Kentucky Derby winner Medina Spirit, who provided trainer Bob Baffert with a record seventh win in the race, has failed a drugs test.
Baffert said he was "shocked" after the horse tested above the limit for betamethasone and insisted he had never administered the substance.
"I got the biggest gut punch in racing for something I didn't do," he said.
Medina Spirit, ridden by John Velazquez, won America's most famous race at Churchill Downs on 1 May.
Baffert, 68, was the first trainer to win the US Triple Crown for 37 years when American Pharoah completed the treble of Kentucky Derby, Preakness Stakes and Belmont Stakes in 2015.
The Hall of Fame trainer repeated the hat-trick with Justify three years later.