The electronic war against shoplifting has come to one of Northwest's busiest supermarkets -- with mixed results.

At Giant Food Inc.'s Columbia Road store, which serves one of the District's most diverse neighborhoods, swinging doors in six checkout aisles guard against thefts that Giant officials say costs the chain thousands of dollars a year in profit.

The machines from Sensormatic Security Corp. work on the same principle as metal security gates at most airports. Customers passing through a checkout lane are "frisked" by a magnetic field between two small, swinging doors; if an item has not been desensitized by a cashier, it can touch off the device's bell alarm.

"It's definitely a deterrent. It's out front and visible, and the folks around here know about it -- as do the shoplifters," said Jim N. Leonard, a part-time manager in the Columbia Road store.

"We know who the shoplifters are," said Leonard, a D.C. police officer who has had the Columbia Road beat for the past six years and who works several nights a week at the store. "Some of them we just ban; we don't let them through the door. But others come in here and try to steal stuff. The machines are good because they act as a deterrent."

But Leonard and Elio Sanchez, another manager at the Columbia Road store, both report that the machines frequently are set off by metal items other than those in the store. Nor does the system prevent all theft: one recent night, several men stole 10 steaks and dashed through the front door--even though it is protected with the same device as those in the checkout lanes, Leonard said.

Despite its faults, the Sensormatic system is well worth the occasional false alarm or late-night ripoff, Giant officials maintain. "It's the way of the future. It's very reliable," said Ronald A. Cook, who has headed security for the 132-store chain for the past three years. "The machines are very obvious to our customers and to the shoplifter. The system is a tremendous aid in cutting down on this type of theft."

Giant officials put their chain-wide theft losses from customers and employees at about $18 million yearly, or 1 percent of the $1.8 billion of sales the chain does annually. That figure, they say, comes close to offsetting the 1 percent profit the company says it earns in 12 months.

"Retailers today are focusing more on expenses, and shoplifting is certainly an expense," said Cook. In the first week of Sensormatic's test period at the Columbia Road store, the anti-theft alarms sounded 53 times, "and each time, if the person had an item on them, they were embarrassed and just paid for it right away," said Cook.

In subsequent weeks, the number of alarms dropped off, rose again, and then fell to 28 in the week of Sept. 20. Because shoplifters promptly pay for an item when nabbed by the machines, there have been no arrests at the Columbia Road store, saving Giant officials the time and expense of a lengthy prosecution.

"It also saves on the cost of a detective. With these, people are watched 24 hours a day, seven days a week," Cook said.

Giant is one of the leaders in the new war the Greater Washington Board of Trade launched against shoplifters. The Columbia Road store, in the heart of a mixed neighborhood of rich and poor, whites, blacks and Hispanics, was selected to test the Sensormatic devices because of a history of shoplifting losses.

The system was installed at the end of August and is valued at roughly $35,000.

"The costs of shoplifting to the grocery stores and the drug and hardware stores are staggering," said Jack Bogaski, the son of a grocer, who has run Sensormatic's Washington office since 1975. "The machines are going to work. It's not really the electronic theory. It's the deterrent that's the thing."

Bogaski envisions the day when detectives and roving cameras will be all but obsolete, when "everything from pharmacies to liquor stores" will have sensing devices.

"Once they get the kinks out, it'll be really good," said Leonard. "I was here the other night and was watching this one guy in line. Just then, the alarm went off in another aisle.

"This guy reached into his pocket, pulled out an item and put it right onto the conveyor belt. He was no fool."