A jury in U.S. District Court in Washington awarded $400,000 yesterday to a 23-year-old Fairfax County man who said he suffered serious genital abnormalities and emotional trauma as a result of the drug DES his mother took while she was pregnant with him.

Lawyers involved in the case said it was apparently the first involving the son of a woman who took the drug, which was supposed to prevent miscarriages. Daughters of women who took DES risk developing cancer caused by the experimental drug, which has been withdrawn from the market.

The award by the six-member jury against Eli Lilly & Co. of Indianapolis, the company that manufactured the drug, was far smaller than the multimillion-dollar amount sought by the man, identified for privacy reasons as Scott Doe.

It could not be determined last night whether Lilly would appeal the verdict in the case, which began June 10.

Lilly was one of a host of drug companies originally named as defendants in the suit, which was filed in 1982. The other firms were dropped when it was determined that they had not manufactured the drug that Doe's mother had taken.

Doe's attorney, Leland Van Koten of Baltimore, argued that DES caused the serious genital defects and led to subsequent emotional anguish and humiliation. Because of his physical condition, Doe, who is mildly retarded, lives at home and works in a sheltered workshop where he earns 95 cents an hour. Were it not for the damage DES caused, his attorney argued, Doe could function independently and hold a higher-paying job.

Van Koten argued that Lilly was negligent in its failure to warn women of possible damage that could result from taking DES and of marketing a product that was unsafe. The jury deliberated for six hours before agreeing on both counts.

Lilly's attorney, Russell Beatie of New York City, argued that Doe's injuries, while unfortunate, were not DES-related. Beatie argued that DES does not cause defects in males and that even if it did, Doe's mother's doctor had overprescribed the drug.

Beatie's witnesses included several medical experts, including Dr. William Scott Sr., a professor emeritus at Johns Hopkins Medical School, and Dr. Richard J. Worley, a professor at the University of Utah Medical Center.

In 1980, the Supreme Court allowed DES victims to sue all makers of the drug, which was taken by as many as 3 million pregnant women between 1941 and 1971. More than 200 companies made the drug, but 90 percent of it was produced by five large pharmaceutical companies, including Lilly.