Strikeforce light heavyweight Muhammed Lawal tested positive for the banned substance drostanolone at an event earlier this month in Las Vegas, according to the Nevada State Athletic Commission.
Lawal (9-1) finished Lorenz Larkin via knockout in the second round of their Jan. 7 fight on the "Strikeforce: Rockhold vs. Jardine" card at the Hard Rock Hotel.
The outcome will be changed to a no-contest, barring a successful appeal from Lawal. The fighter also faces a potential one-year suspension and fine.
"I'm very surprised about this. I am very careful about what I put into my body," Lawal said Tuesday. "I've never tested positive for a performance-enhancing drug. I've gone through, and still welcome, Olympic-style testing."
"I will do whatever is necessary to get to the bottom of this."
It is the first time Lawal, known to many as "King Mo", has tested positive for a banned substance. Drostanolone is an anabolic steroid, most frequently seen in professional bodybuilding.
A former champion, Lawal had moved himself back into title contention with knockout wins in his last two fights. He was expected to face another former titleholder, Gegard Mousasi, in 2012 for the Strikeforce 205-pound belt.
In all, the NSAC tested 15 of the 18 fighters on the roster that made up the event. Lawal's was the only test that came back positive.
"I'm not a cheater," Lawal said. "I might cheat at video games, but I never cheat at fighting."
Brett Okamoto covers mixed martial arts for ESPN.com. ESPN.com MMA writer Franklin McNeil contributed to this report.