SAN FRANCISCO -- Major League Baseball will not retest the approximately 500 samples remaining from last year's drug tests for the steroid THG, a baseball official said Monday.

Results of the drug tests on Barry Bonds and several other players are being sought by federal prosecutors probing a San Francisco-area nutritional supplements lab. It is not clear if the samples for Bonds and those other players are among the ones that still exist.

The urine samples were taken from players last year as part of baseball's effort to determine the scope of steroid use. When more than 5 percent of samples showed evidence of steroids, new regulations were imposed beginning this season.

"There will be no effort by Major League Baseball to retest the samples because our agreement (with the players' union) does not allow it," said Rob Manfred, baseball's executive vice president for labor relations.

Two samples were taken from each of the more than 1,400 major league players last season. Most were destroyed, but about 500 were saved when a grand jury in San Francisco issued a subpoena for baseball's drug tests.

Since two tests were taken on each major leaguer, the surviving tests could have come from as few as 250 players -- and as many as 500.

The steroid tests did not check for THG, which was undetectable until last summer. Officials did not know of THG's existence when the baseball tests were carried out last season. THG is at the center of the federal probe.

The tests were supposed to remain anonymous. But Manfred said the subpoena seeks urine samples and paperwork showing drug-test results from Bonds, Gary Sheffield, Jason Giambi and a handful of other players as part of the federal probe into the Bay Area Laboratory Co-Operative.

Bonds, Sheffield and Giambi, all of whom have denied steroid use, testified before the grand jury probing BALCO.

The subpoena demands that the material be turned over to federal prosecutors in San Francisco by this Thursday.