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                                                                    EXHIBIT 4(M)

                               Second Amendment to
                            Stock Purchase Agreement

      This Second Amendment Agreement is made as of the 15th of May, 1997 among
Westinghouse Electric Corporation, a Pennsylvania corporation ("WEC"), Rollins
Environmental Services, Inc., a Delaware corporation ("RESI"), and Laidlaw,
Inc., a Canadian corporation ("Laidlaw"). 

                                    Recitals

      WEC and RESI entered into a Stock Purchase Agreement dated as of March 7,
1995 ("Purchase Agreement") wherein RESI purchased from WEC all of the stock of
National Electric, Inc., a Minnesota corporation ("NEI") and Aptus, Inc., a
Delaware corporation ("Aptus"), a wholly-owned subsidiary of NEI.

      WEC and RESI executed an Amendment to Stock Purchase Agreement dated
September 22, 1995 ("First Amendment").

      WEC, subject to the terms of the Purchase Agreement agreed to provide
letters of credit and reimburse RESI for letters pursuant to the terms of
Section 5.8 Closure and Post-Closure Costs and Financial Assurances ("Closure
LOC").

      WEC, subject to the terms of the Purchase Agreement, further agreed to
provide a letter of credit ("Bond LOC") to support repayment of those certain
Variable Rate




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Hazardous Waste Treatment Revenue Bonds ("Rollins Environmental Services, Inc.,
Project"), Series A, issued by Tooele County, Utah such bonds having a face
amount of Forty-Seven Million Five Hundred Thousand Dollars ($47,500,000)
("Tax-Exempt Bonds").

      WEC and RESI entered into that certain Side Letter Agreement dated May 31,
1995 dealing with the Remarketing of the Tax Exempt Bonds ("Side Agreement").

      The Tax Exempt Bonds were originally issued as of June 1, 1990, remarketed
under the First Supplemental Indenture of Trust dated as of May 1, 1995 and
subsequently remarketed on December 5, 1996 supported by an LOC in the original
principal amount of $52,631,167 (U.S. Dollars) issued by ABN AMRO Bank N.W.
acting through its Pittsburgh Branch.

      RESI has entered into a purchase agreement with Laidlaw, Inc. ("Laidlaw")
to purchase all of the stock Laidlaw Environmental Services Inc. ("LESI") and
Laidlaw Environmental Services Ltd. ("LESL"), both of which are wholly-owned
subsidiaries of Laidlaw, Inc. The surviving entity will be renamed Laidlaw
Environmental Services, Inc.

      WEC wishes to be released from the obligations under Sections 5.8, 7.8 and
the Side Agreement and any other obligations associated with the Closure LOC and
the Bond LOC; and



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      NOW THEREFORE, WEC, RESI and Laidlaw for good and valuable consideration
and intending to be legally bound hereby agree as follows:

      1)    WEC is released of any obligation to provide Closure LOCs pursuant
to Section 5.8 of the Purchase Agreement for the facilities of RESI located at
Aragonite, Utah, Coffeyville, Kansas and Lakeville, Minnesota.

      2)    WEC shall be reimbursed for any amounts it has paid in connection
with the providing of such Closure LOCs that has not been previously reimbursed
by RESI to WEC pursuant to 5.8 of the Purchase Agreement.

      3)    RESI and Laidlaw agree to cause the release cancellation and return
of the Closure LOC on the date that RESI and Laidlaw close the purchase
agreement that those parties have entered into for the stock of LESI and LESL
("Closing Date"); provided, however, RESI and Laidlaw may if needed and after
prior written notice to WEC, have an additional 30 days from the Closing Date to
replace such Closure LOC.

      4)    Laidlaw shall indemnify and hold WEC harmless from and against any
damages, liabilities, costs or expenses suffered or incurred by WEC in
connection with the Closure LOC, including such amounts under paragraph 2 above
and, including but not limited to, any claims against the Closure LOC if such
Closure LOC is not replaced on the Closing Date.


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      5)    WEC is released of any obligations to provide Bond LOC pursuant to
Section 7.8 of the Purchase Agreement in connection with the Tax Exempt Bonds.

      6)    WEC shall be reimbursed for any amounts it has paid in connection
with the providing of such Bond LOC that has not been previously reimbursed by
RESI to WEC.

      7)    RESI and Laidlaw agree to cause the release cancellation and return
of the Bond LOC on the Closing Date; provided, however, RESI and Laidlaw may if
needed and after prior written notice to WEC, have an additional 30 days from
the Closing Date to replace such Bond LOC.

