BALTIMORE, SEPT. 11 -- Trainer Bill Boniface has been suspended 15 days after one of his horses tested positive for metabolites of cocaine following a race at Pimlico last month, the stewards announced today.

Lacy Underalls, a 3-year-old filly, was found to contain benzoylecgonine and ecgonine methyl ester -- the major metabolites of cocaine -- after she finished third in the fifth race Aug. 17. She was the 13-10 favorite in the race for $5,000 claimers and earned $660, which later was rescinded.

After the Maryland Racing Commission discovered the metabolites, Boniface exercised his option by having a portion of the urine sample tested by an independent laboratory. Bioquest Laboratories of Seaford, Del., confirmed the finding.

Boniface's suspension began Monday and runs through Sept. 24. During his absence, his son, Kevin, will operate his stable.

Bill Boniface could not be reached today at his home, Bonita Farm, in Harford County.

Cocaine, which in horses serves as a local anesthetic that also is said to stimulate the central nervous system, had not been detected in a Maryland racehorse for several years.

Maryland's absolute insurer rule holds a trainer responsible for a horse's medication violation, regardless of who administered the medication. One scientific expert said a horse could test positive for cocaine by passively ingesting or absorbing even the smallest amount.

"As long as there is cocaine on the backstretch of a race track, then there exists the potential for an accidental administration to the horse," said Tom Lomangino, director of the Maryland lab in College Park.

Boniface, 48, won the 1983 Preakness Stakes with Deputed Testamony and three Maryland Million races on the same program in 1987. He did not have a horse in Sunday's Maryland Million races.No Crying Wolf

Northern Wolf, who joined the upper echelon of thoroughbred sprinters with his victory in the Frank De Francis Memorial Dash last month, is scheduled to run in a $150,000 race Saturday at Philadelphia Park. Trainer Hank Allen said no decision had been made concerning next month's $1 million Breeders' Cup Sprint at Belmont Park, but he appeared reluctant because the colt would not be able to run with Lasix, the bleeder medication prohibited in New York.

"I don't like guessing when you're going into a race," Allen said.

Seventy-seven of the 94 horses on today's program raced with Lasix, nearly 82 percent.