obligations and liabilities of the Parent and the Subsidiary
         Borrowers to the Administrative Agent and the Lenders hereunder on
         terms and conditions first approved (such approval not to be
         unreasonably withheld) in writing by the Administrative Agent and (b)
         otherwise containing terms, covenants and conditions reasonably
         satisfactory in form and substance to the Administrative Agent, as
         evidenced by its prior written approval.

                 `Senior Debt Ratio':  on the date of any determination
         thereof, the ratio of Consolidated Senior Debt on such date to
         Consolidated Adjusted Operating Profit for the four full fiscal
         quarters ending on such date; provided that for purposes of
         calculating Consolidated Adjusted Operating Profit for any period of
         four full fiscal quarters, the Consolidated Adjusted Operating Profit
         of any Person acquired during such period shall be included on a pro
         forma basis for such period of four full fiscal quarters (assuming the
         consummation of each such acquisition and the incurrence or assumption
         of any Indebtedness in connection therewith occurred on the first day
         of such period of four full fiscal quarters and assuming only such
         cost reductions as are related to such acquisition and are immediately
         realizable as of the date of such acquisition).  For purposes of this
         calculation, Consolidated Adjusted Operating Profit of any such Person
         acquired during such period shall be derived from a certificate, in
         form and substance satisfactory to the Administrative Agent, of a duly
         authorized financial officer of the Parent setting forth such
         Consolidated Adjusted Operating Profit.

                 `TA Loan':  the collective reference to the Term Loans and the
Additional Term Loans.

                          (b)     Subsection 1.1 of the Credit Agreement is
hereby amended by deleting  paragraph (2) in the definition of "Interest
Period" in its entirety and substituting in lieu thereof the following:

                 "(2)     any Interest Period in respect of Revolving Credit
         Loans that would otherwise extend beyond the Termination Date, and any
         Interest Period in respect of the Term Loans or the Additional Term
         Loans that would otherwise extend beyond the date final payment is due
         on the Term Loans or the Additional Term Loans, shall end on the
         Termination Date or such date of final payment, as the case may be;"

                 2.       Amendments to Section 2.  Section 2 is hereby amended
                          as follows:

                 (a) by deleting subsection 2.13 thereof in its entirety and
substituting in lieu thereof the following:

                          "2.13.  Procedure for Additional Term Loan Borrowing.
                 The Parent shall give the Administrative Agent irrevocable
                 notice (which notice must be received by the Administrative
                 Agent prior to 10:00 A.M., New York City time, one Business
                 Day prior to the proposed Borrowing Date, if the Additional
                 Term Loans are to be initially ABR Loans, or three Business
                 Days prior to the proposed Borrowing Date, if the Additional
                 Term Loans





<PAGE>   4
                                                                               4




                 are to be initially Eurodollar Loans, in whole or in part)
                 requesting that the Lenders make their portions of the
                 Additional Term Loans on the proposed Borrowing Date.  Upon
                 receipt of such notice the Administrative Agent shall promptly
                 notify each Lender thereof.  Not later than 11:00 A.M. on the
                 proposed Borrowing Date each Lender shall make available to
                 the Administrative Agent at its office specified in subsection
                 11.2 the amount of its relevant portion of its Additional Term
                 Loans in immediately available funds.  The Administrative
                 Agent shall on such date credit the account of NNI on the
                 books of such office of the Administrative Agent with the
                 aggregate of the amounts made available to the Administrative
                 Agent for NNI by the Lenders and in like funds as received by
                 the Administrative Agent."

                 (b)  by deleting each of the references to "Term Loans" in
subsection 2.14 and substituting in lieu thereof references to "Term Loans or
Additional Term Loans."

                 (c)  by deleting in its entirety paragraph (c) of subsection
2.15 thereof and substituting in lieu thereof the following:

                          "(c)  Any prepayment required by the terms of this
                 subsection 2.15 shall be applied first to the reduction of the
                 TA Loans in accordance with the following sentence until such
                 TA Loans shall have been satisfied in full and second to the
                 permanent reduction of the Aggregate Revolving Credit
                 Commitment.  Prepayments of the TA Loans shall be made pro
                 rata between Term Loans and the Additional Term Loans based on
                 the outstanding aggregate principal amount thereof, and
                 prepayment of the Term Loans pursuant to this subsection 2.15
                 shall be applied equally between (i) the outstanding
                 installments of principal scheduled to be paid pursuant to
                 subsection 2.12 immediately succeeding the date of such
                 prepayment and (ii) the last outstanding scheduled
                 installments of principal pursuant to subsection 2.12.
                 Amounts prepaid on account of the TA Loans may not be
                 reborrowed."

