What set them apart - besides the rosebuds in their laples. leftovers from Joan Mondale's afternoon reception - was a kind of air of expectancy.

One of the evening's sponsors came closest, perhaps, by suggesting that the artists at the National Collection of Fine Arts last night weren't only discovering Washington but, more importantly, Washington was discovering them.

"It's marvelous to be here with living American artists in the capital of their country where they ought to be displayed," said Richard Contee of the Minneapolis-based Dayton Hudson Foundation.

Others in the black-tie crowd last night at the National Collection of Fine Acts, where the "First Western States Biennial" exhibition was openning, said that now Washington had recognized these artists, the art gallery clique in New York would have to take notice as well.

For Luis A. Jimenez Jr., the New Mexican sculptor, breaking into the National Collection was almost too good to be true. Not only were two of his huge fiberglass works on loan there for the show but a third, "Man on Fire," has been donated to the permanent collection by Philip Morris Inc.

"They showed an awful lot of insight doing that," Jimenez said, relishing the moment. The show itself, which came here from the Denver Art Museum, was "a big mark" for him.

"People stop me in the streets," Jimmenez said. It's helped my image, that's for sure."

It was Jimenez' "Progress: Part II" that stunned the senses of many of the 375 guests from Hill, media and cultural circles. A gigantic work depicting a cowboy on horseback lassoing a steer, it was a kind of bas relief of what Jimenez defined as old WPA murals. Eyes were light bulbs and when someone asked the artist bow he had the gall to use light bulbs, he replied quited convincingly that if ancient Buddhas had been created today they would have had light bulbs instead of diamonds for eyes.

Ross R. Millhiser, vice chairman of the Philip Morris board which donated the $7,000 that bought Jimenez' "Man on Fire," was introduced to the guests by Dr. Joshua C. Taylor, NCFA director.

"It's not always that a businessman comes to Washington and feels he is among friends," said Millhiser, who had led a delegation here via company jet earlier in the day. It was the 38th time Philip Morris had sponsored a major exhibit.

"In business, you're paid to know what to expect," Millhiser said at the Mondale party. "So you're not surprised. But art is just the opposite. You want heterogeneity - homogeneity is a curse."

Works were nothing if not individual and when the Kennedy Center's Roger L. Stevens spotted the John Buck work of a cutout man against a cutout skyscraper, he was momentarily bemused.

"Well," said Stevens, who once bought and sold the Empire State Building, "it would make somebody very happy who likes skyscrapers."

The preview preceded an al fresco buffet-supper set up in the gallery's courtyard.

"I'm very aware that seeing the exhibition after dinner just doesn't work," said NCFA's Taylor. CAPTION: Picture, Roger L. Stevens, left, Dr. Joshua Taylor and Rep. Sidney Yates, right; by Joe Heiberger - The Washington Post