EXHIBIT 10.21

                          AMENDMENT TO CREDIT AGREEMENT

THIS SECOND  AMENDMENT  ("Amendment")  is made  effective  as of the 17th day of
January,  1997, by and between EMCON  ("Borrower") and Union Bank of California,
N.A. ("Bank").

                                    RECITALS

A.             Borrower is currently  indebted to Bank pursuant to the terms and
               conditions of that certain  Credit  Agreement  dated February 29,
               1996 and amended on September 13, 1996 (the "Agreement");

B.             Borrower  and Bank have agreed to amend the  Agreement to reflect
               certain changes in the terms and conditions set forth therein.

C.             All  references  to the  "Prime  Rate" in all the Loan  Documents
               shall  be  deemed  to be  references  to  the  "Reference  Rate".
               "Reference  Rate" shall mean the rate announced by Bank from time
               to time at its corporate  headquarters as its Reference Rate. The
               Reference  Rate is an index rate  determined by Bank from time to
               time as a means of pricing  certain  extensions  of credit and is
               neither  directly  tied to any external rate of interest or index
               nor  necessarily  the lowest rate of interest  charged by Bank at
               any given time.

NOW, THEREFORE, the parties hereto agree as follows:

1.             Section  5.2(b) of the Agreement  the amount  Forty-One
               Million Dollars ($41,000,000) is deleted and the amount
               Thirty Six Million Dollars ($36,000,000) is substituted
               therefore.

2.             Section  6.8 of the  Agreement  the amount Six  Million
               Dollars  ($6,000,000)  is deleted and the amount  Three
               Million Five Hundred Thousand  Dollars  ($3,500,000) is
               substituted therefore.

3.             Upon the sale of Columbia  Analytical  Services,  Inc.,
               under the  terms of that  Agreement  between  EMCON and
               Columbia Analytical  Services,  Inc., EMCON will reduce
               the outstanding term loan with the Bank by $3,000,000.

                          GENERAL AMENDMENT PROVISIONS

A. Except as specifically provided herein, all terms and conditions of
the  Agreement  remain in full  force and  effect,  without  waiver or
modification.  All terms defined in the Agreement  shall have the same
meaning  when  used in this  Amendment,  and  this  Amendment  and the
Agreement shall be read together as one document. Where any provisions
of the Agreement amended by this Amendment appear in a promissory note
tied to the Agreement,  the same  provisions in said  promissory  note
shall be deemed likewise amended.

B.  Borrower  hereby  confirms  all   representations  and  warranties
contained in the  Agreement  and  reaffirms  all  covenants  set forth
therein.  Further,  Borrower  certifies  that,  as of the date of this
Amendment,  there  exists  no  Event  of  Default  as  defined  in the
Agreement,  nor any  condition,  act or event which with the giving of
notice or the  passage  of time or both would  constitute  an Event of
Default.

IN WITNESS  WHEREOF,  the parties hereto have caused this amendment to
become effective as of the date and year first written above.

Union Bank of California, N.A.            EMCON

By:     /s/ William Hinch                 By:   /s/ R. Michael Momboisse
        --------------------                    ------------------------
        William Hinch                           R. Michael Momboisse

Title:  Vice President                    Title:  Chief Financial Officer &
                         Vice President Legal

                                          Dated:  1/28/97





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