BALTIMORE, OCT. 6 -- Federal narcotics agents seized a record three tons of a chemical used in making the illicit drug speed as it was being hauled by truck near here on the way to suspected clandestine laboratories in Texas, Louisiana and Oklahoma, agents announced today.

The 6,000 pounds of potent phenylacetic acid marked the biggest single seizure of the chemical by Drug Enforcement Administration agents in the eastern United States and, if delivered, could have resulted in the manufacture of 170 million doses of speed, or methamphetamine, according to Carl Jackson, head of the Washington field division.

Speed is a powerful synthetic stimulant similar to cocaine. Phenylacetic acid, a commercially available chemical, is used in the manufacture of perfumes, food additives and fungicides.

DEA spokesman Bob O'Leary said a pickup truck pulling a flatbed trailer filled with drums of the chemical in powder form was stopped by federal agents Monday afternoon on Rte. 1 just south of Baltimore as it was about to transfer the chemicals to another trucking company to complete the trip to points in Texas, Louisiana and Oklahoma.

Two men in the truck were questioned and released, O'Leary said. He said the chemical load was seized under civil impoundment procedures, but agents lacked probable cause to arrest the two men. He said a criminal investigation into the matter has started.

Agents first learned of the attempted delivery when a pharmaceutical manufacturer in Newark alerted them to the purchase of 6,000 pounds of phenylacetic acid -- an unusually large order -- by a customer, O'Leary said.

A truck came to pick up the order, he said, and agents followed it to the Baltimore area where it was stopped Monday.