A federal judge says she might toss some of the strongest evidence against Barry Bonds, a blow to prosecutors trying to prove the home run king lied when he denied knowingly using performance-enhancing drugs.

U.S. District Judge Susan Illston said her "preliminary thoughts" were to exclude from trial three 2000-2001 positive drug tests that prosecutors say belong to Bonds unless there is a direct link that the urine samples came from the former San Francisco Giants slugger.

"If there's no testimony to establish that, I don't think any of them work," Illston said.

The only person who can do that seems to be Bonds' personal trainer, Greg Anderson, who spent more than a year behind bars for refusing to speak to a federal grand jury investigating Bonds. And Anderson's attorney, Mark Geragos, has said his client will not testify against Bonds at the trial, scheduled to start March 2.

If her preliminary thoughts stand, the government's case against Bonds will suffer a significant blow — but not a fatal one, legal analysts said.

"It hurts, but the government still has quite a bit of other evidence," Golden Gate University law professor Peter Keane said.

That includes a recorded conversation between Anderson and Bonds' former personal assistant Steve Hoskins in which they discuss injecting steroids; a conversation Illston said she's inclined to allow at trial.