Four persons were arrested on charges of possessing and passing counterfeit money Monday at Pimlico Race Course after mutuel clerk Frank Guth spotted what appeared to be a phony $100 bill.

Guth summoned guard Nick Monfredo, a former middleweight contender, who held the suspect, James F. Milligan of Mullins, S.C., until agents of the Thoroughbred Racing Protective Bureau could question him.

Within 20 minutes of the arrest, U.S. Secret Service agents had three out-of-state men and one woman in custody and determined that there had been only three bogus bills cashed at the track before Guth spotted the fourth.

A total of $3,700 in phony $100 bills was seized later at a Mitchellville motel, said Joseph Coppola, assistant agent in charge of the Baltimore bureau of the Secret Service. He said it was "one of the largest seizures in Maryland this year."

Guth said a man in his 30s came to the clerk's window on the second floor of the grandstand with two $100 bills in his hand. Guth said the man placed a wager and gave him "the funny-looking bill." Guth, a 75-year-old who wears bifocals, summoned Monfredo.

In addition to Milligan, those arrested were Laura E. Gribble of Lothian, Md., David O. Mercer of Lumberton, N.C., and Earl D. Dail of Shallotte, N.C. The four were arraigned today in Baltimore. d

William Ramsey, track security chief, said, "These suspects obviously have never been to a race track. If they knew how we operated, the last place they would go would be the race track."