Brock Lesnar's UFC comeback could stall after one fight after the former heavyweight champion was flagged for a potential anti-doping violation.
The UFC said Friday it was notified by the USADA of a potential anti-doping policy violation stemming from an out-of-competition sample collection on June 28.
The USADA did not announce what substance may have caused the failure.
The UFC said USADA received the test results Thursday, five days after Lesnar returned from 4 1/2-year layoff and beat Mark Hunt in the co-main event of UFC 200.
Lesnar brushed off doping accusations before the fight, saying, "I'm a white boy and I'm jacked. Deal with it."
Lesnar made a UFC-record $2.5 million guaranteed purse for his first bout since late 2011, returning to the sport where he won and lost the heavyweight title during a meteoric MMA career. Lesnar was given permission by WWE to return to UFC for one fight and he had no other future fights scheduled in the
He was scheduled to return to the WWE and wrestle Randy Orton on Aug. 21 at SummerSlam.