New York Yankees star Jason Giambi has admitted to a federal grand jury that he injected human growth hormone and used steroids, according to a report.
The first baseman's testimony in the Balco hearing was obtained by the San Francisco Chronicle newspaper, which has made its contents public.
Giambi, who formerly played in the Bay Area for Oakland, has always publicly denied using banned substances.
The Balco case, which has rocked US sport, is set for trial next year.
Giambi is one of about 40 athletes asked to testify in the drugs scandal involving Balco - the Bay Area Laboratory Co-Operative.
The nutrition and dietary supplements company is owned by Victor Conte, one of four men charged with distributing illegal performance-enhancing drugs to elite athletes.
The Chronicle detailed how Giambi described injecting human growth hormone in his stomach and testosterone into his buttocks.
The paper said he also told the grand jury how he rubbed an undetectable steroid knows as "the cream" on his body and placed drops of another, called "the clear" under his tongue.
Giambi testified that he obtained several different steroids from Greg Anderson, the personal trainer of San Francisco Giants slugger Barry Bonds.
Anderson is one of Conte's co-defendants, along with Balco vice-president James Valente and veteran athletics coach Remy Korchemny.
All have pleaded not guilty to the charges laid against them.
Giambi won the American League Most Valuable Player in 2000 for Oakland and signed a $120m, seven-year contract with the Yankees after the 2001 season.
He hit 155 homers from 1999-2002 and batted over .300 each season, but has been dogged by injuries in the last two years.