Patrik Sinkewitz tested positive for elevated levels of testosterone before the Tour de France, according to the German cycling federation (BDR).
Sinkewitz has added to T-Mobile's misfortunes in the first week Tour
The T-Mobile rider crashed out of this year's Tour on Sunday when he collided with a spectator after stage eight.
The accident left the 26-year-old with a broken nose and injured shoulder.
Contacted at a Hamburg clinic by German news agency DPA about the positive test, he said: "It's not possible. I know nothing about it."
The BDR said on its website that Germany's National Anti-Doping Agency (NADA) informed it that a test on a first sample Sinkewitz gave on 8 June had shown increased levels of the hormone.
The BDR said it would write to Sinkewitz and he would then have five days to decide if a second sample given on the same date be tested.
If that confirmed the positive result, the BDR would take the case to the federal court for sport.
Sinkewitz has been provisionally suspended by his team, T-Mobile spokesman Stefan Wagner confirmed.
Team general manager Bob Stapleton said: "Patrik is suspended, and if the analysis of the B sample is also positive his contract will be terminated.
"This is an athlete who committed to our code of conduct. If he stepped outside it is extremely disappointing."
German state television stations ARD and ZDF said they would stop broadcasting the Tour until the results of Sinkewitz's B sample were known.
German cycling has been hit by a string of doping confessions, including former Telekom riders Dane Bjarne Riis and German Erik Zabel's admissions that they used banned substances in the 1990s.