New Yorkers can talk all they want about "provincial" Washington, but when it comes to manners, says one of America's leading arbiters on etiquette, too many Big Apple business tycoons behave like country bumpkins.

"For instance, everybody in Washington knows that the honored guest should be the first to leave -- and by 11 o'clock," says Letitia Baldrige, who has spent most of her adult life guiding people through social situations, perhaps most notably as chief of staff to Jacqueline Kennedy at the White House and social secretary to the late ambassador David K.E. Bruce in Paris and to ambassador Clare Boothe Luce in Rome.

"In New York you see heads of corporations having dinner at somebody's house where the guest of honor may be the head of a foreign country. I've seen the Americans look at their watch, say something about having an early day tomorrow and then leap out the door," she says. "Of course, the guest of honor takes offense."

Baldrige now heads her own public relations firm and is a pioneer in instructing corporate executives in how to behave themselves in polite society. Her newest effort at helping people mind their manners is "Letitia Baldrige's Complete Guide to Executive Manners," for which she was honored last night at an Embassy Row Hotel party thrown by her brother, Commerce Secretary Malcolm Baldrige. It is a 519-page book that grew out of a 30-page chapter on business manners she added to the "Amy Vanderbilt Complete Book of Etiquette" when she revised it in 1978. Those 30 pages received all the attention, and right then Baldrige says she knew something was very wrong in executive suites around the country.

At the party, Secretary Baldrige took only minimum credit for his sister's expertise. He said he and his brother used to beat her up all the time when they were children. "That's when she learned kindliness, thoughtfulness and good manners. After a while she began teaching us good manners and that's continued ever since."

Among the guests of Baldrige and his wife Midge were Sargent Shriver, Sen. Claiborne Pell (D-R.I.) and his wife Nuala; Agriculture Secretary John Block and his wife Sue; British Ambassador Oliver Wright and Lady Marjory Wright; J. Carter and Pamela Brown; Nancy Dickerson; Muffie Brandon; Ceci Carusi; Elliot Richardson; and one of the honoree's first bosses, Evangeline Bruce.

Bruce remembered how David Bruce had been with the Marshall Plan and how his appointment as ambassador to France had come almost overnight. "Tish was working in Paris and we decided we'd try each other out," said Evangeline Bruce. "I'd been in diplomacy all my life, but there was a lot to learn from Tish, whose enthusiasm and sense of humor made even the dullest work go well."

In the "Complete Guide," Baldrige theorizes that "good manners are cost-effective because they not only increase the quality of life in the workplace, contribute to optimum employee morale and embellish the company image, but they also play a major role in generating profit. An atmosphere in which people treat each other with consideration is obviously one in which a customer enjoys doing business."

She says it became increasingly apparent that senior executives who minded their manners were taking favorable notice of junior executives who also minded theirs. Those who didn't were beginning to see that their careers could be at stake. A whole new generation also was bringing with it a new set of mores that were raising questions affecting the work place.

Question: How do you handle the live-in boyfriend or girlfriend when the boss asks you to act as a single person at a social function he and his wife are giving?

Answer: Don't foist the housemate off on the boss or his wife. Convey your regrets by making it clear that you live with a very fine person whom you hope to introduce to the boss at a cocktail party (buffet supper/brunch/lunch -- you fill in the blank) you're giving next month.

"Usually when a person is invited by the boss it's because he needs a single woman or man," says Baldrige. "It throws everything off if you say 'I live with somebody -- can I bring him/her along?' And it's no good expecting the housemate to go along with the situation. The one at home will not allow it.

"It's not like the Foreign Service in the old days," she says. "Foreign Service husbands were constantly called upon to serve at ambassadors' parties to fill in between two dowager duchesses."

Another dilemma of the 1980s: What to do if someone on the corporate ladder serves cocaine at a party he's giving?

"I tell a young executive straight out, 'Don't think you have to be one of the guys,' " says Baldrige. "Don't let anyone notice you're not taking it, and make an excuse to leave. It's going to get more and more boring and more uncomfortable, so suddenly say something like 'I've got to get going, I've got somebody calling me from Tokyo [Rome/London/Paris -- take your pick.'"]