                 (d)  by deleting subsection 2.15(e) in its entirety and
substituting in lieu thereof the following:

                          "(e)  The Parent may allocate any prepayment of the
                 Term Loans and the Additional Term Loans pursuant to this
                 subsection 2.15 among the Term Notes and the Additional Term
                 Notes, as the case may be, of the Subsidiary Borrowers as the
                 Parent may determine; provided that the Parent shall give the
                 Administrative Agent prior written notice of such allocation
                 not less than three Business Days prior to such prepayment;
                 and provided, further, that in the event that the Parent does
                 not so notify the Administrative Agent, the Administrative
                 Agent shall allocate such payments as it determines in its
                 sole discretion."

                 (e)  by deleting each of the references in subsections 2.17
and 2.18 to "Term Loans" and substituting in lieu thereof references to "TA
Loans."





<PAGE>   5
                                                                               5




                 (f)  by adding the following subsections at the end of 
Section 2:

                          "2.28  Additional Term Loans.  Subject to the terms
                 and conditions hereof, each Lender severally agrees to make
                 term loans to NNI on the Amendment Effective Date in an
                 aggregate principal amount equal to such Lender's Additional
                 Term Loan Commitment Percentage of the Aggregate Additional
                 Term Loan Commitment (an "Additional Term Loan").  The
                 Additional Term Loans may from time to time be (i) Eurodollar
                 Loans, (ii) ABR Loans or (iii) a combination thereof, as
                 determined by the Parent and notified to the Administrative
                 Agent in accordance with subsections 2.13 and 2.17.  The
                 Additional Term Loans shall mature on the Termination Date.

                          2.29  Additional Term Notes.  NNI, upon the request
                 of an applicable Lender, shall issue a promissory note to
                 evidence the Additional Term Loans made by each Lender,
                 substantially in the form of Exhibit B-1 to this Agreement (an
                 "Additional Term Note"), with appropriate insertions therein
                 as to payee, date and principal amount, payable to the order
                 of such Lender and in a principal amount equal to the lesser
                 of such Lender's Additional Term Loan Commitment Percentage of
                 the Aggregate Additional Term Loan Commitment and the
                 aggregate amount of the Additional Term Loan (or portions
                 thereof) made by such Lender.  An Additional Term Note and the
                 Obligation evidenced thereby may be assigned or otherwise
                 transferred in whole or in part only by registration of such
                 assignment or transfer of such Additional Term Note and the
                 Obligation evidenced thereby in the Register (and each
                 Additional Term Note shall expressly so provide).  Any
                 assignment or transfer of all or part of an Obligation
                 evidenced by an Additional Term Note shall be registered in
                 the Register only upon surrender for registration of
                 assignment or transfer of the Additional Term Note evidencing
                 such Obligation, accompanied by an Assignment and Acceptance
                 substantially in the form of Exhibit I duly executed by the
                 Assignor thereof, and thereupon one or more new Additional
                 Term Notes shall be issued to the designated Assignee and the
                 old Additional Term Note shall be returned by the
                 Administrative Agent to the Borrower marked "cancelled."  No
                 assignment of an Additional Term Note and the Obligation
                 evidenced thereby shall be effective unless it shall have been
                 recorded in the Register by the Administrative Agent as
                 provided in this subsection 2.29.  Each Lender is hereby
                 authorized to record the date and amount of each payment or
                 prepayment of principal of its Additional Term Loan, each
                 continuation thereof, each conversion of all or a portion
                 thereof to another Type and, in the case of Eurodollar Loans,
                 the length of each Interest Period with respect thereto, on
                 the appropriate schedule annexed to and constituting a part of
                 its Additional Term Note (or any continuation thereof), and
                 any such recordation shall constitute prima facie evidence of
                 the accuracy of the information so recorded.  The Additional
                 Term Note of each Lender shall (a) be dated the Amendment
                 Effective Date, (b) be stated to mature on the Termination
                 Date, and (c) provide for the payment of interest in
                 accordance with subsection 2.19."





