The Kazakh rider Dmitriy Fofonov tested positive for the banned stimulant heptaminol after the 18th stage of the Tour de France.
News of the positive test came while riders were sipping champagne on the Champs Elysees after the last stage in cycling's greatest race, in which Spaniard Carlos Sastre won the yellow jersey.
Heptaminol is a vasodilator, sometimes used to treat low blood pressure. It had been erased from the World Anti-Doping Agency's list of banned substance in 2004 but was declared illegal again in 2006.
Credit Agricole rider Fofonov, who was immediately fired by his team, failed the test after the 196.5-km ride from Bourg d'Oisans to St Etienne on Friday.
His positive test is the fourth in this year's Tour after Spaniards Manuel Beltran and Moises Duenas Nevado, as well as Italy's Riccardo Ricco, tested positive for the banned blood-booster EPO.
Fofonov says he bought it on the Internet to help fight cramps and didn't tell the team's doctor.
The softly spoken Fofonov ended the Tour de France in 19th place overall, his best finish in a major tour.