CSFB CREDIT SUISSE FIRST BOSTON

IRWIN HOME EQUITY MORTGAGE TRUST 2005-B

DERIVED INFORMATION [05/26/2005]

IRWIN HOME EQUITY MORTGAGE TRUST 2005-B

[$238,256,918]

Credit Suisse First Boston Mortgage Securities Corp.

Depositor

U.S. Bank National Association

Indenture Trustee

The information contained in the attached materials is referred to as the “Information”.

The Information has been provided by Credit Suisse First Boston. The Information contained herein is preliminary and subject to change.

The Information addresses only certain aspects of the applicable Note’s characteristics and thus does not provide a complete assessment or contain all material information about the Notes. As such, the Information may not reflect the impact of all structural characteristics of the Notes. The assumptions underlying the Information, including structure and collateral, may be modified from time to time to reflect changed circumstances.

Although a registration statement (including the prospectus) relating to the Notes discussed in this communication has been filed with the Securities and Exchange Commission and is effective, the final prospectus supplement relating to the Notes discussed in this communication has not been filed with the Securities and Exchange Commission. There shall not be any offer or sale of the Notes discussed in this communication in any state in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state. Prospective purchasers are

recommended to review the final prospectus and prospectus supplement relating to the Notes (“Offering Documents”) discussed in this communication for definitive Information on any matter discussed in this communication. Offering Documents contain data that is current as of their publication dates and after publication may no longer be complete or current. A final prospectus and prospectus supplement may be obtained by contacting the Credit Suisse First Boston Trading Desk at (212) 538-8373 or from the Securities and Exchange Commission’s website.

The analyses, calculations and valuations herein are based on certain assumptions and data provided by third parties that may vary from the actual characteristics of the pool. Neither Credit Suisse First Boston LLC nor the Depositor have verified these analyses, calculations or valuations or represent that any such valuations represent levels where actual trades may occur.

IRWHE 2005-B

Assumptions:

Draw	0%
Advance	0%
Recovery Lag	0 months
Severity	100%
Triggers	No Stepdown
Optional Redemption	To Maturity

The BreakEven CDR for a class is the maximum percentage of the CDR such that the class will NOT take a writedown at the corresponding scenario assumptions (Percentage of the CDR increment in 0.1%)

Class	PPC (%)	CDR (%)	Libor	Collateral Cum Loss (%)	WAL	DM
1M-1	75	11.3	Fwd	38,873,568.43 (29.95%)	7.04	54
1M-1	100	13.2	Fwd	35,133,771.33 (27.07%)	5.40	55
1M-1	125	15.2	Fwd	32,518,924.30 (25.05%)	4.25	55
1M-1	75	10.4	Fwd + 100	36,509,731.38 (28.13%)	7.15	54
1M-1	100	12.4	Fwd + 100	33,441,933.29 (25.76%)	5.56	55
1M-1	125	14.4	Fwd + 100	31,111,600.24 (23.97%)	4.32	55
1M-1	75	9.5	Fwd + 200	34,046,567.43 (26.23%)	7.27	54
1M-1	100	11.5	Fwd + 200	31,484,407.74 (24.26%)	5.59	54
1M-1	125	13.6	Fwd + 200	29,676,514.75 (22.86%)	4.40	55
1M-1	75	8.7	Fwd + 300	31,768,605.22 (24.48%)	7.64	54
1M-1	100	10.6	Fwd + 300	29,466,875.96 (22.70%)	5.63	54
1M-1	125	12.8	Fwd + 300	28,212,777.28 (21.74%)	4.50	55

CREDIT SUISSE FIRST BOSTON MORTGAGE ACCEPTANCE CORP 8-K