What was wonderful, the artists agreed as they sipped cocktails on the East Building terrace before sitting down to pate de poisson and filet de boeuf jardiniere, was the way in which Morton G. Neumann had managed to stay always on the crest of the artistic wave, visiting young and newly launched artists in their studios, buying their work before critics had pronounced them worthy, taking chances on the unknown.

What was not so wonderful, said his sons, was all the speculation on what their 82-year-old father would be doing with his collection in the future. "This is a happy night," said Hubert Neumann. "Let's not talk about that." "

"Imagine what it's like," said Arthur Neumann, "to walk into a museum and hear people say, 'Your father is in wonderful health,' and then look terribly sad at the idea."

Despite such observations, no one was looking terribly sad at the dinner in honor of the East Building's exhibit of paintings from what is considered one of the most impressive private collections of 20th-century art in the country. The dress was black tie, but the mood was much less formidable, which is probably only natural when the guests include some of the artists whose paintings are included in the exhibit. Such is not always the case, of course; museum exhibits are usually composed of artists who have long since shuffled off such mortal concerns as a satisfied palate.

The artists, naturally, had nothing but hosannas to sing about the exhibit entitled "The Morton G. Neumann Family Collection: Selected Works," and the praise extended to the Morton G. Neumann family, selected members, who were also on hand. "He's really wonderful," said painter Robert Zakanitch, who was dressed in a white dinner jacket and matching white running shoes. "I don't want to call him eccentric, but, well, see for yourself."

As if on cue, the 82-year-old Chicago collector, the friend of painters from Picasso to the practitioners of pop and pattern painting, chose that moment to make his appearance and was soon immersed in a small pool of well-wishers. Tall, thin and a bit hard of hearing, Morton G. Neumann merely beamed heartily when asked what he thought of all this and said, "Tell 'em I'm overwhelmed by the reception and that's it. Put in anything else you like."

Neumann's sons were more than happy to fill in the blanks. "What he did took a lot of courage," said Arthur Neumann, who is a doctor and lives in San Francisco. "People 40 years ago would look at the Picassos he bought and say, 'Aren't they cute?' or 'Nobody has eyes like that.' I remember stopping at a rest area on the Pennsylvania Turnpike once and overhearing one truck driver tell another how he was taking a load of 'Pistachios' up to the Museum of Modern Art. Back then nobody knew the difference."

A few floors below, his father's harvest filled the halls in proud profusion, paintings and sculptures, Picasso, Miro and Klee, Rothko and Warhol, Matisse and Man Ray. "It's an American dream," Arthur Neumann concluded, as the guests filed in to dinner. "To go from life in the back of a tailor shop to this -- imagine. It's a wonderful world."

Morton Neumann took a more prosaic look at his life when he began his dinner toast. "When one gets old," he said, "three things happen. One -- you get very forgetful. Two and three -- I forget what they were."