Alexandria's acting commonwealth's attorney said yesterday he will drop the remaining 23 drug cases brought by an undercover policewoman and her roommate who had made sexual advances to lure men into drug deals.

The undercover officer, already the subject of an internal police investigation that focuses on what prosecutors have said was troublesome vice squad activities, quit yesterday.

Rookie policewoman Cathy Jo Martindale, 26, submitted her resignation to Police Chief Charles T. Strobel, who directed a police probe of her nine-month assignment seeking out drug deals in the city's singles bars. Martindale could not be reached for comment yesterday, but city prosecutor John E. Kloch said she had told police she would not testify in any of the remaining drug cases.

Her former roommate, Janice Picha, 25, who disclosed the undercover work in surprise testimony to a state legislative committee in September, said yesterday that Martindale had resigned under pressure from city police.

"She ended up taking all the heat," Picha said in an interview. "She told me two days ago that although police department and commonwealth's attorney had publicly supported her, she wasn't getting that support in private."

Acting city prosecutor Kloch said yesterday the entire case was an "unfortunate situation" and said he would move for dismissal of the cases that resulted from Martindale's and Picha's work. He cited the officer's refusal to testify as well as questions about her "personal credibility and her use of drugs in the past."

Chief Strobel cited her testimony in the first of the drug cases that resulted from the undercover assignment as a factor in the resignation.

In the Nov. 14 case, the soft-spoken, dark-haired woman testified that she lied on a police-administered polygraph test when she was hired last August. Martindale said she denied using drugs at the time, but admitted at the trial that she had used mescaline, "speed", marijuana, opium and LSD prior to her police employment. Under police regulations, use of LSD alone would automatically bar her as a police applicant.

The drug case was dismissed after the prosecution failed to prove that marijuana Martindale allegedly had purchased from a young man was continously in police possession after the transaction.

Both Martindale and Picha told the court that drugs they purchased were kept in an unlocked nightstand in their Alexandria town house, often for days before being turned over to the vice squad.

"That trial," Kloch said yesterday, "clearly demonstrated the difficulties which exist in all these cases. There is no chance of success where the prosecution's principal witness will not cooperate." he said.

"Establishing chain of custody would require . . . detailed testimony which it is now unrealistic to expect from her (which) lead me to conclude that the remaining cases charging drug sales to Officer Martindale lack prosecutorial merit," Kloch said.

When details of the undercover probe surfaced two months ago, it ignited a heated exchange between the commonwealth's attorney and the police department. Prosecutors blamed the police for lack of communication and said the troublesome activities were the latest in a series of questionable vice squad tactics.

Police Chief Strobel later blamed the vice squad and called it "mismanaged". Vice squad officers said privately that the morale in their unit was "pitiful . . awful."

The police chief said yesterday he was "disappointed" that the inadequacies in the undercover probe "ever came to light, but I wouldn't want to conceal it. I am accepting full responsibility for it. We could have provided Martindale with better supervision."