Tyrone Spong has denied taking a banned substance after he returned an adverse doping test, casting his heavyweight fight against Oleksandr Usyk into doubt.
Spong is set to take on Usyk, who is stepping up to make his heavyweight debut after winning all four world titles at cruiserweight, on Saturday in Chicago.
However, his participation is now up in the air after the 34-year-old's A sample tested positive for a substance banned by the World Anti-Doping Agency (WADA) and the Voluntary Anti-Doping Agency (VADA). The substance is reported to be Clomiphene, something commonly found in female fertility medication.
On Monday, Eddie Hearn confirmed on Twitter that there had been an "adverse finding" in Spong's VADA report, with the promoter adding reserve opponents were on standby as the results of the test were sent to the commission in Illinois.
But Spong insists he has not taken the substance.
"This is a set up," Spong told BoxingScene.com. "I vehemently deny that I have ever taken the substance named in VADA's report.
"I have asked for testing of the B sample at a laboratory of my choosing. Testing performed last week at the request of the Illinois Boxing Commission showed, what I've always known, that I'm a clean fighter."
The fighter's manager, Marcos Gonzales, revealed Spong's team had an uneasy feeling throughout Monday after Usyk elected to cancel a scheduled face-off.
"We received the email [at roughly 19:00 ET]," Gonzales told BoxingScene.com. "Eddie had already tweeted out the news by the time I saw the email notification.
"It's very strange because Tyrone was just informed that he tested negative from the other tests we did.
"We had a face-off [on Monday], and all of a sudden Usyk decided to shut it down. We just had a weird feeling all day that something was off.
"Everything started to make sense after receiving that email from VADA."
Spong was not initially slated to fight the Ukrainian, but accepted an offer to replace Carlos Takam, who withdrew from a bout which was originally planned for May, only for Usyk to pull out with a biceps injury.