Trainer King T. Leatherbury was suspended 15 days by Maryland's racing stewards yesterday because the drug theobromine was found in postrace tests of the gelding Flush N Diamonds, winner of a Dec. 21 race at Laurel.

State Steward J. Fred Colwill, in a statement, said testimony during several private hearings revealed that Leatherbury had bought a vitamin supplement with cocoa flavoring that included the prohibited substance.

Leatherbury, a national training champion, said he "bought this nutritional supplement from . . . a salesman for one of the most reputable companies in the country. In the smallest of writing on the label of the supplement is the ingredient (cocoa) listed. It turns out that cocoa has a caffeine base which comes up in the chemist's findings as a stimulant.

"I am a victim of the (state's) absolute insurer rule which holds a trainer responsible for things like this. I could win in a court of law, but I'm going to take it like everybody else who has to live with that rule."

"I don't believe that there was in any way chicanery in this case," said state chemist Thomas F. Lomangino Jr. "The amounts . . . and circumstances clearly suggest that it was at most negligence."