An Arlington couple who helped organize a national crusade against the makers of a defective infant formula described yesterday in federal court in Alexandria how their infant son's health deteriorated while he was being fed the formula.

Larry and Lynne Pilot are suing Syntex Labs and Pet. Inc. for $25 million in damages, accusing them of negligence and violations of the Virginia Consumer Protection Act for marketing an infant formula called Neo-Mull-Soy.

It was found later to have no salt, a necessary ingredient for healthy development in infants and was ordered recalled from the market by the U.S. Food and Drug Administration in August 1979.

The Pilots have alleged that their son, Bradley, now 6, has suffered speech and language delays, motor development problems and learning disabilities as a result of being fed the formula. The child, who was on the formula from June to August 1979, was "gravely and permanently damaged" by it, the suit asserts.

The Pilots were the first witnesses in what is expected to be a two-week trial before a nine-member jury in U.S. District Court.

Lynne Pilot, a lawyer, was cofounder of a national organization of parents whose children had been affected by the product. The group, called Formula, successfully lobbied for a law to enforce federal testing of infant formulas in 1980. Her husband, Larry Pilot, was an FDA lawyer for 10 years and is a specialist in product liability law.

She said her son stopped normal weight gain, and at one point began to sleep with his eyes half open after he was on the formula a while. Bradley did not crawl until he was a year old and did not walk until he was 15 months old, she testified.