<PAGE>   6
                                                                               6




                 3.       Amendments to Section 4.  Section 4 of the Credit
Agreement is hereby amended as follows:

                 (a)  by deleting subsection 4.16 thereof in its entirety and
substituting in lieu thereof the following:

                          "4.16  Purpose of Loans.  (i) The proceeds of the
                 Term Loans shall be used to finance the general corporate
                 purposes of the Parent and its Subsidiaries, including
                 acquisitions (including the Remel Acquisition) permitted
                 hereunder and to refinance existing indebtedness (as set forth
                 on Schedule 4.16 to this Agreement) and to pay fees and other
                 expenses related thereto, (ii) the proceeds of the Additional
                 Term Loans shall be used to repay outstanding Revolving Credit
                 Loans and (iii) the proceeds of the Revolving Credit Loans,
                 the Swing Line Loans and the CAF Advances shall be used to
                 finance the working capital needs of the Parent and its
                 Subsidiaries and for general corporate purposes, including
                 acquisitions (including the Remel Acquisition) permitted
                 hereunder; provided that no more than $160,000,000 (subject to
                 any post-closing purchase price adjustments in accordance with
                 the Acquisition Documents) of the proceeds of the Loans may be
                 used to finance the Remel Acquisition."

                 (b)  by adding the following subsection 4.26 at the end of
Section 4:

                          "4.26  Year 2000 Matters. The Borrowers have
                 conducted a review of their computer systems and equipment
                 containing embedded microchips to determine whether any
                 reprogramming is required to permit proper functioning of
                 these systems and equipment in and following the year 2000.
                 In respect of the year 2000, the Borrowers have plans in place
                 to complete system upgrades or reprogramming, and testing
                 thereof, by the end of March 1999, and are in the process of
                 communicating with vendors, suppliers and customers to
                 identify any potential issues which may affect the Borrowers.
                 Based upon the information the Borrowers have developed to
                 date, the cost to the Borrowers of such reprogramming,
                 upgrading and testing, and the reasonably foreseeable
                 consequences of year 2000 computer system issues relevant to
                 the Borrowers, will not result in a Default or a Material
                 Adverse Effect.  Except for such of the reprogramming and
                 upgrading referred to in the preceding sentence as may be
                 necessary, the computer and management information systems of
                 the Borrowers are, and with ordinary course upgrading and
                 maintenance will continue for the term of the Agreement to be,
                 sufficient to permit the Borrowers to conduct their business
                 without Material Adverse Effect.

                 4.       Amendments to Section 7.  Section 7 is hereby amended
as follows:

                 (a)  deleting paragraph (c) of subsection 7.1 thereof in its
entirety and substituting in lieu thereof the following:





<PAGE>   7
                                                                               7




                 "(c)  Leverage Ratio.  Permit the Leverage Ratio for any
         period of four consecutive fiscal quarters ending during any period
         set forth below to be greater than the ratio set forth opposite such
         period below:

<TABLE>
<CAPTION>
                             Date                                   Ratio
                             ----                                   -----
                     Amendment Effective Date -
                     <S>                                             <C>
                     9/29/99                                         4.00
                     9/30/99 - 9/29/00                               3.50
                     9/30/00 - 9/29/01                               3.25
                     9/30/01 - thereafter                            3.00."
</TABLE>            

                  (b)  by adding the following at the end of subsection 7.1
thereof:

                          "(d)  Senior Debt Ratio.  Permit the Senior Debt
                  Ratio for any period of four consecutive fiscal quarters
                  ending during any period set forth below to be greater than
                  the ratio set forth opposite such period below:

<TABLE>
<CAPTION>
                             Date                                       Ratio
                             ----                                       -----
                     Amendment Effective Date -
                     <S>                                               <C>
                     9/29/98                                             3.75
                     9/30/98 - 9/29/99                                 3.50
                     9/30/99 - 9/29/00                                 3.00
                     9/30/00 - 9/29/01                                 2.75
                     9/30/01 - thereafter                              2.50."
</TABLE>             

                  (c)  by adding the following paragraph (i) to subsection 7.2
thereof and relettering the current paragraph (i) as paragraph (j):

                          "(i) additional Permitted Indebtedness in an
                  aggregate principal amount not to exceed $300,000,000, so
                  long as at the time of incurrence of such Permitted
                  Indebtedness no Default or Event of Default shall have
                  occurred or would result therefrom; and"

                  5.      Amendments to Section 11.  (a)  Section 11 of the
Credit Agreement is hereby amended as follows:

                  (a)  by deleting subsection 11.6(c) thereof in its entirety
and substituting in lieu thereof the following:

