Former light heavyweight champion Antonio Tarver tested positive for synthetic testosterone, a banned substance, in a drug test before his heavyweight draw with Steve Cunningham on Aug. 14 at the Prudential Center in Newark, New Jersey.
It is the second time in three years that Tarver has tested positive for a banned substance.
Tarver, 46, of Tampa, Florida, issued a statement Monday denying he took a banned substance.
"I have no idea how my urine sample tested positive because I didn't take anything illegal," Tarver said. "Either the test was contaminated or mixed up with another sample. We believe in the process, and I will fully comply. Further analysis will prove I'm 100 percent innocent because I've done nothing wrong."
Sports Illustrated, which was first to report Tarver's positive test, said in its report that when Tarver's "A" sample tested positive, he requested the "B" sample also be tested, and it too came back positive. A source with knowledge of the results confirmed to ESPN.com on Monday that Tarver's "A" and "B" samples were both positive for synthetic testosterone.
Tarver (31-6-1, 22 KOs) will face a fine and suspension from the New Jersey State Athletic Control Board, which did not respond to ESPN.com's request for comment. The result of the fight is likely to be changed to a no-decision.
Premier Boxing Champions, the brainchild of Tarver manager Al Haymon, also did not respond to ESPN.com's request for comment. The 12-round bout was the main event on a PBC card televised on Spike TV.
Tarver also tested positive following a cruiserweight fight with Lateef Kayode in June 2012 in Carson, California. He came up positive for the anabolic steroid drostanolone in a urine test conducted by the California State Athletic Commission.
Tarver and Kayode also had boxed to a draw. The result of the fight, televised on Showtime, eventually was changed to a no-contest. Tarver was fined $2,500 of his $1 million purse and suspended for a year.
Because of that positive test, Showtime fired Tarver as its ringside analyst, and NBC fired him from his assignment to call the 2012 Olympic boxing tournament later that summer. Tarver now serves as the expert ringside analyst for the PBC fights on Spike TV. A spokesman for the network said, "We can't comment at this time until we have all the relevant information."
Tarver returned from the suspension in late 2013 and won two heavyweight fights in a row, including a knockout of fringe contender Johnathon Banks in December. Tarver has repeatedly said he wants to become the oldest fighter in boxing history to win a heavyweight world title and was considered a candidate to land a shot at Haymon-managed titleholder Deontay Wilder.
Cunningham, a former two-time cruiserweight titlist and now a heavyweight contender, was upset by the news of the positive test following a fight he strongly felt he won.
"It could have been a clear victory for me," Cunningham told ESPN.com. "I woke up [Monday] morning to a call that told us he failed on the 'A' and 'B' sample. Where we are now is that we are sitting back and waiting to see what happens, but it looks bad for him. He looks very guilty."
Cunningham said it should lead to questions about Tarver's entire career.
"His stance is, 'I'm clean, I'm clean.' But you already failed a test before, and he has a history," Cunningham said. "With this happening two times in three years, you have to question all of his big victories. What was Antonio Tarver known for? A powerful left hand. He put Roy Jones out with it. He was stopping guys with his left hand. Banks also. You have to question all of his big knockout victories since he has tested positive."