In the last few years, many retail firms that specialize in big-ticket items have opened Washington area stores to tap one of the nation's most affluent markets.

But it hasn't taken their managements long to discover that an unusually high level of spendable income is not the only factor which characterizes the local market. They also have found a level of theft that retailers testify is among the highest in the nation.

For example, at one such store recently some merchandise reportedly was stolen during an opening reception to which were invited only very important people. Although there has been no confirmation by officers, a relaible source said one woman, reportedly a professional shoplifter, made off with more than $3,000 worth of shoes and jewelry in the first days of the store's operation here. And a number of Cartier watches disappeared - from behind lock and key.

Quite neatly, these reports demonstrate the scope of a problem that cost area retailers an estimated $407 million in stolen merchandise for the year ended July 31.

The figures, made public yesterday by the Metropolitan Washington Board of Trade as that group launched a new antishoplifting campaign, mean that theft equals nearly 4 percent of the area's annual sales volume ($11 billion).

To help combat the local retail thefts, which are nearly double the national rate in terms of percentage of overall sales volume, the Washington business community has been conducting annual antishoplifting campaigns since the early 1970s.

Yesterday, Georgetown University French language major and self-styled "upper freshperson" Pearl Bailey took time out from her studies to help the Board of Trade's retail bureau attract media attention for the 1978 campaign.

As could be expected, performer Bailey stole the show, even though jammed downtown traffic delayed her arrival.

Bailey praised the classy posters prepared for this year's antishoplifting drive, featuring the theme: "Shoplifter. A label you'll wear for life."

But, she told businessmen and law enforcement officers in the audience, they should add a brief "P.S." from the Bible: "Thou shalt not steal" (Exodus, xx, 15).

Many people still "are afraid of the Lord," if nothing else, Bailey asserted.

As the theme of this year's campaign suggests, fear of being caught and prosecuted for shoplifting is a central focus. Television and radio spots, played for reporters yesterday, seek to emphasize that an individual's life could be damaged permanently by an arrest record alone.

Bailey siad prosecution or some type of punishment is an essential element in any drive to combat stealing. The other major elements are teaching young children at home not to steal and improving attention to duty by retail clerks, she said.

Looking at executives from Woodward/Lothrop, Hecht's and Sears Roebuck, Bailey blamed inattentive clerks for some of the shoplifting. "A lot of people take things because there is not one around . . . the clerk are off taking about their date the hight before," she told the retailers.

And the inattentive clerks are bad business, too, she noted. If people don't find clerks to help them, they'll either steal or go away angry, she declared.

For their part, the retailers expressed relief that they had spoken first and didn't have to follow the entertainer on the program of yesterday's new conference.

Earlier, they had offered details from the most recent surveys about stealing at stores here. And they expressed some encouragement over the recent figures. Although the total volume of employe theft and shoplifting reached a record $407 million in the most recent 12 months, up 2 percent from a year earlier, the dollar increase was the smallest since antishoplifting campaigns began. Four years ago, the volume of thefts jumped 46 percent in one 12-month period.

Among other findings, reported by retailers Charles Huntley of Sears, Philip Gragan of Woodies, Anthony Harrimab of Neiman-Marcus and Edgar Mangiafico of Hecht's:

Up to 50 percent of all shoplifting occurs between mid-September and the end of December, the busiest time of year for most stores. Items most often stolen are all types of clothing jewelry and cosmetics.

More than 33 percent of all area shoplifters come from housholds with incomes in excess of $20,000. Teenagers account for 35 percent (up 2 percent from last year) and housewives account for 30 percent (also up 2 percent) of the shoplifters.

In terms of education, 18 percent of shoplifters have attended college and 64 percent are high school graduates. About 55 percent of shoplifting occurs on weekends and 60 percent of shoplifting takes place in the afternoons.

The U.S. Commerce Department has estimated that, for every one shoplifter caught, 35 others get away. But Leonard Kolodny, head of the Board of Trade's retail bureau, discounts such estimates as "essentially ridiculous data . . . foder for the press."

Although Kolodny declined yesterday to estimate what percentage of Washington area shoplifters are caught and prosecuted, he obviously believes the figure is much higher than one of 36. All stores that belong to the bureau follow a policy of prosecuting persons caught stealing, he said.

During the 1977 campaign (Sept. 18-Dec. 31), there were 7,917 arrests in the area for sholifting. The year before, for a shorter period, there were 7,143 arrest - pointing to a decline because the 1977 campaign was longer by two months. Kolodny said he expects a decline in arrests this year because retailers are more sophisticated and because of the "powerful, graphic theme" of the tough antishoplifting campaign.

The success of the Board of Trade's program has attrached national attention and, this year, businesses in Frederick, Anne Arundel and Howard counties asked to be included for the first time.

One of the problems facing retailers is the lack of a uniform law enforcement approach to shoplifting by area governments. By far the most agressive stand has been taken by Arthur A. Marshall Jr., state's attorney for Prince George's County, who not only prosecutes all shoplifting cases (90 percent are convicted) but also publishes names of adults convicted and sentenced for shoplifting in weekly newspapers.

Marshall, meantime, suggested another idea he might study. Because he can't publish names of juvenile offenders, he is considerating publishing a list of adults whose children have been convicted - to help emphasize the campaign's theme that being caught at shoplifting creates not only a permanent record for the offender but also an impact on his or her family.