Graham, a former 400m runner, has pleaded not guilty
Mexican steroid dealer Angel Heredia has told a San Francisco court how he provided sprinter Marion Jones with a triple-cocktail of banned substances.
Heredia mailed the blood-booster EPO, growth hormone and insulin to Jones in 2000 at the request of her then-coach Trevor Graham, who is on trial.
Heredia said he also sold banned substances to Tim Montgomery.
Graham is not charged with distributing steroids but lying about them in a June 2004 interview with federal agents.
Former Mexican national discus champion Heredia warned Graham not to allow Jones to combine the three drugs he sent her into a single injection in the weeks before the Sydney Olympics in 2000, where she won five medals.
Heredia also told the jury he sold banned substances to Olympic gold medal sprinters Antonio Pettigrew, Jerome Young and Dennis Mitchell, while Ramon Clay, Duane Ross and Garfield Ellenwood were customers.
Heredia, who said he either sent the doping products to the athletes or to Graham to distribute, is the prosecution's key witness in a trial expected to last six days.
Graham denies meeting Heredia in person, talking to him on the phone since 1997 and obtaining performance-enhancing substances from him or referring athletes to him for drugs.
Jamaican-born Graham once coached star American sprinters Montgomery and Jones, who were both disgraced and banned for steroid use in wake of the Balco scandal.
Graham, who ran for Jamaica at the 1988 Seoul Olympics, sparked the investigation into the Bay Area Laboratory Co-operative (Balco) in 2003.
He anonymously sent a syringe of the anabolic steroid THG, which had been chemically designed to avoid detection, to the US anti-doping authorities.
Graham was initially granted immunity from prosecution on the grounds he would not lie but charges have been brought because federal agents now believe he did so. He has pleaded not guilty.
If found guilty, Graham could face 15 years in jail.
More than a dozen athletes have been suspended or disciplined in connection with Balco.
Balco founder and owner Victor Conte, who has himself served a prison sentence for steroid distribution, claims there are world champions who have used THG but have escaped being exposed.
The scandal ensnared dozens of athletes, including disgraced Olympic champions Jones and Montgomery, both of whom are currently serving prison terms.
Although they are not expected to testify in the trial, it is thought at least five as-yet-unnamed athletes who worked with Graham will give evidence.
Many of Graham's former charges are expected to testify he helped them get human growth hormone, the blood-booster EPO and other performance-enhancing drugs.
The witness list also includes Young and Pettigrew, team-mates in America's 4x400m relay team which won gold at the Sydney Olympics in 2000, and Mitchell, who finished third in the 1992 Olympic 100m final.