When Armstead Barnett finished serving a sit-down dinner for 80 people at the home of Howard University president James E. Cheek recently, he asked one of the guests for her autograph.

It was an unusual breach of decorum for the veteran of White House glamor who for 25 years has been a caterer for some of Washington's most illustrious names.

But this time, for the legendary Lena Horne, Barnett was leading the pack of admirers: "I was holding her chair and that's how it began," he explained. The towering 67-year-old's tone was sheepish, scrambling for excuses. "My son asked for an autograph first and I was standing there, so naturally I got in.

Adoration soon gave way to professionalism, however, and Barnett led the clean-up of dishes. When it was all over, he was pleased because the Cheek dinner - one of the last he will cater - was the kind he most likes to serve: a white-gloved, leisurely, multi-course meal of backfin crab appetizer, Long Island duckling with wild rice, asparagus with browned butter, tossed green salad, strawberries with cream and champagne cookies.

Barnett is now ready to retire after a quarter of a century as one of the city's best-known black businessmen, and for 15 years before that an institution at the White House under the Roosevelts, Trumans and Eisenhowers.

A year ago, worn from "working too hard and these long hours," Barnett sold the catering business that bears his name but agreed to continue working to oversee the transaction. "I decided to pull out while my name is still in good shape," said Barnett. That name will remain in town; he is considering a management job in food service corporation.

On a recent day off, Barnett sat in his living room decorated with baby-blue carpet, Roman statuary and a photograph of Harry S. Truman, and reminisced about life for a black caterer amid the Washington whirl.

His long career began largely by accident. In 1933, on his way from his home in Lynchburg, Va. to Philadelphia, he got off the train in Washington to see his aunt. The next night his cousin, who had been the doorman at the White House since the Taft administration, asked Barnett to help out at a cocktail party here.

"Down South I had been working at the cotton factory, I didn't know what I wanted to do. So my cousin took me along. I learned a lot, I had never been in service before," said Barnett. And chopping carrots was better than baling cotton.

"Service," as he still calls the art of waiting and cooking, seemed to suit him, and within days he had gone to work for a wealthy private family here. In June of 1938 he joined the White House staff as a full-time pantryman.

Despite the long and arduous routine of the White House, in those days the housekeeping staff took on extra jobs as cooks and waiters for prominent families. It was fashionable for Washington hostesses to have executive mansion help; and it was extra income for the help.

"It started out with just a few congressmen, senators and judges asking us to help out at parties," he remembered. "But when it grew and people would call the White House looking for a butler, we had more work than we could fill." He added that the government jobs didn't come easily, and over the years he was constantly obliged to prove his capabilities.

After the war, the outside work became so demanding that Barnett formed a Butler Association of a dozen men. "At the same time," he recalled, "Truman moved me to the Executive Office. There you had to make sure the paper was in the right place, take the coats of guests. And Truman always had a rose on his desk. Truman put me there because he knew we had all those other jobs."

From the outside, Barnett's tenure at the White House seemed privileged, but he was involved in a struggle to maintain his young marriage. "I didn't have any personal life, per se," said Barnett, as his wife of 38 years, Viola, nodded in agreement. "After the White House job, there was no struggle, the struggle had been before - juggling."

In 1954, after a year's leave from the White House, Barnett officially started his firm with six employes, three of them family members.

"Oh, in those early days, the service was great. People preferred the French style, very formal and graceful. And people demanded the whole treatment for six guests or 400 guests," he said, allowing himself a sigh for the old days which have given way to the informal, sometimes budgetary, buffet. Recently, he recalled, the State Department asked him to use napkins for hors d'oeuvres instead of plates. He balked: "Never."

For a long time the Barnett clientele was almost exclusively white, since only a few black families entertained on a grand scale. And, looking back, he feels he was well-treated by all his clients.

"Discrimination, no, not even in southern Maryland. We were treated finely. No incidents. When we got out on our own, people would say Vi and I were 'not like the rest of colored pople," said Barnett, grimacing at the slur. Government jobs were harder to come by: "Government was the only place we met some resistance."

The largest party Barnett ever handled was a racetrack supper given by the Postmaster General for 25,000 in 1968. "All the big caterers said I couldn't do it. Jealousy," said Barnett. But once he inspected the ovens at the track and arranged the 12 buffet tables, cooking 24 steamship rounds of beef was a breeze.

On one recent Thursday, he worked three jobs at the State Department, starting with a coffee for 200 people, cocktails for 500, and a dinner for 10. The next evening he was saluted by a cadre of people in his neighborhood, Ward 7. "I have always tried to be more than a businessman. We have given lefrovers to needy families. When the teen-agers came by, we gave them money for peeling potatoes or going to the store," said Barnett. "At the stores we have sold tickets for charity, had meetings for the politicians when they were running. Also I have donated help at times, paid waiters out of my pocket."

At the urging of his family, Barnett intends to pass on his traditions. His first plan is to establish a school in Washington. "Because black folks just have to learn the arts of cooking and serving," he said, leaning back in his easy chair.

His second plan is to write a cook-book, and here he looked very sheepish. He has discussed this project over the years, always reluctant to write down his favorite recipes, some of which appear below. But he has vowed this time he will put together a book of Southern-inspired Barnett staples.

"Now I have to decide what to call it," he said. "I guess it should say 'White House' or 'Southern' in the title to sell."

SWEET POTATO PIE (6 servings) 1/3 cup butter 1 cup sugar 1 can (1 pound) mashed sweet potatoes or 1 3/4 cups cooked and mashed sweet potatoes 3 eggs 2 tablespoons dark rum 2/3 cup evaporated milk One 10 inch unbaked pie shell

Cream butter, add suger gradually. Add potatoes and mix well. Beat in eggs one at a time. Add milk and rum, beat until smooth. Pour into pie shell. Bake at 450 degrees 10 minutes, reduce heat to 350 and bake 20 to 25 minutes or until firm.

CHICKEN ROSEMARY (4 to 6 servings) 3 chicken breasts, 1 pound each, halved, skinned and boned Salt and pepper to taste 5 or 6 tablespoons butter 1 cup chicken stock (make with skin and bones from above) 1/4 cup flour plus flour for dredging 1 teaspoon rosemary 1 cup white wine

Sprinkle chicken with salt and pepper, dredge lightly in flour. Heat 2 to 3 tablespoons butter over medium heat, saute chicken only until golden on both sides. Remove from pan, add remaining 3 tablespoons butter to pan, add flour and mix well. Add stock, stirring constantly until thick. Stir in wine and remove from heat. Butter a 1 1/2 quart casserole, sprinkle rosemary in corners. Place chicken in casserole and pour sauce over. Bake at 350 degrees 30 minutes. CAPTION: Picture 1, Armstead Barnett, by Tom Allen - The Washington Post; Picture 2, no caption; Picture 3, Dwight Eisenhower; Picture 4, Harry Truman.