Alexandria's chief prosecutor expressed fear yesterday that an undercover police woman's admission that she had lied to the officers who recruited her may jeopardize efforts to prosecute more than 20 drug cases.

Acting Commonwealth's Attorney John Kloch said the statements by vice squad officer Cathy Martindale that she had conceealed her prior drug use from police administrators has damaged her credibility as "the main witness" in the drug cases.

"I'm very displeased with the way the investigation was handled," said Assistant Commonwealth's Attorney David O'Brien, following a meeting between the city prosecutors and Police Chief Charles T. Strobel.

The meeting came a day after dismissal of the first drug case that resulted from undercover work by Martindale and her former roommate, a women who has told a Virginia legislative committee she would make sexual advances to young men in an effort to complete drug transactions.

Chief Strobel also voiced concern over Martindale's testimony and said, "I'm having the entire episode reviewed." The chief said he had met yesterday with the young officer, but declined to say what action he might take.

Use of LSD - one of five drugs Martindale testified she had used prior to joining the force - "automatically" disqualifies any police candidate, Strobel said. Martindale said in Alexandria Circuit Court that she had used speed, mescaline, marijuana, opium, and LSD before she joined the force in August 1977, but said she had lied about her drug usage when she took a police polygraph test.

Her testimony was the latest in a series of what prosecutors have said are troubling disclosures about activities of Alexandria's vice squad, a police detail that Stroble said yesterday had been "mismanaged."

Separately it was learned that prosecutors earlier this year had to drop two heroin cases after they discovered that their informant had "planted" a powdery substance that turned out not to be heroin on the property of two suspects.

Martindale, 26, was recruited with her civilian roommate Janice Picha, 25, to solicit drugs in the city's singles bars. Picha has said in a sworn deposition that the women smoke marijuana to protect their cover and made sexual advances to obtain drugs.

The first of those drug cases was dismissed by Alexandria Circuit Court Judge Franklin P. Backus on Tuesday after the prosecution failed to prove that marijuana purchased by the two women was continually in possession of the police after the transaction.

Strobel also declined to comment yesterday when asked whether the Alexandria Police Department was aware that Martindale had lied on the polygraph test.

"We don't know everything about everyone when we hire them," Strobel said. "We do the best we can."

But the head of the police personnel department. Mark Conoyer, said had Martindale's use of LSD been known, she would not have been hired. "LSD concerns us because of the danger of flashbacks," said Conoyer. The term refers to the possibility of sudden recurrences of a drug-induced state months after the initial "trip."

Any recent use of LSD would be enough to reject an applicant. We've been investigating the effects of LSD, and you just don't know what's going to happen," said Conoyer.

Martindale's immediate superior at the time of the investigation was Sgt. Larry Black, who resigned as head of the vice squad unit in May after refusing to take a polygraph test. Black was the subject of an internal investigation into the vice squad unit.

I can't blame Cathy Martindale for this," Strobel said yesterday. "She may not have been given the supervision she needed. The mismanagement in the vice squad was the problem."

Black said yesterday. "Had I known of her (Martindale's) drug use. I would not have given her the undercover assignment."

The vice squad unit has come under fire recently for its use of undercover officers and civilian informants in drug and prostitution crackdowns. In dismissing one of the cases, a judge said he could not decide who was the "bigger where" - the masseuse or the civilian undercover informant.