THIS SECOND  SUPPLEMENTAL  INDENTURE,  effective as of May 7, 1999, among
Safety-Kleen  Services,  Inc. (formerly LES, Inc.), a Delaware  corporation (the
"Company"),  Safety-Kleen Corp. (formerly Laidlaw Environmental Services, Inc.),
a  Delaware  corporation  (the  "Parent  Guarantor"),  SK  Services,  L.C.  ("SK
Services") and SK Services (East),  L.C. ("SK Services East"),  both SK Services
and SK Services East are Utah limited liability companies,  and The Bank of Nova
Scotia Trust Company of New York, as trustee (the "Trustee").

       WHEREAS,  the Company,  the  Guarantors,  and the Trustee entered into an
Indenture dated as of May 29, 1998 (the "Indenture") to provide for the issuance
of the Company's 9 1/4 % Senior Subordinated Notes due 2008;

       WHEREAS,  the Company,  the Parent Guarantor,  the Trustee and SK Europe,
Inc.  entered into the First  Supplement  Indenture dated December  18,1998 (the
"First Supplemental Indenture");

       WHEREAS,  Safety-Kleen  (Delaware),  Inc., a  wholly-owned  subsidiary of
Safety-Kleen  (Lone  and  Grassy  Mountain),   Inc.,  which  is  a  wholly-owned
subsidiary of the Company,  acquired an  additional  20% ownership of each of SK
Services  and SK Services  East so that it now holds 100% of each of SK Services
and SK Services East;

       WHEREAS, pursuant to Sections 1307 and 1308 of the Indenture, SK Services
and SK Services East, as Subsidiary Guarantors,  are each required to enter into
this Second Supplemental Indenture (the "Second Supplemental Indenture");

       WHEREAS, the Company, the Parent Guarantor, SK Services, SK Services East
and the Trustee are authorized to enter into this Second Supplemental Indenture;

       NOW,  THEREFORE,  for and in consideration of the premises and the mutual
covenants contained in this Second Supplemental Indenture and for other good and
valuable  consideration,  the  receipt  and  sufficiency  of  which  are  herein
acknowledged,  the Company, the Parent Guarantor,  SK Services, SK Services East
and the  Trustee  hereby  agree  for the equal and the  ratable  benefit  of all
Holders of the Securities as follows:


                                   ARTICLE ONE

                                   Definitions
                                   -----------

       1.1 Definitions.  For purposes of this Second Supplemental Indenture, the
terms defined in the recitals  shall have the meanings  therein  specified;  any
terms  defined  in the  Indenture  and not  defined  herein  shall have the same
meanings  herein as therein  defined;  and  references  to  Articles or Sections
shall,  unless the context  indicates  otherwise,  be  references to Articles or
Sections of the Indenture.




                                   ARTICLE TWO

                  GUARANTEES OF SECURITIES AND OTHER PROVISIONS
                  ---------------------------------------------

       2.1 SK Services Guarantee.
           ---------------------

       (a)  SK  Services  hereby  jointly  and  severally,   fully,  absolutely,
unconditionally  and  irrevocably  guarantees  to  each  Holder  of  a  Security
authenticated  and delivered by the Trustee,  and to the Trustee for its benefit
and the benefit of each Holder, the punctual payment and performance when due of
all Indenture Obligations which, for purposes of its Securities Guarantee, shall
also be  deemed to  include  all  commissions,  fees,  charges,  costs and other
expenses (including  reasonable legal fees and disbursements of counsel) arising
out of or  incurred  by the  Trustee  or the  Holders  in  connection  with  the
enforcement of any Securities Guarantee.  Without limiting the generality of the
foregoing,  SK Services'  liability  shall extend to all amounts that constitute
part of the  Indenture  Obligations  and  would be owed by the  Company  to such
Holder or the Trustee  under the  Securities  or the  Indenture but for the fact
that they are unenforceable, reduced, limited, suspended or not allowable due to
the existence of a bankruptcy,  reorganization or similar  proceeding  involving
the Company.

       (b) SK Services and by its  acceptance  of a Security  each Holder hereby
confirms  that it is the  intention of all such parties that the guarantee by SK
Services  pursuant to its  Securities  Guarantee  not  constitute  a  fraudulent
transfer or conveyance for purposes of the Federal  Bankruptcy Code, the Uniform
Fraudulent  Conveyance Act, the Uniform  Fraudulent  Transfer Act of any similar
federal or state law or the  provisions  of its local law relating to fraudulent
transfer or conveyance.  To effectuate the foregoing intention,  the Holders and
SK Services  hereby  irrevocably  agree that the  obligations  of such Guarantor
under its Securities  Guarantee  shall be limited to the maximum amount as shall
after giving effect to all other contingent and fixed liabilities of SK Services
and after giving effect to any collections from or payments made by or on behalf
of any other  Guarantor in respect of the  obligations  of such other  Guarantor
under its  Securities  Guarantee or pursuant to paragraph (c) of Section 1301 of
the  Indenture,  result in the  obligations  of SK Services under its Securities
Guarantee not constituting a fraudulent  conveyance or fraudulent transfer under
federal or state law.