                          "(c) Any Lender may, in the ordinary course of its
                  commercial banking business and in accordance with applicable
                  law, at any time and from time to time assign to any Lender
                  or any Affiliate thereof or, with the consent of the
                  Administrative Agent and the Borrower (which shall not be
                  unreasonably withheld), to an additional bank or financial
                  institution ("an Assignee") all or any part of its rights and
                  obligations under this Agreement and any Note (provided





<PAGE>   8
                                                                               8





                  that any such assignment (i) must be in a minimum amount of
                  the lesser of (A) $10,000,000 and (B) the full amount of the
                  assigning Lender's applicable Commitment and (ii) with
                  respect to assignments of Term Loans or Revolving Credit
                  Commitments (other than assignments to an Affiliate of the
                  assigning Lender), shall be a sale of all or a ratable
                  portion of each of the Term Loans or Revolving Credit Loans
                  of the Subsidiary Borrowers, as the case may be, held by such
                  assigning Lender and its Affiliates) pursuant to an
                  Assignment and Acceptance, substantially in the form of
                  Exhibit I, executed by such Assignee, such assigning Lender
                  (and, in the case of an Assignee that is not then a Lender or
                  an Affiliate thereof, by the Administrative Agent and the
                  Borrower) and delivered to the Administrative Agent for its
                  acceptance and recording in the Register.  Upon such
                  execution, delivery, acceptance and recording, from and after
                  the effective date determined pursuant to such Assignment and
                  Acceptance, (x) the Assignee thereunder shall be a party
                  hereto and, to the extent provided in such Assignment and
                  Acceptance, have the rights and obligations of a Lender
                  hereunder with a Commitment as set forth therein, and (y) the
                  assigning Lender thereunder shall, to the extent provided in
                  such Assignment and Acceptance, be released from its
                  obligations under this Agreement (and, in the case of an
                  Assignment and Acceptance covering all or the remaining
                  portion of an assigning Lender's rights and obligations under
                  this Agreement, such assigning Lender shall cease to be a
                  party hereto)."

                  (b)  by deleting subsection 11.6(e) thereof in its entirety
and substituting in lieu thereof the following:





<PAGE>   9
                                                                               9




                          "(e) Upon its receipt of an Assignment and Acceptance
                  executed by an assigning Lender and an Assignee (and, in the
                  case of an Assignee that is not then a Lender or an Affiliate
                  thereof, by the Administrative Agent and the Borrower)
                  together with payment to the Administrative Agent by the
                  assigning Lender or such Assignee of a registration and
                  processing fee of $4,000 (or $1,000 in the case of an
                  Assignee that is already a Lender), the Administrative Agent
                  shall (i) promptly accept such Assignment and Acceptance and
                  (ii) on the effective date determined pursuant thereto record
                  the information contained therein in the Register and give
                  notice of such acceptance and recordation to the Lenders and
                  the Parent.  On or prior to such effective date, each of the
                  Borrowers, at its own expense, shall execute and deliver to
                  the Administrative Agent (in exchange for the Revolving
                  Credit Note, CAF Advance Note, Term Note or Additional Term
                  Note of the assigning Lender) a new Revolving Credit Note,
                  CAF Advance Note, Term Note or Additional Term Note, as the
                  case may be, to the order of such Assignee in amounts
                  reflecting the Revolving Credit Commitment, Term Loan or
                  Additional Term Loan, as the case may be, assumed by it
                  pursuant to such Assignment and Acceptance and, if the
                  assigning Lender has retained a Revolving Credit Commitment
                  or Term Loan or Additional Term Loan hereunder, a new
                  Revolving Credit Note, CAF Advance Note, Term Note or
                  Additional Term Note, as the case may be, to the order of the
                  assigning Lender in amounts reflecting the Revolving Credit
                  Commitment, Term Loan or Additional Term Loan, as the case
                  may be, retained by it hereunder.  Such new Notes shall be
                  dated the Amendment Effective Date, and shall otherwise be in
                  the form of the Note replaced thereby";

                  (c)  by deleting the reference to "Term Loans" in subsection
11.7(a) and substituting in lieu thereof a reference to "Term Loans, Additional
Term Loan".

                          6.      Amendment to Schedules to the Credit
Agreement.  Schedules I, IV, 4.15A, 4.15B and 4.19 to the Credit Agreement are
hereby amended by deleting such Schedules in their entirety and substituting in
lieu thereof the Schedules attached hereto as Annex A.

