Belmont Stakes winner Union Rags has been retired because of a tendon injury in his left front leg.
The announcement on Saturday came a week after the colt's veterinarian had said he would be out with the injury but that his prognosis was "excellent" for a return to the races next year.
Russell Jones, bloodstock adviser to the colt's owner Phyllis Wyeth, says Union Rags is "a pretty attractive stallion prospect" and that there has been a high level of interest in the horse with his retirement.
Trained by Michael Matz, Union Rags won the Saratoga Special as a 2-year-old, the Champagne Stakes and was beaten by a head by Hansen after a troubled trip in the Breeders' Cup Juvenile.
As a 3-year-old, he won the Fountain of Youth Stakes, finished third in the Florida Derby and seventh in the Kentucky Derby after being squeezed at the start of this year's race.
The colt skipped the Preakness and then won the 1 1/2-mile Belmont after wearing down pacesetter Paynter to take the final leg of the Triple Crown under jockey John Velazquez.
Jones said no decision has been made on where Union Rags will stand as a stallion, but he said Wyeth prefers the horse remain in the U.S.
"It's a love affair she had with this horse, and she doesn't want him to go somewhere where she can't find him," Jones said.
Kentucky Derby and Preakness winner I'll Have Another was retired with a tendon injury the day before the Belmont, which ended his chance to become the first Triple Crown winner in 34 years. The colt was sold to Japanese interests and will begin his stallion career overseas.