Saul 'Canelo' Alvarez says it "surprises and bothers" him that traces of clenbuterol were found in a drugs test, but his rematch against Gennady Golovkin will still go ahead on 5 May.
A statement from Alvarez's promoters blamed it on "meat contamination that has impacted dozens of athletes in Mexico over the last years".
Alvarez will now move his training camp from Mexico to the USA.
The first fight in Las Vegas ended in a controversial draw last September.
Clenbuterol is used to treat asthma but can also help build lean muscle mass and burn off fat.
Alvarez said: "I am an athlete who respects the sport and this surprises me and bothers me because it had never happened to me.
"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."
Daniel Eichner, director at a World Anti-Doping Agency-accredited laboratory that conducted the tests, said: "These values are all within the range of what is expected from meat contamination."
Kazakh Golovkin, 35, will again put his WBA, WBC and IBF titles on the line against Mexican Alvarez, 27, in Las Vegas.