LAS VEGAS (AP) — Middleweight fighter Julio Cesar Chavez Jr. could be stripped of his $3 million purse, depending on the explanation he offers to Nevada boxing regulators about a positive test for marijuana after his first professional loss last weekend in Las Vegas, officials said Wednesday.

Top Rank Inc. spokesman Lee Samuels said promoter Bob Arum was aware the 26-year-old Chavez tested positive for the banned substance.

"Julio Cesar Chavez Jr. will have the opportunity to explain the situation to the Nevada State Athletic Commission," Samuels said in a statement he read to a reporter. Samuels declined further comment.

Commission executive Keith Kizer wouldn't immediately provide the name, but said only one of 32 fighters who took part in bouts at two Las Vegas venues last Saturday tested positive for a banned substance, marijuana.

Kizer said the commission was awaiting results of pre-fight tests for banned steroids, diuretics and masking agents before releasing all pre-fight and post-fight test results for all 32 fighters.

The commission can ban a fighter for one year and penalize him for up to 100 percent of his purse for a failed test, Kizer said.

Chavez was guaranteed $3 million for the WBC title fight against Sergio Martinez.

Martinez dominated the first 11 rounds in the 160-pound fight at the Thomas & Mack arena, but had to withstand a frenzied 12th round effort by Chavez to win a unanimous decision. Martinez ran his record to 50-2-2.

Chavez fell to 46-1-1 with 32 knockouts.

He was suspended before, for seven months, and his 10-round unanimous decision win in November 2009 over Troy Rowlands was changed to a no-contest after he tested positive for a banned diuretic.

Chavez was arrested on drunken driving charges Jan. 22 in Los Angeles, and struggled to make weight for a championship bout in February against Marco Antonio Rubio in San Antonio, Texas.