EXHIBIT 11
 
                  OPINION OF WILLIAM BLAIR & COMPANY, L.L.C.
 
                                                              November 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 fairness, from a financial point of
view, to the shareholders (the "Shareholders") of Safety-Kleen Corp. (the
"Company") of the consideration to 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 Company, SK Parent Corp. ("Parent") and SK
Acquisition Corp., a wholly-owned subsidiary of Parent ("Purchaser"). Pursuant
to the terms of, and subject to the conditions of, the Merger Agreement,
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
(the "Transaction").
 
  We have acted as financial advisor to the Company in connection with the
Transaction. In connection with our review of the Transaction and the
preparation of our opinion herein, we have: (a) reviewed the terms and
conditions of the Merger Agreement and the financial terms of the Transaction
as set forth in the Merger Agreement; (b) analyzed the historical revenue,
operating earnings, net income, dividend capacity and capitalization, of both
the Company and certain other publicly held companies in businesses we believe
to be comparable to the Company; (c) analyzed certain financial and other
information relating to the prospects of the Company provided to us by the
Company's management, including financial projections; (d) discussed the past
and current operations and financial condition and prospects of the Company
with senior executives of the Company; (e) reviewed the historical market
prices and trading volume of the common stock of the Company; (f) reviewed the
financial terms, to the extent publicly available, of selected actual business
combinations we believe to be relevant; and (g) performed such other analyses
as we have deemed appropriate.
 
  We have assumed the accuracy and completeness of all such information and
have not attempted to verify independently any of such information, nor have
we made or obtained an independent valuation or appraisal of any of the assets
or liabilities of the Company. With respect to financial information, we have
assumed that it has been reasonably prepared on bases reflecting the best
currently available estimates and judgments of the Company's management, as to
the future financial performance of the Company. We assume no responsibility
for, and express no view as to, such forecasts or the assumptions on which
they are based. Our opinion relates to financial fairness only, and we express
no opinion as to the appropriateness of the financial structure or the
soundness of the financial condition of the Company subsequent to the
consummation of the Merger. We understand that other professionals who are
expert in those areas will be providing advice on those subjects. Our opinion
is necessarily based solely upon information available to us and business,
market, economic and other conditions as they exist on, and can be evaluated
as of, the date hereof.
 
  In rendering our opinion, we have assumed that the Transaction will be
consummated on the terms described in the Merger Agreement, without any waiver
of any material terms or conditions by the Company and that obtaining the
necessary regulatory approvals for the Transaction will not have an adverse
effect on the Company.
 
  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. For our services, including the
rendering of
 
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this opinion, the Company will pay us a fee, a significant portion of which is
contingent upon consummation of the Transaction, and indemnify us against
certain liabilities. William Blair & Company has provided investment banking
and financial advisory services to the Company in the past for which we have
received customary compensation. Edgar D. Jannotta, Sr., Senior Director of
William Blair & Company, serves as a member of the Board of Directors of the
Company.

  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 mailed to shareholders
by the Company with respect to the Transaction. 
 
  Based upon and subject to the foregoing, it is our opinion as investment
bankers that, as of November 20, 1997, the consideration to be paid to the
Shareholders of the Company in the Transaction pursuant to the Merger
Agreement is fair, from a financial point of view, to such Shareholders.
 
                                          Very truly yours,
 
                                          William Blair & Company, L.L.C.
 
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