Only once in history has a Maryland-bred horse won the Kentucky Derby. It could happen again this spring.

Deputed Testamony, a humbly bred colt who was running in claiming races last fall, indicated that he might be one of the outstanding members of his generation when he won the $111,200 Federico Tesio Stakes at Pimlico today.

He swooped around the leaders on the final turn to score a 2 1/4-length victory over Dixieland Band, with Island Champ third. Even more impressive than his running style--almost unbelievable, in fact--was his running time. On a very slow racing surface, he covered 1 1/16 miles in 1:42 4/5, a full second faster than some of the best older horses in the state ran only a half-hour earlier. Few, if any, 3-year-olds in America have run a race so impressive this year.

While other Derby-age colts across the country were taking turns making headlines this winter, Deputed Testamony was largely unnoticed, because trainer Billy Boniface planned it that way.

The colt had shown promise as a 2-year-old, winning one minor stakes in the fall, and Boniface took him to North Carolina for the winter with one seemingly implausible goal. "Our objective was the Kentucky Derby," he said. "I wanted to race him three times before the Derby, and I didn't want him to get geared up too soon."

Deputed Testamony made his 3-year-old debut with a second-place finish in a sprint two weeks ago, but today he would finally be asked to show how good he is. Even Boniface wasn't sure what to expect. "I know he'll get a distance," the trainer said before the race. "But we'll find out in his next two starts if he has class."

He found out today. Under jockey Herb McCauley, Deputed Testamony broke alertly but was jostled and carried wide going into the first turn. Dixieland Band set the pace on the rail, holding off challenges by two long shots, while Deputed Testamony sat about six lengths behind him. On the turn Deputed Testamony accelerated strongly, three wide, and moved toward the leader as they turned into the stretch.

On the basis of breeding it should have been a mismatch. Dixieland Band is a son of the world's greatest thoroughbred stallion, Northern Dancer, and a great mare, Mississippi Mud. Deputed Testamony's sire stands for a $1,000 stud fee and his dam won one race at Charles Town.

But Deputed Testamony evidently didn't know he comes from the wrong side of the tracks, for he made the kind of run through the stretch that can win Kentucky Derbies. Drawing abreast of Dixieland Band an eighth of a mile from the finish line, he covered the last furlong in 12 1/5 seconds and drew away steadily.

The horses behind the two leaders were playing their version of bumper pool in the stretch, causing four foul claims and the disqualification of the fourth-place finisher, Hardi.

Deputed Testamony paid $5.20, $2.60, $2.20. Dixieland Band returned $2.80 and $2.60 and Island Champ paid $7.80 to show.

Boniface will ship Deputed Testamony and his stablemate Parfaitment to Lexington, Ky., for the Blue Grass Stakes on April 28, which has been his intention ever since he planned the colt's schedule months ago. Trainer Charlie Peoples will send Dixieland Band there, too, for a rematch. Kentuckians who distain Maryland-breds, knowing that Kauai King was the only one to win the Derby, may be in for a big surprise.