SAN FRANCISCO — The government's case against Barry Bonds has suffered a setback.

A federal judge ruled Thursday that prosecutors cannot show jurors three positive steroid tests and other key evidence in the slugger's trial next month.

U.S. District Judge Susan Illston said the urine samples that tested positive for steroids are inadmissible because prosecutors cannot prove conclusively that they belong to Bonds. The judge also barred prosecutors from showing jurors so-called doping calendars that Bonds' personal trainer, Greg Anderson, allegedly maintained for the slugger.

The judge said prosecutors need direct testimony from Anderson to introduce such evidence. Anderson's attorney said the trainer will refuse to testify at Bonds' trial even though he is likely to be sent to prison on contempt of court charges.

Prosecutors could not immediately be reached to determine whether they planned an appeal, which would delay the start of the scheduled March 2 trial.

Bonds has pleaded not guilty to lying to a grand jury on Dec. 4, 2003 when he denied knowingly using performance-enhancing drugs.

Prosecutors allege Anderson collected the urine samples and delivered them for testing to the Bay Area Laboratory Co-Operative.

During a September 2003 raid, federal investigators seized the positive test results that they allege belong to Bonds along with 21 other blood and urine samples that tested negative.

Prosecutors wanted to use all the tests to show that Bonds was a knowledgeable steroids consumer because he was a frequent customer of BALCO, the center of a massive sports doping ring.

Prosecutors said the three key tests show positive results in 2000 and 2001 for the steroids nandrolone and methenolone. Sports Illustrated has reported that Alex Rodriguez also tested positive for methenolone in 2003.

The samples themselves do not identify the source, but prosecutors said business records seized in the BALCO raid tie Bonds to the positive tests.

The ruling was not a complete loss for prosecutors. The judge said that they could play parts of a recording that Bonds' former personal assistant, Steve Hoskins, secretly made of a conversation he had with Anderson in front of the slugger's locker in San Francisco in March 2003.

In that conversation, Anderson discussed how he was helping Bonds avoid infections by injecting him in different parts of his buttocks rather than in one spot.

Bonds testified before the grand jury that no one but his doctor ever injected him.

In the recording, Anderson appears to boast about injecting Bonds with a steroid designed to evade detection.

"But the whole thing is," Anderson is quoted as saying, "everything that I've been doing at this point, it's all undetectable."

Prosecutors also have a fourth test showing Bonds used steroids that they will be allowed to show a jury. In 2003, Major League Baseball tested all of its players for steroid use. The results of those tests were to remain confidential and were to be used only to determine if MLB had a drug problem that needed to be addressed.

The lab that MLB hired to conduct its testing found that Bonds tested negative for steroid use. But in 2004, federal agents seized Bonds' urine sample and had it retested for the designer drug THG, which they said turned up positive.

Bonds' lead attorney, Allen Ruby, didn't return a telephone call late Thursday night. But other attorneys on Bonds' legal team have said that the MLB positive test jibes with the player's grand jury testimony that he took substances he later determined were designer steroids supplied by his trainer without explanation.