When the Illinois Racing Board slapped trainer Robert Holthus with a stiff suspension this week, the ruling made few headlines outside of Chicago. But it will have important reverberations in Maryland and the rest of the racing world.

Holthus was punished after four of his horses were found to have been treated with Sublimaze, a drug that until now had been as undetectable as it is potent. Just two months ago, a state chemist told an assemblage of trainers at a Maryland Racing Commission hearing that there was no way to test for the drug. Across the country, trainers were taking advantage of the golden opportunity.

But the Holthus suspension, which came as the culmination of a year of laboratory research, should mark the beginning of the end of Sublimaze's widespread illegal use.

Sublimaze, known generically as fentanyl, was created in the laboratory of a Belgian pharmaceutical firm a decade ago. It was devised principally to relieve postoperative pain in humans. But when it started appearing at the nation's race tracks three or four years ago, unscrupulous veterinarians and trainers found that it was almost the perfect illegal drug.

"Fentanyl stimulates the central nervous system and produces in horses an urge to run," said Dr. Thomas Tobin, professor of toxicology at the University of Kentucky. "It is also an analgesic. This combination is a little unusual and it is obviously very useful. Besides, the narcotic has no long-term negative effects, unless you were using it on an injured horse. You could administer it once a week to a race horse and he'd be sound and healthy at the end of the year."

Best of all, Sublimaze was practically impossible to detect in a horse's postrace urinalysis. The amounts injected in a horse were very small, as little as one-one thousandth of the dose of a medication like Butazolidin. Looking for Sublimaze in the system of a half-ton thoroughbred was like searching for a needle in a haystack.

In the summer of 1977, however, the Illinois Racing Board decided to start hunting for that needle. "We'd heard all the rumors about Sublimaze," said Jewel Klein, counsel for the Board. "We made it our No. 1 priority."

Illinois started working in cooperation with other racing states (Maryland was not among them) determined to end the Sublimaze epidemic. Louisiana found one technique for identifying the drug, but its methods were so new and untested that they probably would not have stood up in court. It was John McDonald, director of the Illinois lab, who came up with the clincher.

McDonald purchased some radioactive Sublimaze, which had what he called "a light bulb that emitted radioactive particles." He injected a horse with enormous quantitites of the drug and then looked to see if he could find the light bulb shining somewhere.

He found that even after a horse had been given such a large dosage, his lab's $150,000 computer could not find the presence of Sublimaze. But McDonald found something else.

"When you inject a drug into a system," he explained, "that system has a detoxification mechanism to help get rid of the drug because it is a foreign entity. The body generates what are called metabolites to help try to eliminate it. We could see those metabolites giving off light. And we finally worked out a method to isolate them."

Those metabolites led to Holthus' suspension.

The Illinois lab not only found Sublimaze violations in its own back yard, but tested urine specimens sent to it by other states.A trainer at Waterford Park received a 60-day suspension this week for Sublimaze use. And there are more to come.

"This is the first time different jurisdictions have worked together and thrown a net over a large number of trainers," McDonald said. "From now on, only the most brazen trainers are going to commit violations with Sublimaze."

But that hardly means that illegal drugs have been banished from racing. Far from it.

"They'll just start abusing a new drug now," McDonald said. "As soon as we wipe out one they start on something else. In fact, they're beyond Sublimaze already. And the new drugs are even more sophisticated and harder to detect."