Victor Conte is founder and president of Bay Area Laboratory Co-operative (Balco), the San Francisco-based company which the United States Anti-Doping Agency says developed the banned steroid THG (tetrahydrogestrinone).
Conte shows off one of his nutritional supplements
This puts him at the very centre of the doping scandal that is threatening to blow athletics apart.
Britain's Dwain Chambers and up to 20 high-profile American athletes - including Olympic champions and world record holders - are believed to have tested positive for THG.
53-year-old Conte founded Balco 20 years ago after a career in the music business.
He was a bassist with the nickname "Walking Fish" who played with soul group Tower of Power and jazz pianist Herbie Hancock, as well as in a band called Pure Food and Drug Act.
His company analyses blood and urine from athletes and then prescribes a series of supplements to compensate for vitamin and mineral deficiencies.
Its client list includes athletes Marion Jones, Tim Montgomery and Kelli White, baseball star Barry Bonds and the NFL's Bill Romanowski.
Conte and Balco are being investigated by a US federal grand jury. The hearing began on Thursday behind closed doors and is expected to focus on alleged tax evasion and money-laundering relating to payments made by athletes for banned substances.
Chambers' lawyer Graham Shear said on Wednesday that Chambers had been advised by his Ukrainian coach Remi Korchemny to use Conte as his nutritionist.
Shear said that after Chambers tested positive he "immediately challenged Mr Conte on the finding and was categorically assured that all supplements he had been given were within the IAAF rules.
"I understand that currently Mr Conte continues to deny that
the supplements he prepared for my client and other athletes
contain any illegal substances," said Shear.
Bonds has been listed as a client of Balco
Conte worked with Korchemny, who coached Valery Borzov to his 1972 Olympic sprint double, at his ZMA Track Club in San Francisco.
It was while Chambers was training with this group that his positive test was taken.
ZMA is an acronym drawn from the zinc and magnesium included in
one of Balco's nutritional supplements.
Conte promoted ZMA on the internet through a company called SNAC Systems, whose web site included former tennis champion Ivan Lendl, Jones and Bonds as clients.
Conte has estimated that gross retail sales of ZMA during the
past four years total $100 million worldwide.
He won an athletics scholarship to study at Fresno State College
but had no medical background when he founded Balco.
Balco's offices in Burlingame, California, have been raided by officials from the US Anti-Doping Agency, the Internal Revenue Service, the Food and Drug Administration and the San Mateo County narcotics taskforce.
Conte is no stranger to controversy. He was described as CJ Hunter's "nutritionist" three years ago when the American shot-putter and former husband of Marion Jones tested positive for nandrolone.
Lawyers for Conte said on Wednesday that their client was the
victim of malicious rumours.