WASHINGTON — The Federal Reserve has auctioned $7.2 billion in safe Treasury securities to big investment firms, part of an ongoing effort to ease credit stresses.

The auction — the eighth of its kind — was held yesterday and drew bids less than the $25 billion being made available. That could be viewed as a sign of some improvements in credit conditions.

In exchange for the 28-day loan of Treasury securities, bidding firms can put up as collateral more risky investments, including certain shunned mortgage-backed securities and bonds backed by federally guaranteed student loans. Bidders’ identities are not made public. The program began March 27.

In yesterday’s auction, investment firms paid an interest rate of 0.1000% for a slice of the securities. The auction program is intended to make investment houses more inclined to lend to each other. The program also is aimed at providing relief to the distressed market for mortgage-linked securities and for student loans.

To help shore up the shaky student loan market, the Fed recently agreed to let financial institutions put up bonds backed by federally guaranteed student loans as collateral.

Spreading credit problems have forced more than 60 lenders to stop making federally guaranteed student loans, either temporarily or permanently.