About 18 months ago, the Entenmann family of Bay Shore, Long Island, decided Washingtonians were ready for some of their 40,000 crumb cakes and 130,000 chocolate chip cookies, to mention just a very few of the 145 different sweets they bake each week.

So they sent down their plain white boxes with simple cold-fashioned blue lettering and cellophane windows. And they never breathed a word of advertising about their arrival.

Voila! Instant stardom. About 200,000 boxes a week - everything from 89 cent chocolate fudge squares to chocolate layer cakes for $2.29, trucked into the Baltimore-Washington area and sold at all but one of the major supermarkets, plus dozens of convenience stores and independent markets.

Customers like the man who was seen furtively putting 15 boxes of Entenmann's apricot pastries into his shopping cart are a sign of the company's rapid, and somewhat surprising, success here

Those plain white boxes are part of the mystique. Robert Entenmann, a grandson of the founder, is proud that some people their think box is "homely. We like it and everyone has imitated it. Take a look at one of your local bakeries in Washington and you'll see what I mean."

At a time when most products are shouting at customers from four-color packages with expensive graphics, Entenmann's is selling itself as modest and wholesome, qualities which marketing specialists believe are becoming increasingly desirable.

Even the company's stockholders report exudes hominess.There's a picture of a grandmother and some cakes on the cover. no expensive model was hired to pose. She is Martha Entenmann, the daughter-in-law of the company's founder, and according to her three sons, the person who has kept the business together. The company went public in 1976 "for estate planning," but the family retains 80 percent of the stock, which split three for two last month.

Grandpa William Entenmann opened a bakery in Brooklyn in 1898, a few years after he immigrated from Stuttgart, Germany. Several years later, when his son, William Jr., had rheumatic fever, the doctor recommended a move at least 50 miles out of the city for "fresh air and the country." Which is how the bakery ended up in Bay Shore, an hour out of Penn Station, but hardly the country anymore.

In the best tradition of family bakeries, William learned his trade on the job as a child and met his wife across a bakery counter in Bay Shore where she was selling and keeping the books.

"He fell right in love with me," says the Hobokenborn, 71-year-old matriarch of the firm, who only recently reduced her 5 a.m. to 9 p.m. schedule at the plant. Now she leaves at noon. Mrs. Entenmann, whom everyone at the plant calls Mrs. E., was always a "career woman," although that's not what they were called in 1926. She never wanted to stay home, partly she says, because "I don't like housework."

Had Mrs. Mrs. Entenmann not been involved in the bakery'd day-to-day operations, it probably would have been sold in 1951 when her husband died. "She was advised to sell it," according to one of her sons, "but she said she'd save if for the boys," who were serving in the Korean war.

"It wasn't easy," Mrs. Entenmann recalls. "But with the help of my brothers, and sister-in-law and father-in-law," who had retired by that time, "we made it. The problem was managing about 50 people 24 hours a day, seven days a week - the front, the back, the books. . . ."

Soon after the sons returned from the war, the bakery went wholesale. It gave up house-to-house selling and its retail operations. And even if it hasn't been "a piece of cake" since then, the business is thriving.

Mrs. Entenmann still checks in the drivers, takes the cash and rates new products. She has no plans to retire. "A lot of people say, 'Why are you working?' But they just stay home and gossip. I don't think I'll ever retire.I may just cut down my hours. I love to travel."

Even when Mrs. Entenmann travels, she stops in all the bakeries she sees. "I look around, compare prices, check the top seller, like chocolate chip cookies. Betty Furness scored ours tops," she noted.

A day doesn't pass without Mrs. Entenmann eating some of what her company produces. Vacationing this winter in Florida, which got its own Entenmann's bakery 2 1/2 years ago to serve displaced New Yorkers, she would go to the store, just like everyone else, and buy her daily quota.

Each of the sons is responsible for a different department in the 500,000-square-foot plat which employs 2,200 people. There is always an Entenmann on the premises. Every day between 30 and 40 items are produced and every day a sample of all those items is cut-up, weighed, tasted and examined by one or more of the owners and several trained employes. The items are given a rating from 10 to 10-, any that fail to get 6 or higher are investigated all the way back to the flour barrel.

Every month three new items are added to the line and three removed. "Right now we're kicking around a raisin cookie, and we've almost gotten a new muffin line the way we like it." They also are looking carefully at the cookies of one of their competitors, Pepperidge Farm. They admit to failure for only one item: lemon meringue pie. "We could never make it so the meringue didn't slide off the filling."

Entenmann's competition includes a few nationwide bakery manufacturers, small local bakery stores and the in-house bakeries of stores like Giant. To insure that supplies for the Balitmore-Washington area, four hours away, are fresh, they are trucked down each day from Long Island to a depot near Baltimore. Route vans, which distribute to the stores, begin leaving the depot around 3 a.m.

The bakery has been so successful in this area the company opened a thrift store in Rockville on March 1. It sells outdated merchandise and what is so colorfully known in the trade as "cripples," fresh items that don't look perfect. For the thrift store they have done some advertising.

Boston could be the next market. Then who knows? "Maybe Germany, Switerland or Belgium." Asked if that wasn't like taking coals to Newcastle, William Entenmann said: "Everybody eats our cake. We test marketed there and they were very successful. Maybe we could put the cakes on a 747 and fly them over and still be competitive."

Not everyone thinks Entenmann's is "as good as it used to be," but Gael Greene, food-maven-turned-author, who is a steadfast fan, has put her seal of approval on it. "It's high class junk food," she says, and who doesn't like that.