A federal agency's agreement with an accounting firm marks the agency's first major enforcement action against auditors accused of failing to report lax savings and loan practices.

The Office of Thrift Supervision announced Wednesday that Coopers & Lybrand, one of the nation's largest accounting firms, has agreed to tighten its auditing procedures.Experts have said that accountants played an important role in the $500 billion savings and loan scandal, but until now regulators have focused their attention mainly on S&L operators and directors.

The charges against Coopers & Lybrand stemmed from its 1986 audit of Silverado Banking, Savings and Loan Association of Denver, whose board of directors included President Bush's son Neil. The collapse of Silverado in December 1988 is expected to cost taxpayers $1 billion.

OTS had accused Coopers & Lybrand and Arthur L. Knight, who headed the firm's team that handled the Silverado audit, of:

-Accepting an unjustified understatement of Silverado's allowance for loan losses on its $1.3 billion loan portfolio.

-Relying on appraisers' judgments that were not justified by market conditions or by financial statements applying to the properties behind the loans.

-Not following accounting principles.

The loan loss allowance should have been $35 million higher, OTS said.

The auditors "did not possess the expertise required to properly complete the Silverado audit," the office asserted in a legal brief released with its announcement.

Coopers & Lybrand agreed, among other things, to ensure that employees on S&L audits are competent, and to comply with accounting principles for those audits.