By Laird Harrison

SAN FRANCISCO (Reuters) - Baseball's home run king admitted to using steroids and even complained about the pain from receiving injections, an old friend of Barry Bonds testified in the tarnished star's perjury trial on Wednesday.

Steve Hoskins, a childhood friend of the slugger and former business partner, said he first became aware that the former Major League Baseball player was using steroids when Bonds asked him to investigate their health effects.

"He complained that his butt was sore from the injections," Hoskins told the court.

Dressed in a grey suit, Bonds, 46, who has pled not guilty, took notes as Hoskins testified about a relationship that dates back to a friendship between their fathers, who were professional athletes in San Francisco.

The Bonds case is one of the last strands in a lengthy investigation into the use of performance-enhancing drugs in sports. Doping revelations have tarnished the reputation of baseball, known as America's national pastime.

Hoskins said he saw Bonds and his personal trainer Greg Anderson go into a bedroom and when Anderson emerged he was holding a syringe. In another instance, Bonds said if Anderson would not inject him that he would do it himself, according to Hoskins.

Later, prosecutors played a recording that Hoskins secretly made of Anderson talking about injecting athletes.

In the recording, Hoskins asks why it's important to change injection sites, and Anderson says injecting too much in one place can cause cysts. "Is that why Barry didn't just shoot it in his butt all the time?" Hoskins asks.

"I never just go there. I move it all over the place," Anderson responds.

The soft-spoken Hoskins also testified that Bonds asked him to investigate the side effects of the steroid Winstrol.

He said that he and Bonds were reintroduced by their mothers around 1992 and that the former player helped Hoskins make connections for a business in which Hoskins painted lithographs of professional athletes. Hoskins also said he worked as Bonds' personal assistant.

The two shared a business account and a safe in Hoskins' office stocked with over $100,000 in cash from which Hoskins made payments to two of Bonds' mistresses and to the trainer, Hoskins said.

In cross examination, defense attorney Allen Ruby suggested that the partnership broke apart after Bonds accused Hoskins of forging Bonds' signature on sports memorabilia, and reported Hoskins to the FBI.

Hoskins denied using Bonds' signature without his permission, but he insisted he never held a grudge against the slugger.

"Barry's a good friend," he said. "He's a very good person, and he's one of the best baseball players that's ever going to be, and that's the reason why I tried to stop him from taking steroids -- because I thought it wasn't good for him."

Hoskins rejected Ruby's suggestion that he had made a separate recording of a Bonds' doctor in order to extort Bonds. He said he had planned to play the recording to Bonds' father, former San Francisco Giant Bobby Bonds, in hopes that the older Bonds would convince his son to stop using steroids.

The charges against Bonds stem from the player's appearance before a U.S. grand jury in 2003 when Bonds testified that he did not use steroids or growth hormones, and that Anderson, did not inject him with any.

Bonds told a grand jury he did not knowingly use steroids or growth hormones and never questioned the flaxseed oil, vitamins, protein shakes and creams that Anderson supplied.

Anderson told the court on Tuesday he refused to testify, even after U.S. District Judge Susan Illston threatened to send him to jail. He was taken into the custody of a U.S. marshal.

As a member of the San Francisco Giants, Bonds broke Hank Aaron's 33-year-old career home run record in August 2007, his last season in Major League Baseball.

Three months later, a grand jury indicted him for making false statements and for obstruction of justice.

Bonds also set the single-season home run record with 73 in 2001.

The case in U.S. District Court, Northern District of California is United States of America v. Barry Lamar Bonds, 07-cr-732.

(Editing by Frank Pingue)