       (c) SK  Services,  the  Trustee and each  Holder by its  acceptance  of a
Security  hereby agrees that the  Securities  Guarantee of SK Services  provided
hereunder  shall be subject  to all  terms,  provisions  and  conditions  in the
Indenture that relate to a Securities Guarantee (including,  without limitation,
Articles XIII and XIV of the Indenture).  SK Services further agrees to be bound
by, and to comply with, all provisions of the Indenture and Securities Guarantee
that are applicable to a Subsidiary Guarantor.



       3.2 SK Services East Guarantee.
           ---------------------------
       (a) SK Services East hereby  jointly and  severally,  fully,  absolutely,
unconditionally  and  irrevocably  guarantees  to  each  Holder  of  a  Security

authenticated  and delivered by the Trustee,  and to the Trustee for its benefit
and the benefit of each Holder, the punctual payment and performance when due of
all Indenture Obligations which, for purposes of its Securities Guarantee, shall
also be  deemed to  include  all  commissions,  fees,  charges,  costs and other
expenses (including  reasonable legal fees and disbursements of counsel) arising
out of or  incurred  by the  Trustee  or the  Holders  in  connection  with  the
enforcement of any Securities Guarantee.  Without limiting the generality of the
foregoing,  SK  Services  East's  liability  shall  extend to all  amounts  that
constitute part of the Indenture Obligations and would be owed by the Company to
such Holder or the Trustee  under the  Securities  or the  Indenture but for the
fact that they are unenforceable,  reduced, limited,  suspended or not allowable
due to the  existence  of a  bankruptcy,  reorganization  or similar  proceeding
involving the Company.

       (b) SK  Services  East and by its  acceptance  of a Security  each Holder
hereby  confirms that it is the intention of all such parties that the guarantee
by SK Services  East  pursuant to its  Securities  Guarantee  not  constitute  a
fraudulent  transfer or conveyance for purposes of the Federal  Bankruptcy Code,
the Uniform Fraudulent  Conveyance Act, the Uniform  Fraudulent  Transfer Act of
any similar  federal or state law or the provisions of its local law relating to
fraudulent transfer or conveyance.  To effectuate the foregoing  intention,  the
Holders and SK Services East hereby  irrevocably  agree that the  obligations of
such Guarantor  under its Securities  Guarantee  shall be limited to the maximum
amount  as  shall  after  giving  effect  to  all  other  contingent  and  fixed
liabilities of SK Services East and after giving effect to any collections  from
or  payments  made by or on behalf  of any other  Guarantor  in  respect  of the
obligations of such other Guarantor  under its Securities  Guarantee or pursuant
to paragraph (c) of Section 1301 of the Indenture,  result in the obligations of
SK Services East under its  Securities  Guarantee not  constituting a fraudulent
conveyance or fraudulent transfer under federal or state law.

       (c) SK Services  East, the Trustee and each Holder by its acceptance of a
Security  hereby  agrees  that the  Securities  Guarantee  of SK  Services  East
provided  hereunder shall be subject to all terms,  provisions and conditions in
the  Indenture  that  relate  to  a  Securities  Guarantee  (including,  without
limitation,  Articles XIII and XIV of the  Indenture).  SK Services East further
agrees to be bound by, and to comply with,  all  provisions of the Indenture and
Securities Guarantee that are applicable to a Subsidiary Guarantor.

       3.3 Execution and Delivery of Guarantee.
           ------------------------------------

       The  delivery of any Security by the  Trustee,  after the  authentication
thereof under the  Indenture,  shall  constitute  due delivery of the Securities
Guarantee on behalf of SK Services and SK Services East.



       3.4 Amendment of Schedule A.
           ------------------------

       In order to  reflect  the  Securities  Guarantee  of SK  Services  and SK
Services  East,  Schedule  A of  the  Indenture  and  Schedule  A of  the  First
Supplemental  Indenture is hereby replaced by Schedule A attached to this Second
Supplemental Indenture.

       3.5 No Personal Liability.
           ----------------------

       No  stockholder,  officer,  director,  employee  or  incorporator,  past,
present or future,  of SK Services or SK Services East, as such,  shall have any
personal  liability under the Securities  Guarantee by reason of his, her or its
status as such stockholder, officer, director, employee or incorporator.

                                  ARTICLE THREE

                                  Miscellaneous
                                  -------------

       4.1 Effect of the Second Supplemental Indenture. This Second Supplemental
           --------------------------------------------
Indenture  supplements  the First  Supplemental  Indenture and the Indenture and
shall be a part and  subject to all the terms  thereof.  Except as  supplemented
hereby,  the  Indenture,  the First  Supplemental  Indenture and the  Securities
issued thereunder shall continue in full force and effect.

       4.2 Counterparts.  This Second Supplemental  Indenture may be executed in
           ------------
counterparts,  each of which shall be deemed an original, but all of which shall
together constitute one and the same instrument.