      8)    Laidlaw shall indemnify and hold WEC harmless from and against any
damages, liabilities, costs and expenses suffered or incurred by WEC in
connection with the Bond LOC, including such amounts under Paragraph 6 above
and, including but not limited to, any claims against the Bond LOC if such Bond
LOC is not replaced on the Closing Date.

      9)    The failure to comply with the provisions of this Second Amendment
shall be a default under that Certain Promissory Note of even date herewith
having WEC as payee and RESI as obligor in the original principal amount of
$60,000,000.



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      10)   This Agreement shall be binding upon the successors and assigns
whether by contract, merger or operation of law of WEC, RESI and Laidlaw;
provided, however, Laidlaw shall not be permitted to transfer or assign its
obligations hereunder without the prior written consent of WEC.

      11)   RESI represents and warrants that:

            (a) Corporate Existence and Authority. RESI is a corporation validly
incorporated and existing in good standing under the laws of its jurisdiction of
incorporation and is duly qualified and authorized to do business in each
jurisdiction where such qualification is necessary or where failure to be
qualified would have a material adverse effect on its doing business.

            (b) Corporate Authority. RESI has the legal right corporate power
and corporate authority to enter into this Agreement and to do all acts and
things and execute and deliver all other documents and instruments as are
required hereunder or thereunder to be done, observed or performed by it in
accordance with the terms hereof and thereof.

            (c) Valid Authorization of Credit Documents. RESI has taken all
necessary corporate action to authorize the execution, delivery and performance
of this Agreement and the execution, delivery and performance of this Agreement
or any other Credit Document by such Borrower will not (i) require the consent
or approval of any governmental entity having jurisdiction over it, except such
as have already been obtained and are in full force and effect; (ii) contravene
or conflict with the articles, by-



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laws or resolutions of directors or shareholders of RESI or the provisions of
any judgment, order, statute, regulation, indenture, instrument, agreement or
undertaking to which RESI is a party or by which RESI is or may become bound; or
(iii) oblige RESI to grant, or will result in the creation of, security interest
in favour of any Person.

            (d) Validity and Binding Effect. This Agreement constitutes a valid
and legally binding obligation of RESI enforceable against it in accordance with
their terms.

      12)   Laidlaw represents and warrants that:

            (a) Corporate Existence and Authority. LAIDLAW is a corporation
validly incorporated and existing in good standing under the laws of its
jurisdiction of incorporation and is duly qualified and authorized to do
business in each jurisdiction where such qualification is necessary or where
failure to be qualified would have a material adverse effect on its doing
business.

            (b) Corporate Authority. LAIDLAW has the legal right, corporate
power and corporate authority to enter into this Agreement and to do all acts
and things and execute and deliver all other documents and instruments as are
required hereunder or thereunder to be done. observed or performed by it in
accordance with the terms hereof and thereof.

            (c) Valid Authorization of Credit Documents. LAIDLAW has taken all
necessary corporate action to authorize the execution, delivery and performance
of this Agreement and the execution, delivery and performance of this agreement
or any other Credit Document by such Borrower will not (i) require the consent
or approval of any 


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governmental entity having jurisdiction over it, except such as have already
been obtained and are in full force and effect; (ii) contravene or conflict with
the articles, bylaws or resolutions of directors or shareholders of LAIDLAW or
the provisions o~ any judgment, order, statute, regulation, indenture,
instrument, agreement or undertaking to which LAIDLAW is a party or by which
LAIDLAW is or may become bound; or (iii) oblige LAIDLAW to grant, or will result
in the creation of ? security interest in favour of any Person.

            (d) Validity and Binding Effect. This Agreement constitutes a valid
and legally binding obligation of LAIDLAW enforceable against it in accordance
with their terms.

      13)   This Agreement shall be construed in accordance with and governed by
the laws of the State of New York to agreements made and to be performed wholly
within such state. All of the parties agree to the exclusive jurisdiction and
venue in the courts of the United States sitting in the City of New York, New
York.

      IN WITNESS HEREOF, the Parties have executed this Agreement as of the date
first written above.

                    WESTINGHOUSE ELECTRIC COROCORPORATION (WEC)
                    a Pennsylvania corporation
     
                    By: /s/ C.E. Mort
                       -------------------------------------
                    Its: C.E. Mort
                        ------------------------------------
                        Vice President

                    ROLLINS ENVIRONMENTAL SERVICES, INC. (RESI)
                    A Delaware corporation
                    
                    By: /s/ John Rollins, Jr.
                        -----------------------------------
                        John Rollins, Jr.

                    Its:
                         ----------------------------------


                    LAIDLAW INC. (Laidlaw)
                    a Canadian corporation

                    By: /s/ Ivan Cairns
                       -----------------------------------
                    Its: Senior Vice President
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