"We celebrate 12 rich contributions to the American arts," President Reagan told guests at a White House luncheon yesterday to honor National Medal of Arts winners, "and in a wider sense, we celebrate American culture."

Most of the honorees this year are well known and well honored, some legendary: composer Aaron Copland, 85; choreographer Agnes de Mille, 80; opera singer Marian Anderson, 84; author Eudora Welty, 77; painter Willem de Kooning, 82; film director Frank Capra, 89. One honoree is a corporation: Exxon. Others are actress Eva Le Gallienne, 87; folklorist Alan Lomax, 71; critic Lewis Mumford, 90; Buffalo arts patron Seymour Knox, 88; and Houston arts patron Dominique de Menil.

The award was conceived by Reagan in 1983 and given out for the first time last year. The president noted that now, "in matters of culture, Americans look not so much to the Old World as to the new, America itself, and they do it with pride."

During the presentation at the East Room luncheon, Nancy Reagan spoke about the accomplishments of each honoree and President Reagan then handed each a sterling silver medal encased in a wooden box.

"Well, I'm very proud indeed," said Eudora Welty as she walked out of the White House after the lunch. "I've never gotten anything like this before." Welty has, however, received the Pulitzer Prize, the Presidential Medal of Freedom and many other awards and honorary degrees.

Last night, the honorees were feted at a reception given by Ambassador at Large for Cultural Affairs Daniel J. Terra and his wife Judith at the Diplomatic Rooms of the State Department.

Folklorist Alan Lomax at 71 is the youngest of this year's medal winners. "I spend my life talking to the nicest people in the world -- the folk artists," said Lomax last night, sporting in his jacket lapel a tiny pink rose like that seen on other medalists, "and that's what's really kept me young."

Buffalo arts patron and medalist Seymour Knox, better known to good friends as "Shorty" ("That was my nickname at Yale") said of his award, "I think it's wonderful. I didn't know it was going to be such a big event here. They're taking very good care of me."

Six of the 12 honorees did not attend, due to illness or difficulty in traveling. Only four of the six honorees who attended the White House luncheon came to the party. Mumford, according to his daughter Alison Morss, "doesn't travel at all. He's in residence" -- at Amenia, N.Y., 80 miles north of New York City -- "and that's where he stays," she said. Capra was also not well enough to leave his Palm Springs home for yesterday's events. "He's had a series of strokes," said his son, Frank Capra Jr., "and he really can't travel."

Sandra Grymes, who accepted the medal for her cousin, Marian Anderson, who lives in Danbury, Conn., similarly said, "She doesn't travel anymore."

It was generally lamented yesterday that more of the honorees could not attend.

*"We should have one in their fifties, three in their sixties, and the rest whatever you want," National Council on the Arts member Jacob Neusner said. "It's wonderful that people in their nineties can get awards, but let's honor the artists while they still . . . benefit from the honor."

The NEA solicited "as many nominations as possible" for the medal, NEA Chairman Frank Hodsoll said. A committee of the National Council on the Arts -- the NEA's advisory board -- then reviewed several hundred nominations and the council as a whole voted on the winners. (Neusner, by the way, abstained.)

"I think that first one has to look at awarding these medals to the most distinguished artists and patrons," said Hodsoll, "and there was a feeling that we shouldn't just let a few slip away because they are too old or infirm to attend. However we would hope in the future more honorees would be able to attend. But looking at that list, there's not one I wouldn't want."

At the luncheon, accepting the medal for de Kooning was his wife, Elaine, also a painter; for Le Gallienne her friend and colleague Anne Kaufman-Schneider; for Lewis Mumford, his daughter, and for Copland, his friend and biographer, Vivian Perlis.

A second Capra son accepted for his father. "It's dear to us," said Tom Capra, a television news director in Los Angeles, of the medal. "It's kind of nice to get it, with Liberty Weekend so close."

