A Cambridge sting operation cracking down on alcohol sales to minors has discovered numerous bars and restaurants in the 02139 zip code which are violating state law, according to a police report released last weekend.

Eighteen of the 29 bars and restaurants targeted by the investigation's first round in early May sold liquor to undercover constables who are under the age of 21. In the second round of the investigation, which targeted first-round violators, 17 of the 18 stores sold to another underage constable.

Richard V. Scali, executive officer of the Cambridge License Commission, said he was surprised by the number of establishments which repeatedly violated the law.

"I think [the first round] was about right, maybe a little higher than expected. I was very surprised by the second round," he said.

But the manager of the Drum- lins bar, one of the violators, says that there may be a reason why both rounds of the investigation yielded the same results. The manager, Dan C. Steinkrauss, says both constables purchased the beers on the same day of the week during the same bartender's shift.

"Both times they came in on Tuesday early in the evening," Steinkrauss said. "They went after one particular person. They went back same time, same shift, same bartender."

Steinkrauss also said that while he and the owner are "assuming it did happen," they mistrust the accuracy of the investigation. He said his bar does not sell beer at the prices quoted in the city's summary of the results of the investigation. "This is supposed to be a serious investigation and he's misrepresenting prices," he said.

Officials at several other pouring establishments contacted refused to comment.

The License Commission will vote at a June 16 hearing on disciplinary action for the bars and restaurants. Scali said the police officer in charge of the investigation, Michael D. McMahon, will probably recommend temporary license suspensions of three days for the sole one-time violator, and penalties ranging from a week to 15 days suspension for the two-time violators, depending on their previous records.

McMahon is on vacation and could not be reached for comment.

The investigation parallels last December's city-wide investigation of stores selling packaged alcohol to minors. In both cases, violators were not immediately notified that they had sold to an undercover officer, and another underage constable was sent to try to purchase alcohol again, Scali said.

In last month's investigation, a constable who worked as a security guard for Cambridge City Hospital would enter a bar and attempt to buy a beer, Scali said. If he was served, he would leave the beer on the counter and exit the bar. If he was asked for identification, he would decline and leave the bar.

The area targeted so far includes Central Square, Cambridgeport and Inman Square. The rest of the city, however, will be investigated as well. "We're going to go on to another zip code," Scali said.

The pouring establishment investigation will eventually target 150 out of the 208 licensed stores in the city, costing about $600, Scali said.

The remaining 58 stores will not be investigated because they are not considered "high risk places." Scali said the city has determined that underage people are not very likely to be served at those places. "You'd have to be sat down and served a meal to actually get a drink," he said.

The package store investigation cost the city about $200. The beer bought by the constables, which is still being held as evidence, will be donated to a charitable cause as soon as the last appeal is decided, Scali said