                          7.      Exhibits to the Credit Agreement.  Exhibits
D, E, F and I to the Credit Agreement are hereby amended by deleting such
Exhibits in their entirety and substituting in lieu thereof Exhibits A, B, C
and D hereto.  Exhibit B-1 to the Credit Agreement is hereby added thereto in
the form of Exhibit E hereto.

                  III.  Conditions to Effectiveness.  This Amendment shall
become effective on the date (the "Amendment Effective Date") on which all of
the following conditions precedent have been satisfied or waived:

                  1.  The Parent, the Subsidiary Borrowers, the Administrative
Agent and the Lenders (as required by the Credit Agreement) shall have executed
and delivered to the Administrative Agent this Amendment.





<PAGE>   10
                                                                              10




                  2.  The Administrative Agent shall have received a copy of
the resolutions, in form and substance satisfactory to the Administrative
Agent, of the Board of Directors of each of the Parent and the Subsidiary
Borrowers authorizing the execution, delivery and performance of this
Amendment, certified by the Secretary or an Assistant Secretary of such party
as of the Amendment Effective Date, which certificate shall state that the
resolutions thereby certified have not been amended, modified, revoked or
rescinded as of the date of such certificate.

                  3.  The Administrative Agent shall have received, to the
extent that it has not theretofore received, a certificate of the Secretary or
Assistant Secretary of the Parent and the Subsidiary Borrowers, dated the
Amendment Effective Date, as to the incumbency and signature of each of the
officers signing this Amendment, and any other instrument or document delivered
by such party in connection herewith, together with evidence of the incumbency
of such Secretary or Assistant Secretary.

                  4.  The Administrative Agent shall have received a written
legal opinion of counsel to the Parent and the Subsidiary Borrowers in form and
substance satisfactory to the Administrative Agent and its counsel.

                  5.  The Administrative Agent and the Lenders shall have
received all fees due from the Parent.

                  IV.     General.

                  1.      Representation and Warranties.  To induce the
Administrative Agent and the Lenders parties hereto to enter into this
Amendment, the Parent and the Subsidiary Borrowers hereby represent and warrant
to the Administrative Agent and all of the Lenders as of the Amendment
Effective Date that the representations and warranties made by the Loan Parties
in the Loan Documents are true and correct in all material respects on and as
of the Amendment Effective Date, before and after giving effect to the
effectiveness of this Amendment, as if made on and as of the Amendment
Effective Date and no Default or Event of Default shall have occurred and be
continuing.

                  2.   Payment of Expenses.  The Parent and the Subsidiary
Borrowers agree to pay or reimburse the Administrative Agent for all of its
out-of-pocket costs and reasonable expenses incurred in connection with this
Amendment, any other documents prepared in connection herewith and the
transactions contemplated hereby, including, without limitation, the reasonable
fees and disbursements of counsel to the Administrative Agent.

                  3.      No Other Amendments; Confirmation.  Except as
expressly amended, modified and supplemented hereby, the provisions of the
Credit Agreement and the Notes are and shall remain in full force and effect.
This Amendment shall constitute a Loan Document.

                  4.      Governing Law; Counterparts.  (a)  This Amendment and
the rights and obligations of the parties hereto shall be governed by, and
construed and interpreted in accordance with, the laws of the State of New
York.





<PAGE>   11
                                                                              11




                  (b)     This Amendment may be executed by one or more of the
parties to this Agreement on any number of separate counterparts, and all of
said counterparts taken together shall be deemed to constitute one and the same
instrument.  This Amendment may be delivered by facsimile transmission of the
relevant signature pages hereof.

                  5.      Confirmation of Guarantees and Security Documents.
The Loan Parties hereby confirm and agree that the obligations of NNI in
respect to the Additional Term Loans are guaranteed and secured as Obligations
to the fullest extent permitted by the Guarantees and Security Documents.





<PAGE>   12
                                                                              12



         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed and delivered by their respective proper and duly authorized
officers as of the day and year first above written.

