Owners or employes of 21 liquor outlets in Montgomery County, including clerks in three county-operated liquor stores, have received summonses for allegedly selling alcohol to a 17-year-old boy who was working as a volunteer police undercover agent.

Montgomery County police said two plainclothes officers accompanied the youth to 31 retail liquor outlets in the Rockville-Potomac area and watched as he successfully purchased beer or other alcoholic beverages at 21 of them.

The action, which occurred during the weekend of Nov. 12-13, was the Montgomery police's first major crackdown on violations of the higher drinking age that went into effect on July 1. Legislation passed by the General Assembly this year raised the legal drinking age for wine and beer from 18 to 21.

Police said merchants were issued summonses after a 17-year-old member of Police Explorers, an organization of young people interested in law enforcement, was able to buy alcoholic beverages at 21 places without being required to show identification.

Owners of the bars and wine and beer stores who are accused of selling or serving the teen-ager have been summoned to appear in District Court Dec. 16 to face misdemeanor charges of selling alcohol to minors.

Several of the store owners complained that the method employed by police to discover violators amounted to entrapment.

"I'm very upset that the county is in the business of entrapment," said Mark Goldstein, owner of the Delly Den in Potomac, one of the places cited. "People around here know that we normally check age in this store, but in all the cases where the police got this sleek-looking kid to go in and make a purchase, they waited for the busy hour. We got set up so the county could show off numbers."

Goldstein said he was skeptical of an enforcement program aimed at penalizing merchants and not the consumer. "When they fine a kid $200 or take away his driver's license, that's when we'll see juvenile drinking go down," he said.

Cpl. Philip Caswell, the department's information officer, said the undercover operation was "part of a routine series of checks in a stepped-up enforcement of the liquor laws. We have sent juveniles in the past to purchase liquor. We checked more stores this time because there is a greater emphasis on drunk driving prevention and there is a new minimum age for drinking wine and beer."

The legal minimum age in Maryland for drinking hard liquor has always been 21, but since July 1, buyers of beer and wine also must be 21. The new law was "grandfathered," meaning that only people who turned 18 after July 1 will be prevented from purchasing beer and wine, while their friends one or more days older can drink . . . or buy alcohol for their younger friends.

John McGraw, chief of retail liquor operations for the county, which has a monopoly on the sale of liquor by the bottle, said the summoned clerks would not be fired. "About all we can do to prevent this sort of thing is training and instruction of our clerks," McGraw said. He emphasized that because the stores are county-operated, clerks do not make any profits from illegal sales.

Caswell said he thinks the new laws and stricter enforcement, including roadblocks, already have produced positive results. "It's hard to correlate the statistics," Caswell said, "but the fact is that we haven't had one teen-age fatality due to drunk driving in the last three months." There have been 17 alcohol-related fatalities in the county so far this year, but Caswell said he did not know how many involved teen-agers.