Journalist I. F. Stone offered to reveal his incision. Artist Jim Rosenquist arrived in pink and peach Hawaiian attire, then said the party was a lot like a cattle roundup. Pianist Van Cliburn said he was glad he had the night off. Actor Theodore Bikel claimed Washingtonians are actually beginning to look normal to him.

And Toby Goldberg came all the way from Freehold, N.J., because after all, it was Uncle Joe's 80th birthday. Besides, said her husband, Sy, "Who has birthday parties like this at home?"

Who indeed. But then, who else besides Joseph H. Hirshhorn, the tiny man who says he ate garbage as an immigrant in Brooklyn but grew up to give away Picassos and Calders, would celebrate his 80th by having 1,000 friends over for a little dancing and cake?

On Saturday night, a somewhat diverse mob descended on Hirshhorn's namesake museum for a giant, noisy party the neighbors would have complained about had there been any. The crowd included socialites, museum directors, politicians and lots of New York art types who spiced up the evening for Washingtonians used to more mundane affairs where everybody dresses alike.

A favorite activity of the locals was to stand at the bottom of the escalator and watch the costumes slide by. "Now there's an outfit for you," said Marilyn Tublin, a museum docent who was escalator-watching. "That woman is dressed in birds and skulls."

And so she was, with feathers sprouting from her forehead and an animal skull draped around her neck. Another outfit popular among the Washingtonians, who had carefully heeded the black-tie dress code, was a Scottish kilt. But that skirt actually ran a poor second to a black-sequined minidress and black fishnet stocking ensemble worn by a woman who danced with at least two men at a time. "We always do this," she said of her trio.

As for the guest of honor, he wore a baby-blue shirt, a black tuxedo and a pink rose and pronounced everything absolutely "great" from the three-tiered birthday cake that was hacked to bits by the five-deep crowd around it to the poster present that said "Happy Birthday Joe." It was printed with the signatures of Henri Matisse, Henry Moore, Willem DeKoonig and other artists whose works the Latvian-born collector has given to the museum.

Hirshhorn arrived after the party was well under way from a private, 120-person dinner at the Smithsonian Castle. He was just about squashed by the well-wishers, photographers and reporters who swooped down on him. The band, which had been playing George Gershwin earlier and would play the Bee Gees later, struck up "For He's a Jolly Good Fellow."

And so the little man whose first meal in America was on Ellis Island inched through the crowd, laughing, accepting kisses, shaking hands and looking the 60 years old he said he felt like.

"This is so important for my dad," said Gordon Hirshhorn, the only son of the six children. "There's more sentiment for him now than there was at the opening of the museum five years ago. The place has mellowed, and he's mellowed with it."

Hirshhorn spent the rest of the evening in the rain sitting next to his "baby" sister on the terrace, shaking hands with more party guests.

"Hey, Olga," he yelled to his wife every few minutes or so, "it's drizzling out here. Let's go inside."

Said sister Dora: "But it's good for you."

Hirshhorn: "Oh yeah?"

Dora: "Yeah. It'll make you grow."

Olga Hirshhorn, who isn't much taller than her husband's 5 feet 4 inches herself, didn't seem to mind the drizzle and chattered away about the merits of older men.

"I'll tell you," said Olga, 59, "all of you women who wonder what it's like to be married to an 80-yar-old man, well, it's really fantastic. What do I like about older men? It's funny. I guess it stems back to when I was a little girl. My movie idol was Leslie Howard, and he was an older man."

In between his wife's revelations, Hirshhorn had a few of his own - like what he's going to do with the art he hasn't given to the museum yet.

"I'm going to cook it," he said, more or less disgusted that he'd been asked that old question again. But he added: "I'm going to sell some, sell a lot of the duplicates."

And then it was back to the handshaking and back-slapping. "Holy mackerel," he told a member of the small army of Hirshhorns who turned up for the biggest birthday party the family had ever been to, "I haven't seen you in 40 years."

Inside, away from the drizzle, the Hirshhorn birthday talk and liquor flowed. Lots of art luminaries showed up, like Livingston Biddle, chairman of the National Endowment for the Arts; S. Dillon Ripley, secretary of the Smithsonian; Peter Marzio, director of the Corcoran; Warren Robbins, director of the Museum of African Art; Mary Ann Tighe, deputy chairman of the National Endowment, and Martin Friedman, director of the Walker Art Center in Minneapolis. Everybody had praise for Hirshhorn.

"He could be anybody's Jewish grandfather," said Jerry Loube, the husband of a museum docent. "He's a real down-to-earth guy. He's cute."

Izzy Stone, on the other hand, thought Hirshhorn's wife was cuter and said so. Stone also said he himself was nowhere near 80 years old but rather "a juvenile delinquent on parole. I've barely passed 30 for the second time."

At this point, Stone, who was in good party form, asked: "Would you like to see my incision?" He then made off into the crowd, threatening as he vanished, "Don't mix up any of my jokes now, or I'll sue you."

Other luminaries at the party included artist Larry Rivers, who has done a portrait of Hirshhorn that he claims the great collector keeps hidden away. "He doesn't show it to anybody," Rivers said. "I didn't make him handsome enough."

Rivers spent a lot of the evening harassing his old friend John Ashbery, the poet and critic who was in town reviewing the Russian art exhibit at the Renwick Gallery.

"Why aren't you home writing poetry?" Rivers asked Ashbery.

"Because I'm having a drink," responded the poet.

"He needs some adverse publicity," Rivers complained. "He always gets good publicity."

By this time, the band had long abandoned waltzes and foxtrots and was well into Donna Summer and the soundtrack from "Saturday Night Fever."

"What is this?" asked a Washington woman. "American Bandstand?"

A young man standing nearby just shrugged.

"Well," said the woman, "I'd give it about an 80. It's got a good beat, but you can't dance to it." CAPTION: Picture 1, Joseph H. Hirshhorn blowing out birthday candles, watched by S. Dillon Ripley, right, by Fred Sweets - The Washington Post; Picture 2, Mr. and Mrs. Joseph Hirshhorn at 80th birthday party, by Fred Sweets