French jockey Olivier Doleuze, who is currently based in Hong Kong, was suspended from riding for an indefinite period after testing positive for a forbidden substance at Sha Tin, the Australian Associated Press reported.
The sample will now go to a second laboratory and if the result is confirmed, Doleuze will face a full inquiry from the Hong Kong Jockey Club (HKJC).
The rider was replaced on five mounts at Happy Valley racecourse in Hong Kong last night.
The AAP said Doleuze is the first jockey to test positive for a banned substance since former German champion Andrasch Starke was handed a six-month suspension for testing positive for cocaine during the Hong Kong International meeting in 2001.
A year earlier Starke had been banned for a month after a positive alcohol test.
Doleuze burst onto the Hong Kong scene with a memorable first season in 2002, winning 26 races -- mostly for trainer Derek Cruz -- a 20% winning strike rate.
Given a five-month contract by the HKJC this year, he had ridden 19 winners up until the time of last Sunday's events.