UFC star Anderson Silva failed a drugs test ahead of his comeback fight against Nick Diaz.
Diaz himself also tested positive for Marijuana in the aftermath of the pair's fight at UFC 183 in Las Vegas - won by Silva via unanimous decision.
Brazilian fighter Silva - widely regarded as the UFC's greatest ever competitor - made his comeback to the octagon on Saturday after spending over a year out of action due to a broken leg suffered in a middleweight title fight against Chris Weidman in December 2013.
A UFC statement read: "On February 3, 2015, the UFC organization was notified by the Nevada State Athletic Commission that Anderson Silva tested positive for Drostanolone metabolites on his Jan. 9 out of competition drug test. UFC’s understanding is that further testing will be conducted by the Commission to confirm these preliminary results.
"Anderson Silva has been an amazing champion and a true ambassador of the sport of mixed martial arts and the UFC, in Brazil as well as around the world. UFC is disappointed to learn of these initial results.
"The UFC has a strict, consistent policy against the use of any illegal and/or performance enhancing drugs, stimulants or masking agents by its athletes."
Silva signed a 15-fight contract with the UFC in October, despite calls for him to retire due to his horrific injury.
The UFC also released a statement on Diaz: "As a result of his positive test, Diaz has been informed that he has violated the UFC Fighter Conduct Policy and Promotional Agreement with Zuffa, LLC.
"The UFC organisation will fully respect the Commission's decision relating to Diaz at a hearing set for February 17."