Boxing star Antonio Tarver says he "vehemently refutes" reports that he failed a drug test before his August fight with Steve Cunningham ... saying, "I am 100% innocent."
Tarver is responding to an article on SI.com -- which says he tested positive for synthetic testosterone, which is considered a banned substance.
But Tarver says he "categorically denies taking any banned substance before his fight with Cunningham."
"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% innocent because I've done nothing wrong."
Tarver has failed a drug test in the past -- testing positive for an anabolic steroid in 2012, for which he was fined $2,500 and given 1 year suspension.