Trainer Mac Robertson had his provisional suspension lifted Friday, allowing him to resume racing at Canterbury Park after a split sample failed to confirm a positive drug test in one of his horses.

Robertson was suspended July 3 by the Horseracing Integrity and Welfare Unit (HIWU), which conducts drug testing at U.S. tracks. HIWU said an initial drug test of the gelding Johnny Up showed the presence of altrenogest, a banned substance in male horses. Robertson exercised his right to request testing of the B sample, and that test was negative.

In a letter sent Friday to Robertson, the Minnesota Racing Commission and horse owner John Mentz, HIWU said it was immediately vacating the provisional suspensions of Robertson and Johnny Up in light of the negative test.

"No further disciplinary action will be taken against you or Johnny Up in connection with this matter,'' the letter read. "HIWU is closing this matter as fully resolved.''

The all-time leading thoroughbred trainer at Canterbury Park, Robertson trains one of its largest stables, with about 70 horses under his care. He is currently third in the trainer standings with 13 victories this season. Trainer Nate Quinonez oversaw Robertson's horses during the suspension.

Robertson was not allowed on the Canterbury Park grounds while he was suspended. Track officials said Robertson can return immediately, and they will accept entries from him beginning Saturday. Horses can run under his name beginning July 29.

In a phone interview Friday, Robertson said Johnny Up initially tested positive for 25 picograms of altrenogest, a drug given to female horses to regulate their reproductive cycles. A picogram is one trillionth of a gram. The positive test came after Johnny Up won a claiming race at Canterbury on June 4.

While a few fillies in the Robertson barn were receiving the medication, he said he was confident his staff had not mistakenly given the drug to Johnny Up or handled it carelessly. He was glad to see his staff exonerated, and he expressed no ill will toward HIWU or its mission to crack down on drugs in horse racing.

"I questioned my help about whether we made a mistake, and they said no,'' said Robertson, who is Canterbury's all-time leader in wins (1,038) and purse earnings ($22.9 million). "Most of them have worked for me for 20 years, and I believed them.

"Horse racing needs change. It needs less drugs. I agree with that. But I've always been faithful to my horses, and I'm happy to see them again.''

Under HIWU rules, a positive test for a banned substance triggers an automatic provisional suspension. If it is upheld, the penalty is a two-year suspension, plus a fine of as much as $25,000.