                                             SYBRON INTERNATIONAL CORPORATION


                                             By:                  
                                                -------------------------------
                                                Title:


                                             ORMCO CORPORATION


                                             By:                  
                                                -------------------------------
                                                Title:




                                             KERR CORPORATION


                                             By:                  
                                                -------------------------------
                                                Title:




                                             NALGE NUNC INTERNATIONAL
                                               CORPORATION


                                             By:                  
                                                -------------------------------
                                                Title:




                                             ERIE SCIENTIFIC COMPANY


                                             By:                  
                                                -------------------------------
                                                Title:

<PAGE>   13
                                                                              13


                                             BARNSTEAD THERMOLYNE CORPORATION



                                             By:                  
                                                -------------------------------
                                                Title:


                                    
                                             THE CHASE MANHATTAN BANK, as     
                                              Administrative Agent and as a   
                                              Lender                          
                                                                              
                                                                              
                                             By:                              
                                                -------------------------------
                                                Title:                       
                                                                             
                                                                             
                                             ABN AMRO BANK N.V.              
                                                                             
                                                                             
                                                                             
                                             By:                             
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             BANK OF AMERICA NATIONAL TRUST AND
                                              SAVINGS ASSOCIATION              
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                             BANK OF MONTREAL                  
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               




<PAGE>   14
                                                                              14



                                             BANK ONE, WISCONSIN               
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                                                               
                                             THE BANK OF NOVA SCOTIA           
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             BANK OF SCOTLAND                  
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             NATEXIS BANQUE/BFCE               
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                  





<PAGE>   15
                                                                              15



                                             PARIBAS                           
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             CREDIT AGRICOLE INDOSUEZ          
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             COMERICA BANK                     
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               
                                                                               
                                                                               
                                             CREDIT LYONNAIS CHICAGO BRANCH    
                                                                               
                                                                               
                                                                               
                                             By:                               
                                                -------------------------------
                                                Title:                         
                                                                               





<PAGE>   16
                                                                              16



                                        DG BANK DEUTSCHE GENOSSENSCHAFTSBANK   
                                         CAYMAN ISLAND BRANCH                  
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        U.S. BANK NATIONAL ASSOCIATION         
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        FLEET NATIONAL BANK                    
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        THE FUJI BANK, LIMITED                 
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                                                               
                                                                               
                                                                               
                                        THE LONG-TERM CREDIT BANK OF JAPAN,    
                                         LTD.                                  
                                                                               
                                                                               
                                                                               
                                        By:                                    
                                           -------------------------------     
                                           Title:                              
                                       
                                       
                                       







<PAGE>   17
                                                                              17





                                  
                                        MELLON BANK, N.A.                 
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                         
                                        THE BANK OF TOKYO-MITSUBISHI, LTD.,
                                         CHICAGO BRANCH                   
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE MITSUBISHI TRUST AND BANKING  
                                         CORPORATION                      
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE FIRST NATIONAL BANK OF CHICAGO
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE SAKURA BANK, LIMITED          
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         







<PAGE>   18
                                                                              18




                                        SOCIETE GENERALE                  
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE SUMITOMO BANK, LTD. CHICAGO   
                                         BRANCH                           
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE BANK OF NEW YORK              
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE SANWA BANK, LIMITED           
                                                                          
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                  








<PAGE>   19
                                                                              19




                                        BANQUE NATIONALE DE PARIS CHICAGO 
                                         BRANCH                           
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        BHF BANK AKTIENGESELLSCHAFT GRAND 
                                         CAYMAN BRANCH                    
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        FIRST UNION NATIONAL BANK         
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                        FIRSTAR BANK MILWAUKEE, N.A.      
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        THE INDUSTRIAL BANK OF JAPAN,     
                                          LIMITED CHICAGO BRANCH          
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                  
                                  







<PAGE>   20
                                                                              20





                                        BANCA COMMERCIALE ITALIANA        
                                          CHICAGO BRANCH                  
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                        BANK HAPOALIM, B.M. PHILADELPHIA  
                                         BRANCH                           
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         
                                                                          
                                                                          
                                        By:                               
                                           -------------------------------
                                           Title:                         



<PAGE>   21
                                                                              21



Acknowledged and Agreed:

