SAN FRANCISCO — Barry Bonds admits using steroids during his baseball career, his lawyer told a jury today. The catch is that Bonds’ personal trainer misled him into believing he was taking flax seed oil and arthritis cream.

“I know that doesn’t make a great story,” Allen Ruby said during his opening statement at the home run leader’s perjury trial. “But that’s what happened.”

Assistant U.S. Attorney Matt Parrella called such claims “ridiculous and unbelievable” and portrayed Bonds as a liar during his first chance to present the government’s position.

And so the crux of the criminal case against Bonds was laid before an eight-woman, four-man jury as the testimony phase of the trial got under way. Bonds has pleaded not guilty to four charges of lying to a grand jury in 2003 when he denied knowingly taking steroids and one count of obstruction.

Parrella started the day by saying Bonds lied to the grand jury even though the government promised not to prosecute him for drug use if he testified truthfully.

“All he had to do was tell the truth, Parrella said. “That’s all, but he couldn’t do it.”

Parrella tried to show a deep connection between Bonds and the Bay Area Laboratory Co-Operative, known as BALCO, the Burlingame company at the center of an international sports doping ring that the grand jury was investigating. Five men, including BALCO’s founder Victor Conte and Bonds’ personal trainer Greg Anderson, pleaded guilty to steroids distribution after a 2003 government raid on BALCO.

Today Parrella displayed a photograph taken from a magazine of Bonds, Conte and Anderson and called the trio the “Three Musketeers of BALCO,” drawing an objection from Ruby.

Dressed in a dark suit with a light blue shirt, Bonds sat slouched in his chair, his long legs crossed at the ankles and poking out the other side of the defense table, as he watched Parrella tell jurors that a childhood friend of Bonds will discuss watching him inject steroids.

Parrella promised other witnesses will talk about conversations they had with Bonds regarding his steroid use, while others will discuss their deep suspicions.

Ruby, Bonds’ lead attorney, countered by trying to discredit some of the government witnesses scheduled to testify during a trial that is expected to last between two and four weeks.

He said at least two prosecution witnesses have axes to grind because of bitter fallouts with the man who hit 762 career home runs, a Major League Baseball record. He also holds the mark for home runs in a single season, with 73 in 2001.

Ruby alleged that Bonds’ ex-girlfriend, Kimberly Bell, and former business partner, Steve Hoskins, were “facing the loss of the financial benefit that Barry provided to them over the years” when Bond ended his relationships with them in 2003.

Hoskins and Bell are key government witnesses.

Bell plans to testify that Bonds admitted to her he took steroids. She will also testify to physical and mental changes she says Bonds experienced and that prosecutors attribute to steroid use.

But in a deep baritone, Ruby told the jury that “after the break up Ms. Bell was extremely unhappy,” suggesting she has motivation to unfairly denigrate Bonds.

Ruby, a former professional wrestling announcer now with a prestigious law firm, said Hoskins has somewhat similar motives as Bell.

But there is one crucial government witness who won’t testify at all — Anderson, who prosecutor allege supplied Bonds with steroids and detailed instructions on how to use them. Anderson was taken to jail today after he told U.S. District Judge Susan Illston he was refusing to testify against Bonds, who he grew up with in San Mateo County.

Bonds looked away when Anderson and his attorney Mark Geragos entered the courtroom and again when U.S. Marshals led him away to jail, where he will remain until he changes his mind or the trial ends.

Anderson has been held in contempt before. He served more than a year in prison for refusing to testify in 2006 before a grand jury investigating Bonds.

The judge plans to give the jury an instruction later explaining Anderson’s absence from the trial.

After lunch, lead investigator Jeff Novitzky was called to the witness stand.

Prompted by questioning from Assistant U.S. Attorney Jeff Nedrow, Novitzky recounted for the jury the start of his BALCO investigation. After receiving a tip, and making preliminary Internet searches and examining BALCO finances, Novitzky said he began to root through the lab’s trash every Monday night for about a year and found incriminating evidence tying famous athletes to BALCO and steroid use.