Chemists in four states have determined 18 horses were helped this summer by a powerful stimulant drug banned for all but experimental use, according to United Press International.

Numerous racing industry sources say positive laboratory findings of Clenbuterol in the horses' urine are "the tip of the iceberg." Use of the previously undetectable drug probably has influenced scores of races.

Some trainers apparently have obtained Clenbuterol, a respiratory aid, through researchers conducting clinical investigations on horses under authority from the Food and Drug Administration.

Federal sources say investigators --including agents of the FDA, FBI, Drug Enforcement Administration, Customs Service and New Jersey State Police, are looking into a reported operation to illegally import Clenbuterol and other illegal drugs used at race tracks.

In the last two months, Clenbuterol has been detected in seven harness horses that ran in New York, two harness horses entered in New Jersey, a thoroughbred that won in Pennsylvania, and eight harness horses in Illinois.

The discoveries are the latest turn in an effort by racing laboratories and law enforcement authorities to halt use of illegal drugs on racehorses.

Rep. John Conyers (D-Mich.), seeking a tougher crackdown on the use of illegal drugs in racehorses, plans to head a House Judiciary subcommittee hearing next week on legislation calling for federal intervention.