BARNSTEAD THERMOLYNE CORPORATION
ERIE SCIENTIFIC COMPANY
ERIE SCIENTIFIC COMPANY OF
  PUERTO RICO
EVER READY THERMOMETER CO., INC.
RICHARD-ALLAN SCIENTIFIC COMPANY
NEW ENGLAND REAGENT LABORATORY, INC.
CASCO STANDARDS, INC.
THE NAUGATUCK GLASS COMPANY
NALGE NUNC INTERNATIONAL CORPORATION
SYBRON DENTAL SPECIALTIES INC.
SAC/ORMCO, INC.
ORMCO CORPORATION
ALLESEE ORTHODONTIC APPLIANCES, INC.
SAC/KERR, INC.
KERR CORPORATION
METREX RESEARCH CORPORATION
BELLE DE ST. CLAIRE, INC.
SAC/COMMONWEALTH, INC.
SYBRON COMMONWEALTH HOLDINGS, INC.
SYBRON TRANSITION CORP.
MEXOSERV COMPANY
REMEL INC.
DIAGNOSTIC REGENTS, INC.
SYBRON LABORATORY PRODUCTS CORPORATION
OWL SEPARATION SYSTEMS, INC.
ALEXON-TREND, INC.
VIRO RESEARCH INTERNATIONAL, INC.
CARR-SCARBOROUGH MICROBIOLOGICALS, INC.
CLINICAL STANDARDS LABS, INC.
LIDA MANUFACTURING CORPORATION
LRS ACQUISITION CORP.
"A" COMPANY ORTHODONTICS
NALGE PROCESS TECHNOLOGIES GROUP, INC.
CHASE SCIENTIFIC GLASS, INC.
NATIONAL SCIENTIFIC COMPANY
SUMMIT BIOTECHNOLOGY, INC.



-----------------------------
By:  John J. Buono, Assistant
Treasurer





<PAGE>   22
                                                                      EXHIBIT I
                                                        TO THE CREDIT AGREEMENT

                       FORM OF ASSIGNMENT AND ACCEPTANCE

     Reference is made to the Credit Agreement, dated as of April 25, 1997 
(as amended, supplemented or otherwise modified from time to time, the "Credit
Agreement"), among Sybron International Corporation, a Wisconsin corporation
(the "Parent"), Ormco Corporation, a Delaware corporation, Kerr Corporation, a
Delaware corporation, Nalge Nunc International Corporation, a Delaware
corporation, Erie Scientific Company, a Delaware corporation, Barnstead
Thermolyne Corporation, a Delaware corporation, the Lenders named therein, Chase
Securities Inc., as arranger, and The Chase Manhattan Bank, as administrative
agent for the Lenders (in such capacity, the "Administrative Agent"). Unless
otherwise defined herein, terms defined in the Credit Agreement and use herein
shall have the meanings given to them in the Credit Agreement.

     ________ (the "Assignor") and ________ (the "Assignee") agree as follows:

     1.   The Assignor hereby irrevocably sells and assigns to the Assignee
          without recourse to the Assignor, and the Assignee hereby irrevocably
          purchases and assumes from the Assignor without recourse to the
          Assignor, as of the Effective Date (as defined below), a ____%
          interest (the "Assigned Interest") in and to the Assignor's rights
          and obligations under the Credit Agreement with respect to those
          credit facilities contained in the Credit Agreement as are set forth
          on SCHEDULE 1 (individually, an "Assigned Facility"; collectively,
          the "Assigned Facilities"), in a principal amount for each Assigned
          Facility as set forth on SCHEDULE 1.

     2.   The Assignor (A) makes no representation or warranty and assumes no
          responsibility with respect to any statements, warranties or
          representations made in or in connection with the Credit Agreement or
          the execution, legality, validity, enforceability, genuineness,
          sufficiency or value of the Credit Agreement, any other Loan Document
          or any other instrument or document furnished pursuant thereto, other
          than that it has not created any adverse claim upon the interest
          being assigned by it hereunder and that such interest is free and
          clear of any such adverse claim; (B) makes no representation or
          warranty and assumes no responsibility with respect to the financial
          condition of the Parent, any of its Subsidiaries or any other obligor
          or the performance or observance by the Parent, any of its
          Subsidiaries or any other obligor of any of their respective
          obligations under the Credit Agreement or any other Loan Document or
          any other instrument or document furnished pursuant hereto or
          thereto; and (C) attaches the Note(s) held by it evidencing the
          Assigned Facilities and requests that the Administrative Agent
          exchange such Note(s) for a new Note or Notes payable to the Assignee
          and (if the Assignor has retained any interest in the Assigned
          Facility) a new Note or Notes payable to the Assignor in the
          respective amounts which reflect the assignment being made hereby
          (and after giving effect to any other assignments
 
<PAGE>   23
                                                                               2



     which have become effective on the Effective Date).

