Triple Crown champion American Pharoah has had a spirited gallop at Keeneland in preparation for his swan song in the Breeders' Cup Classic.

He is the 4-5 favourite for Saturday's race with Beholder, the horse judged his main opposition, ruled out.

American Pharoah arrived from California on Tuesday and walked around his barn Wednesday because of wet conditions. On Thursday (Friday AEDT), he galloped on the synthetic surface of the training track.

Trainer Bob Baffert had planned to use the main dirt track but changed his mind after days of rain left the surface muddy.

Several hundred fans watched the work-out with exercise rider Georgie Alvarez aboard the horse and Baffert was happy with his condition.

Beholder, who was the early 3-1 second choice, was taken out by trainer Richard Mandella after a scope revealed she had bled.

Mandella attributed the bleeding to the fever Beholder developed when she arrived in Kentucky last week. He said the five-year-old mare would be fine, but it would be too risky to run her Saturday.

"If I put her under the pressure of the race, she might do some serious damage," Mandella said. "There's obviously some irritated lung tissue there."