Golovkin and Alvarez are scheduled to meet on May 5 (HBO PPV) at the T-Mobile Arena in Las Vegas in a rematch of their hugely controversial draw from September. It is unclear whether the positive test will affect the fight.

As part of the deal for the first fight, as well as for the rematch, both boxers agreed to be randomly tested by the Voluntary Anti-Doping Association.

Alvarez, who is training in his hometown of Guadalajara, Mexico, provided urine samples on Feb. 17 and Feb. 20, and both tests came back positive for trace amounts of clenbuterol, according to the letter VADA president Dr. Margaret Goodman sent to those involved on Monday that was obtained by ESPN.

Clenbuterol has shown up in drug tests for many Mexican athletes in recent years because of meat contamination in the country. Alvarez also said meat contamination caused his positive test.

"As part of the voluntary testing program that Canelo Alvarez insisted on ahead of his May 5 fight, one of his results came back positive for trace levels of clenbuterol, consistent with meat contamination that has impacted dozens of athletes in Mexico over the last years," Golden Boy Promotions, Alvarez's promoter, said in a statement addressing the positive test. "As Daniel Eichner, director of [the Sports Medicine Research and Testing Laboratory], the WADA-accredited lab that conducted the tests stated in his letter [Monday], 'These values are all within the range of what is expected from meat contamination.' Upon receiving this information, Golden Boy immediately notified the Nevada State Athletic Commission and Gennady Golovkin's promoter, Tom Loeffler."

In the letter VADA sent disclosing the positive test, Goodman said she had spoken to Nevada State Athletic Commission executive director Bob Bennett and that "the current plan is to continue to test Mr. Alvarez so that the Nevada State Athletic Commission can make a final determination. Mr. Alvarez has the right to promptly request analysis of the 'B' samples at his expense."

Alvarez usually does the bulk of his training for fights in San Diego, and Golden Boy said he would immediately move his camp there "and will submit to any number and variety of additional tests that VADA deems necessary ahead of and after May 5."

Loeffler was notified of the positive test and said Golovkin (37-0-1, 33 KOs) is also aware of it but that they are waiting to hear more from the Nevada commission before making an assessment.

"We're not really going to comment until we get more detail from VADA and the commission. We'll reserve comment," Loeffler told ESPN. "But Gennady has always insisted on VADA testing for any of his fights. He punches so hard that [trainer] Abel [Sanchez] wanted nobody to have any doubt about his training program. Gennady feels very strongly about a clean sport and level playing field for both guys. He can't comment on this test specifically, but that's his position. It's premature to say anything more without knowing all of the details."

Alvarez said he was embarrassed by the positive test.

"I am an athlete who respects the sport and this surprises me and bothers me because it had never happened to me," Alvarez said in a statement. "I will submit to all the tests that require me to clarify this embarrassing situation and I trust that at the end the truth will prevail."

According to Golden Boy Promotions president Eric Gomez, Alvarez (49-1-2, 34 KOs) has undergone random testing administered by VADA or the U.S. Anti-Doping Agency for his past 12 fights dating to his 2012 decision win over Shane Mosley.

"He's been tested many times and never had findings, nothing. And every fight they test him 10, 15 times, at least a dozen times," Gomez said. "This is something Canelo insists on it. We're very confident that everything is going to be fine."

Bennett told ESPN that the Nevada commission would investigate the situation before ruling on whether the fight would take place.

"We will do our due diligence, and once it's done, we'll let it be known whether we're going to move forward [with the fight]," Bennett told ESPN.

This is not the first time a prominent Mexican fighter has tested positive for clenbuterol in the lead-up to a fight.

In 2012, Erik Morales blamed contaminated meat for his positive tests ahead of his rematch with then-junior welterweight world champion Danny Garcia in New York. The fight went ahead, and Garcia sent Morales into retirement with a fourth-round knockout.

In 2016, then-junior lightweight world titleholder Francisco Vargas, also of Mexico, tested positive for clenbuterol during his training camp for a defense against former titleholder and countryman Orlando Salido. The California State Athletic Commission, in consultation with both fighters' camps, allowed the fight to go on because the panel gave Vargas the benefit of the doubt that the bad test was because he ate tainted meat in Mexico.

Vargas was, however, subject to much more rigorous random drug testing for the remainder of his camp. The fight went on in Carson, California, and Vargas and Salido battled to a hellacious draw in the 2016 fight of the year.

In 2016, former Houston Texans left tackle Duane Brown tested positive for clenbuterol. Brown, who now plays for the Seattle Seahawks, was not disciplined, and the drug program's independent administrator sent a letter warning players that consumption of too much meat in Mexico and China could cause a positive test for the anabolic substance.

Bennett said that "naturally there will be increased testing" for Alvarez and that the case "appears to be a situation that runs parallel to Mr. Vargas'."