3.   The Assignee (A) represents and warrants that it is legally authorized to
     enter into this Assignment and Acceptance; (B) confirms that it has
     received a copy of the Credit Agreement, together with copies of the
     financial statements delivered pursuant to subjection 4.1 thereof and such
     other documents and information as it has deemed appropriate to make its
     own credit analysis and decision to enter into this Assignment and
     Acceptance; (C) agrees that it will, independently and without reliance
     upon the Assignor, the Administrative Agent or any other Lender and based
     on such documents and information as it shall deem appropriate at the time,
     continue to make its own credit decisions in taking or not taking action
     under the Credit Agreement, the other Loan Documents or any other
     instrument or document furnished pursuant hereto or thereto; (D) appoints
     and authorizes the Administrative Agent to take such action as agent on its
     behalf and to exercise such powers and discretion under the Credit
     Agreement, the other Loan Documents or any other instrument or document
     furnished pursuant hereto or thereto as are delegated to the Administrative
     Agent by the terms thereof, together with such powers as are incidental
     thereto; and (E) agrees that it will be bound by the provisions of the
     Credit Agreement and will perform in accordance with its terms all the
     obligations which by the terms of the Credit Agreement are required to be
     performed by it as a Lender including, if it is a Foreign Lender, its
     obligation pursuant to paragraph 2.20(b) of the Credit Agreement.

4.   The effective date of this Assignment and Acceptance shall be ________ ,
     19___ (the "Effective Date"). Following the execution of this Assignment
     and Acceptance, it will be delivered to the Administrative Agent for
     acceptance by it and recording by the Administrative Agent pursuant to
     subsection 11.6 of the Credit Agreement, effective as of the Effective Date
     (which shall not, unless otherwise agreed to by the Administrative Agent,
     be earlier than five Business Days after the date of such acceptance and
     recording by the Administrative Agent).

5.   Upon such acceptance and recording, from and after the Effective Date, the
     Administrative Agent shall make all payments in respect of the Assigned
     Interest (including payments of principal, interest, fees and other
     amounts) to the Assignee whether such amounts have accrued prior to the
     Effective Date or accrue subsequent to the Effective Date. The Assignor and
     the Assignee shall make all appropriate adjustments in payments by the
     Administrative Agent for periods prior to the Effective Date or with
     respect to the making of this assignment directly between themselves.

6.   From and after the Effective Date, (A) the Assignee shall be a party to the
     Credit Agreement and, to the extent provided in this Assignment and
     Acceptance, have the rights and obligations of a Lender thereunder and
     under the other Loan Documents and shall be bound by the provisions thereof
     and (B) the Assignor shall, to the extent provided in this Assignment and
     Acceptance, relinquish its
<PAGE>   24
                                                                              3

          rights and be released from its obligations under the Credit 
          Agreement.

     7.   This Assignment and Acceptance shall be governed by and construed in
          accordance with the laws of the State of New York.

     IN WITNESS WHEREOF, the parties hereto have caused this Assignment and
Acceptance to be executed as of the date first above written by their
respective duly authorized officers on Schedule 1 hereto.
<PAGE>   25
                                                                              3

                          TO ASSIGNMENT AND ACCEPTANCE
         RELATING TO THE CREDIT AGREEMENT, DATED AS OF APRIL 25, 1997,
                                     AMONG
     SYBRON INTERNATIONAL CORPORATION, ORMCO CORPORATION, KERR CORPORATION,
         NALGE NUNC INTERNATIONAL CORPORATION, ERIE SCIENTIFIC COMPANY,
                       BARNSTEAD THERMOLYNE CORPORATION,
                           THE LENDERS NAMED THEREIN,
                    CHASE SECURITIES INC., AS ARRANGER, 
                                      AND
               THE CHASE MANHATTAN BANK, AS ADMINISTRATIVE AGENT
                                FOR THE LENDERS

Name of Assignor:

Name of Assignee:

Effective Date of Assignment:

           Credit               Principal                 Term Loan
  Facility Assigned ERROR!   Amount Assigned     Commitment Percentage and/or
                                 ERROR!      Revolving Credit Commitment Percent
                                                     Assigned(1) ERROR!
                               $________                ________%

             [Name of Assignee]                   [Name of Assignor]

  By                                         By
  Name:                                      Name:
  Title:                                     Title:

  Accepted:

          THE CHASE MANHATTAN BANK,          SYBRON INTERNATIONAL
           as Administrative Agent           CORPORATION

  By                                         By
  Name:                                      Name:
                                             Title:
<PAGE>   26
                                                                              4

Title:

     Calculate the relevant Commitment Percentage that is assigned to at least
15 decimal places and show percentage of the aggregate revolving credit
commitments and/or term loan commitments of all the Lenders.