Aside from the loss of more than 1,200 jobs, perhaps the biggest impact of the 12 area Memco stores closing three days from now will be felt in greater Washington's fiercely competitive retail industry.

To be sure, the closings will affect market shares in the food, general merchandise and retail drug store sectors of the industry. Memco's market shares in all three categories are up for grabs. And so are its customers.

Who benefits most from the demise of Memco won't be determined for some time. It's apparent, however, that Giant Food Inc. isn't leaving anything to chance and has seized the initiative in a bid to expand its share of the food and drug store markets.

The more than $85 million in food sales that Memco recorded last year represents a sizable chunk of the Washington market, which is dominated by Safeway and Giant. And the contest for Memco customers could narrow the gap between Safeway and Giant or add to the edge that Giant holds.

The latest survey by Food World, a regional trade journal based in Columbia, shows Giant with a 39.2 percent share of the market and Safeway with about 30 percent.

Memco ranked fifth among leading food chains in the greater Washington market. Its percentage of the market, although small compared with Giant's and Safeway's, is a very attractive portion, nevertheless. And Giant already has begun a determined bid for Memco's food and pharmacy business.

In a two-pronged campaign to get the jump on competitors, Giant has purchased the pharmaceutical inventory and customer prescription files in all 12 Memco pharmacies.

Thus, Giant has assured former Memco pharmacy customers that they can have their prescriptions refilled without interruption of service at the nearest Giant pharmacy.

In the meantime, Giant is interviewing Memco pharmacists for employment. "We have agreed to interview those people Memco pharmacists and take as many as we can," said Morris H. Bortnick, Giant's senior vice president in charge of professional services.

Although Giant and Safeway dominate the area's supermarket competition, Giant also has emerged as a formidable competitor among leading drug chains operating in the Washington region.

The Landover-based supermarket chain has added 71 pharmacies to its operations since opening the first in 1962. In recent years, all new stores added to the Giant chain have been food-pharmacy combinations.

Giant remains committed to those combinations except in those cases where the company takes over a smaller, free-standing store, a spokesman said.

The spokesman also pointed out that Giant pioneered the one-stop-shopping concept in the Washington area. Although it has worked for Giant--notwithstanding the chain dropping its Super Giant department stores in 1978--the concept apparently enabled Memco to be only marginally profitable in a strongly competitive market.

And when Memco officials decided to abandon the Washington market, they approached Giant with an offer to sell the pharmacy departments because they apparently were confident of working out a satisfactory deal quickly, Bortnick explained.

"We made a deal within a day," Bortnick recalled. "We gave them a fair price for their products."

Giant made no such deal for Memco's grocery inventory, but again it has taken the initiative in competing for volume and profits by going after Memco shoppers.

In a highly visible campaign, Giant's newspaper ads and circulars sent through the mail welcome Memco customers to Giant supermarkets. Pending approval of new check-cashing courtesy cards, Giant will accept Memco customers' personal checks for purchases when they present their Memco membership cards.

Although Giant has taken similar steps previously when a major retailer has gone out of business, that had been done mostly in the Baltimore market, an official noted. "They have a lot of cash, and they can afford to do a lot of things," Food World's associate editor David Uchic said of Giant. "They can afford to take chances. I don't see anyone else doing anything like that."

Nonetheless, Uchic thinks Memco customers, like most price-conscious consumers, will shop at the supermarket that's most convenient to them.

All of the Memco stores, coincidentally, are near Giant pharmacy-supermarket combinations.