<PAGE>
$1,293,888,780
Washington Mutual Bank, FA
Seller and Servicer
Credit Suisse First Boston
Mortgage Securities Corp.
Depositor
Mortgage-Backed Pass-Through Certificates, Series 1999-WM3
Issuer
Supplement dated January 31, 2000
to
prospectus supplement dated December 28, 1999
to
prospectus dated December 3, 1999
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Capitalized terms used herein and not otherwise defined herein have the meanings
assigned in the Prospectus Supplement dated December 28, 1999.
The Prospectus Supplement is hereby revised as follows: The second sentence of
the second paragraph under "Federal Income Tax Consequences" is replaced by the
following:
The prepayment assumption that will be used in determining the rate of accrual
of original issue discount, market discount and premium, if any, for federal
income tax purposes will be based on the assumption that, subsequent to the date
of any determination, the Mortgage Loans will prepay at a rate equal to 185% of
the Prepayment Assumption in the case of the Class 1 A Certificates and the
Class 1 M-1, Class 1 M-2 and Class 1 M-3 Certificates, and the Mortgage Loans
will prepay at a rate equal to 250% of the Prepayment Assumption in the case of
the Class 2 A Certificates and the Class 2 M-1, Class 2 M-2 and Class 2 M-3
Certificates.
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Credit Suisse First Boston
January 31, 2000
Dealers will be required to deliver a supplement, prospectus supplement
and prospectus when acting as underwriters of the certificates offered hereby
and with respect to their unsold allotments or subscriptions. In addition, all
dealers selling the Class A and Class M Certificates, whether or not
participating in this offering, may be required to deliver a supplement,
prospectus supplement and prospectus until March 27, 2000.
