TJ Dillashaw says he has vacated the UFC bantamweight title after failing a drug test after his last bout against Henry Cejudo.
The American lost to flyweight champion Cejudo in 33 seconds after stepping down a division to take the fight.
He says the United States Anti-Doping Agency and the New York State Athletic Commission told him about the finding.
Dillashaw added he was "voluntarily relinquishing" his title "while I deal with this matter".
In a message on Instagram, he wrote: "While words can't even begin to express how disappointed I am at this time, please know that I'm working with my team to understand what has occurred and how to resolve this situation as quickly as possible."