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, the same steroids Alex Rodriguez said he believed he took for three years ending 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.