Bay Area Laboratory Cooperative (Balco) boss Victor Conte has pleaded guilty to distributing steroids and laundering money at a court in San Francisco.
Conte had been answering the charge of giving performance-enhancing substances to more than 30 sports stars.
"I agreed with others to distribute steroids," he said on Friday.
It is now unlikely that athletes Marion Jones and Tim Montgomery, who had both been linked to Balco, will testify about alleged doping in this case.
Three other men charged with Conte were also expected to plead guilty to reduced charges, a law enforcement source said on condition of anonymity.
These include Greg Anderson, personal trainer of baseball superstar Barry Bonds.
"I knew at the time that steroid distribution was an unlawful activity," said Conte.