Shortly after hitting the go-ahead home run that beat Oakland 8-5, Ortiz responded to a story on The New York Times ' Web site that he and former teammate Manny Ramirez tested positive for performance-enhancing drugs six years ago.
"I've just been told that the report is true," Ortiz said in a statement after contacting the union. "Based on the way I lived my life I'm surprised to learn I tested positive."
The popular Big Papi, who had never been linked to drugs, said he intended to find out what was in his system and would tell the Red Sox and the public.
"You know me -- I will not hide and I will not make excuses," he said.
Alex Rodriguez, Barry Bonds, Roger Clemens and Sammy Sosa are among the All-Stars tainted by the cloud of steroids and drugs, which has called into question some of the sport's greatest achievements over the past two decades. Ortiz and Ramirez led the Red Sox to the World Series title in 2004 -- their first in 86 years -- and another title in 2007.
Ramirez, now with the Los Angeles Dodgers, recently served a 50-game suspension for violating baseball's drug policy.
Asked about this second alleged doping violation, he told reporters in St. Louis: "You want more information, I'm pretty sure you guys got the phone number to the union. Call the union, and they can explain that to you guys."
Red Sox manager Terry Francona said the news "blindsided everybody," including Ortiz.
In 2003, 104 major leaguers tested positive for performance-enhancing drugs; the results were supposed to be anonymous and are now under court seal.
The Times story cited lawyers involved in pending litigation over the testing results who spoke anonymously because the information is under seal. The newspaper did not say what the players tested positive for.