"Yum," said a man as he tasted the dip on the end of his celery stick. "I believe it's mucilage." Certainly not an unusual observation for a Washington cocktail party, but it was especially appropriate last night. As one observer standing nearby pointed out, what would American taxpayers have said if the staff of public television's "MacNeil-Lehrer Report" had celebrated their 1,000th telecast last night with caviar and sour cream.

It was a night for the jovial and the frivolous sides of one of public television's most serious and durable public affairs programs, now playing to over 3 million households five nights a week. As the names of the more than 3,000 guests who have appeared on the program -- among them the Ayatollah Khomeini, Anastasio Somoza and Jimmy Carter -- rolled by on the video screens, the 200 celebrants mixed and mingled in the WETA studio.

Jim Lehrer, the Washington anchorman and interviewer for the show, stood watching the scene happily on the sidelines. Asked if he foresaw any changes in the program, which began in the summer of 1976, he laughingly answered, "How can you improve on perfection?'

Across the room, Jo Franklin, one of five producers of the "MacNeil-Lehrer Report," and Peggy Whedon, producer of ABC's "Issues and Answers," chatted amicable about the difficulties of scheduling people for their shows.

"If someone doesn't show or bugs out . . . " began Franklin.

"Hubert Humphrey was always my standby," said Whedon. "One time he was playing baseball and I got him on his walkie talkie about an hour before showtime. He came in wearing his baseball uniform with mud on his knees. We gave him a coat and tie and filmed him from the waist up."

Several of those who have faced Lehrer's questions came for the celebration, including Sen. Strom Thurmond (R-S.C.), Jim Flug of Energy Action, and Ben Fernandez, Republican candidate for president.

Thurmond, looking dapper in his tie bearing the number 96 (the number of the legislative session), modestly said that his wife, Nancy, who has come out with a book this year, does better on talk shows than he does. "In the first place, she's better looking. And she's also articulate," he said.

Benjamin Fernandez said he was traveling 5,000 miles and putting in 100 hours a week in his quest for the presidency. "I think the country needs an economist in the White House," added Fernandez, who is one.

A little later, after all had a chance to take advantage of the open bar, most of the crowd wandered into the broadcasting room where the New York half of the show could be seen celebrating on numerous video screens strategically placed around the room.

Robin MacNeil, the blond Canadian who began the show, appeared on the screen, mike in hand, to say a few words from New York. "We feel, perhaps a little arrogantly, that we are near the cutting edge of trends," he said.

"The bread and butter of the show will always be intelligent people," he added, "who can resist the temptation to pick their nose and make obscene gestures." Such are the costs of TV journalism.

President Carter's campaign manager, Evan Dobelle, who said he schools himself daily on the issues by watching the "MacNeil-Lehrer Report," put in an appearance, along with his wife Kit, just named director of Rosalynn Carter's East Wing staff. Kit Dobelle declined to go into detail about her new job, except to say, "I want to be sure that everything Mrs. Carter does is well organized."

After a suitable amount of chitchat, the night's show got underway. The subject of the half-hour broadcast was Andy Young's resignation, and on screen all was proclamation and interpretation.

But most of the partyers seemed uninterested, at least for the night, in public affairs. A few were clustered around the TV, watching the program. Most whispered quietly about nothing of earthshaking importance. But when a beer bottle clattered to the floor, a chorus of shushes greeted it.

And somebody said chidingly, "It's a live show, so if you cough it'll be heard across the country."

"The cough heard 'round the world," said another.