ANNEX B     
       
                    [LETTERHEAD OF WILLIAM BLAIR & COMPANY]
 
                                                              December 20, 1997
Board of Directors
Safety-Kleen Corp.
One Brinckman Way
Elgin, IL 60123-7857
 
Dear Directors:
 
  You have requested our opinion as to the superiority, from a financial point
of view, of the consideration which would be received pursuant to the terms of
the amended proposed offer ("Proposed LLE Offer") made by Laidlaw
Environmental Services, Inc. ("LLE") as compared with the consideration which
would be received pursuant to the terms of the Agreement and Plan of Merger
dated as of November 20, 1997 (the "Merger Agreement") by and among the
Safety-Kleen Corp. (the "Company"), SK Parent Corp. ("Parent") and SK
Acquisition Corp., a wholly-owned subsidiary of Parent ("Purchaser") (the
"Merger"). Based on the advice of your counsel, we understand that the
Proposed LLE Offer and the Merger should be compared based on their respective
values upon consummation.
 
  Pursuant to the terms of the Proposed LLE Offer, LLE and a subsidiary
propose to exchange, for each outstanding Common Share of the Company, cash in
the amount of $15.00 less certain expenses (no less than $1.28 per share and
estimated by LLE to be up to $2.14 per share) plus that number of LLE common
stock equal to the Exchange Ratio of not less than 2.8 shares and no greater
than 3.5 shares. Pursuant to the terms of, and subject to the conditions of,
the Merger Agreement, in the Merger, Purchaser will be merged into the Company
in a merger in which each of the outstanding shares of common stock of the
Company will be converted into a right for the Shareholder to receive $27.00
per share of common stock in cash.
   
  We have acted as financial advisor to the Company in connection with the
Merger and the Proposed LLE Offer. In connection with our review of the Merger
and the Proposed LLE Offer and the preparation of our opinion herein, we have:
(a) reviewed the terms and conditions of the Merger Agreement and the
financial terms as set forth in the Merger Agreement and the Preliminary Copy
of the Proxy Statement dated November 26, 1997 as filed by the Company with
the Securities and Exchange Commission ("SEC"); (b) reviewed the terms and
conditions of the Proposed LLE Offer and the financial terms as set forth in
the Amendment No. 2 to the Exchange Offer as filed by LLE with the SEC
("Amended Exchange Offer"); (c) analyzed the historical revenue, operating
earning, net income, dividend capacity and capitalization of both LLE and
certain other publicly held companies we believe to be comparable to LLE; (d)
analyzed certain publicly available financial and other information relating
to LLE and the pro forma combination analysis in the Amended Exchange Offer
and performed a sensitivity analysis on such pro formas based upon variable
synergy assumptions; (f) reviewed the historical market prices and trading
volume of the common stock of LLE as well as its stock ownership and analyzed
factors which could influence the trading price of the common stock of LLE on
the anticipated closing date for the proposed LLE Offer; (g) together with the
Company's management met with Jim Bullock, Chairman of LLE; and (h) performed
such other analyses as we have deemed appropriate.     
 
  Our opinion with respect to the Proposed LLE Offer reflects only limited
access to LLE management and no access to internal LLE projections.
 
  In rendering our opinion, we have assumed that the Merger or the Proposed
LLE Offer would be consummated on the terms described in the Merger Agreement
or the Amended Exchange Offer, respectively, without any waiver of any
material terms or conditions by the Company and that obtaining the necessary
regulatory approvals for the Merger or Proposed LLE Offer would not have an
adverse effect on the Company.

     
Board of Directors                                            December 20, 1997
Safety-Kleen Corp.
   
  William Blair & Company has been engaged in the investment banking business
since 1935. We undertake the valuation of investment securities in connection
with public offerings, private placements, business combinations, estate and
gift tax valuations and similar transactions. During August of this year we
were retained by the Company to render financial advisory and investment
banking services in connection with the evaluation of its strategic
alternatives. The Company has paid us a fee in connection with rendering our
fairness opinion as it relates to the Merger. Upon the consummation of either
the Merger or the Proposed LLE Offer, the Company will pay us a transaction
fee. The amount of such fee increases as the consideration received by the
Company's stockholders increases.     
 
  Our engagement and the opinion expressed herein are for the benefit of the
Company's Board of Directors. It is understood that this letter may not be
disclosed or otherwise referred to without our prior written consent, except
that this opinion may be included in a proxy statement or other disclosure
document mailed to shareholders by the Company with respect to the Merger or
the Proposed LLE Transaction, as the case may be.
 
  Based upon and subject to the foregoing, it is our opinion as investment
bankers that, as of December 20, 1997, we do not have a basis for concluding
that the Proposed LLE Offer is superior to the Merger from a financial point
of view.
 
                                          Very truly yours,
 
                                          William Blair & Company, L.L.C.
 
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