Yesterday, four days after Fairfax County police announced with fanfare the arrests of 90 persons on charges of selling alcohol to minors, a General District Court judge dismissed one-third of the charges.

Judge Richard T. Horan dismissed charges against 30 of the sales clerks accused of alcohol sales violations, saying he would not have reason to suspect the 20-year-old police cadet used in the undercover police operation was under the legal age of 21, based on her appearance.

Assistant Commonwealth's Attorney Raymond F. Morrogh said Horan issued his decision after asking police cadet Sarah Wright to step down from the witness stand so he could take a closer look at her.

Morrogh, who prosecuted the cases, said Virginia law requires that he demonstrate that the sales clerks knew, or should have known, that the customer was under the age of 21. Morrogh said he could not prove beyond a reasonable doubt that the clerks should have known that the police cadet was under 21 years of age.

"It was totally ridiculous," Steven A. Merril, who represented six of the sales clerks charged with alcohol sales violations, said of the three-week police undercover operation. "I mean it was probably one of the dumbest police operations I've seen in a long time. It was an absolute waste of taxpayers' money. That girl looked like she could be 30."

"I knew those cases had to be in trouble," said Commonwealth's Attorney Robert F. Horan Jr., brother of Judge Horan. The prosecutor, who does not take cases before his brother, said he recognized potential problems with the case when he learned that one of the decoys was 20 years old and saw her photograph in a newspaper. He said that judges frequently have dismissed cases when the decoys used by police were near the legal age limit.

Police spokesman Warren Carmichael said the police department had used decoys effectively in the past and planned to continue the enforcement effort.

"During the course of the operation, the majority of the clerks had decided that the age was questionable enough to ask for identification," said Carmichael. "It was our opinion that the individuals looked young enough that they should have aroused suspicion."

The police department had dispatched two underage cadets into 200 stores throughout the county to purchase wine and liquor during a three-week period in May.

Yesterday, 40 of the 90 cases were scheduled for court action, according to court administrator Paul S. Rice. Ten of those cases were continued until Aug. 26 and two of the persons charged pleaded guilty. After dismissing 28 other cases, the judge tossed out the two convictions, Rice said.

"The action by the court reinforces our belief that we have not violated any law," said Sue Challis, a spokeswoman for Giant Food, where arrests were made at six of 18 stores checked in the county. Challis said three of the six charges were dismissed and the others were not heard.

"I think it's a setback," Lou Herzog, president of the Northern Virginia chapter of Mothers Against Drunk Drivers, said of the dismissals. "The kids don't have to go to D.C. [where the legal age for drinking and purchasing alcohol is 18] anymore. They can just stay in Fairfax and drink now."