In her remarks, Nancy Reagan called honoree Capra "a pioneer of the art of film . . . one of the truly great artists of a unique American art form." Capra directed such classic Hollywood films as "It Happened One Night" "Mr. Deeds Goes to Town," and "You Can't Take It With You" -- all of which won him Academy Awards for best direction. But perhaps his most memorable films are "It's a Wonderful Life" and "Mr. Smith Goes to Washington."

Of Agnes de Mille, Nancy Reagan said, "Her name is synonymous with the art of dance." Known for her ballet work as well as her theatrical work, she is the choreographer of Aaron Copland's "Rodeo" as well as the Broadway classics "Carousel," "Oklahoma!" and "Brigadoon."

De Mille, who was present yesterday, was accompanied by two White House military escorts to receive her award from the president and helped back to her chair, all to continuous applause from the guests.

Of Marian Anderson, Nancy Reagan remarked, "Arturo Toscanini said, 'A voice like hers comes only once in a lifetime.' " One of the world's leading contraltos, Anderson didn't make her debut at the Metropolitan Opera House in New York until 1955, because of discrimination against blacks. She was the first black to appear there.

Copland was cited as one of America's greatest composers, known for his "Lincoln Portrait," "Billy the Kid," "Rodeo" and "Appalachian Spring."

De Kooning, said Nancy Reagan, "influenced all modern painting." He is one of the central figures of the American abstract expressionist movement, and his work is exhibited in most of the major museums in the United States and abroad.

Le Gallienne, "a great actress, director, producer and teacher," said Nancy Reagan, has starred in such plays as "The Corn is Green," Schiller's "Mary Stuart" and "The Madwoman of Chaillot" and directed Chekhov's "The Cherry Orchard" and Ibsen's "A Doll's House."

Alan Lomax was praised by Nancy Reagan for enriching "our understanding of the cultures of the world." Lomax has collected and preserved folk music of this country and other parts of the world. In the '30s and '40s, he presented the then-unknown music of Burl Ives, Pete Seeger, Leadbelly and Woody Guthrie on national radio. Lomax is also credited with discovering the jazz music pioneer Jelly Roll Morton.

Lewis Mumford was called "one of our most distinguished critics" by Nancy Reagan. An urban critic, he has long warned against the proliferation of big buildings at the expense of the livability of cities and the comfort of their inhabitants.

Welty was praised as a writer of literary importance and popular appeal. She is the author of 12 novels and collections, including "The Optimist's Daughter," which won her a Pulitzer Prize in 1973.

Dominique de Menil and her late husband John built one of the greatest privately owned art collections in the world. Together they were responsible for developing the Houston Museum of Fine Arts, the Contemporary Arts Museum and other museum collections. They also built the Rothko Chapel, the ecumenical chapel in Houston constructed around the paintings of Mark Rothko.

Knox is one of the main patrons of Buffalo's Albright-Knox Gallery, which has one of the world's foremost collections of contemporary art.

Jack Clarke, senior vice president and director of Exxon, accepted the medal bestowed upon his corporation, which, Nancy Reagan said, is "known by millions for its promotion of arts on television." For the past 12 years, Exxon has sponsored the public television series "Great Performances."

*Ironically, Exxon has decided to reduce funding to "Great Performances" during the 1987-88 season. However, Exxon is funding the coming season at the current level.

"We're cutting back on contributions in general as a reflection of our business," Clarke said last night. "We've cut back on exploration . . . as well as employes as a reflection of what's going on in the oil business. We're still going to be involved on a significant level. We're still supporting 'Live From Lincoln Center' . . . I think the award is a recognition of what we have done . . . We're not getting out of supporting the arts."

At the Terras' reception, Secretary of Defense Caspar Weinberger and Attorney General Ed Meese were among the minglers, most of whom were connected with various arts organizations here and across the country.

Before Secretary of State George Shultz could praise the artists for their part in carrying out the American spirit and so forth, he turned his attention to Daniel Terra, who just married his fiance'e of several years. "I can't tell you what a pleasure it is to hear Daniel Terra speak of Mrs. Terra," he said to cheers and chuckles from the crowd.