       4.3 GOVERNING LAW. THIS SECOND  SUPPLEMENTAL  INDENTURE SHALL BE GOVERNED
           -------------
BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

       4.4 Recitals. The Trustee shall not be responsible for any recital herein
           --------
(other than the sixth  recital as it applies to the  Trustee)  as such  recitals
shall be taken as statements of the Company, or the validity of the execution by
SK Services  or SK Services  East of this  Second  Supplemental  Indenture.  The
Trustee  makes no  representations  as to the  validity or  sufficiency  of this
Second Supplemental Indenture.

       IN  WITNESS   WHEREOF,   the  parties  hereto  have  caused  this  Second
Supplemental Indenture to be duly executed on this 9th day of August 1999.

                                SAFETY-KLEEN SERVICES, INC.

                                By: /s/ Kenneth W. Winger
                                   -------------------------
                                Name:   Kenneth W. Winger
                                Title:  President






                                SAFETY-KLEEN CORP.

                                By: /s/ Kenneth W. Winger
                                   -------------------------
                                Name:   Kenneth W. Winger
                                Title:  President and Chief Executive Officer


                                SK SERVICES, L.C.

                                By:   /s/ Kenneth W. Winger
                                   -------------------------
                                Name:   Kenneth W. Winger
                                Title:  President and Sole Member
                                        Safety-Kleen (Delaware), Inc.


                                SK SERVICES (EAST), L.C.

                                By: /s/ Kenneth W. Winger
                                   -------------------------
                                Name:   Kenneth W. Winger
                                Title:  President and Sole Member
                                        Safety-Kleen (Delaware), Inc.

                                THE BANK OF NOVA SCOTIA TRUST
                                COMPANY OF NEW YORK, as Trustee

                                By:   /s/ George E. Timmes
                                   -------------------------
                                Name:   George E. Timmes
                                Title:  Vice President




                                   SCHEDULE A

                              SUBSIDIARY GUARANTORS

Safety-Kleen Systems, Inc.
Safety-Kleen (Altair), Inc.
Safety-Kleen (Aragonite), Inc.
Safety-Kleen (Bartow), Inc.
Safety-Kleen (Baton Rouge), Inc.
Safety-Kleen (BDT), Inc.
Safety-Kleen (Bridgeport), Inc.
Safety-Kleen (Buttonwillow), Inc.
Safety-Kleen (California), Inc.
Safety-Kleen (Chattanooga), Inc.
Safety-Kleen (Clive), Inc.
Safety-Kleen (Colfax), Inc.
Safety-Kleen (Crowley), Inc.
Safety-Kleen (Custom Transport), Inc.
Safety-Kleen (Deer Park), Inc.
Safety-Kleen (Deer Trail), Inc.
Safety-Kleen (Delaware), Inc.
Safety-Kleen (Encotec), Inc.
Safety-Kleen (FS), Inc.
Safety-Kleen (Gloucester), Inc.
Safety-Kleen (GS), Inc.
Safety-Kleen Holdings, Inc.
Safety-Kleen (La Porte), Inc.
Safety-Kleen (Lone and Grassy Mountain) Inc.
Safety-Kleen (Los Angeles), Inc.
Safety-Kleen (Minneapolis), Inc.
Safety-Kleen (Mt. Pleasant), Inc.
Safety-Kleen (Nashville), Inc.
Safety-Kleen (NE), Inc.
Safety-Kleen (Pecatonica), Inc.
Safety-Kleen (Pinewood), Inc.
Safety-Kleen (Plaquemine), Inc.
Safety-Kleen (PPM), Inc.
Safety-Kleen (Puerto Rico), Inc.
Safety-Kleen (Roebuck), Inc.
Safety-Kleen (Rosemount), Inc.
Safety-Kleen (San Antonio), Inc.
Safety-Kleen (Sawyer), Inc.
Safety-Kleen (TG), Inc.



Safety-Kleen (Tipton), Inc.
Safety-Kleen (TS), Inc.
Safety-Kleen (Tulsa), Inc.
Safety-Kleen (San Jose), Inc.
Safety-Kleen (Sussex), Inc.
Safety-Kleen (White Castle), Inc.
Safety-Kleen (Wichita), Inc.
Safety-Kleen (Westmorland), Inc.
Safety-Kleen (WT), Inc.
Safety-Kleen OSCO Holdings, Inc.
Safety-Kleen Chemical Services, Inc.
SK Europe, Inc.
SK Services, L.C.
SK Services (East), L.C.
Ninth Street Properties, Inc.
Chemclear, Inc.  of Los Angeles
USPCI, Inc.  of Georgia
GSX Chemical Services of Ohio, Inc.
LEMC, Inc.
Dirt Magnet, Inc.
The Midway Gas & Oil Company
Elgint Corp.
Safety-Kleen Envirosystems Company
Safety-Kleen Envirosystems Company of Puerto Rico, Inc.
Petrocon, Inc.
Phillips Acquisition Corp.
ViroGroup, Inc.
SK Insurance Company
SK Real Estate, Inc.
Safety-Kleen International, Inc.
Safety-Kleen Oil Recovery Co.
Safety-Kleen Oil Services, Inc.
The Solvents Recovery Service of New Jersey, Inc.