The largest and most important exhibition of Korean art ever to come to the United States, "5,000 Years of Korean Art," will begina two-year tour of seven American museums May 1 at the Asian Art Museum of San Francisco.

Though it will also make stops at the Chicago Art Institute, the Boston Museum of Fine Arts and New York's Metropolitan Museum of Art, the 345-work exhibition -- much of it classified as Korean National Treasure -- will not be seen in Washington.

The National Gallery of Art said that "prior commitments" had made scheduling here impossible. In addition, the National Gallery has a policy requiring all major exhibitions in which they participate to be shown there firsrt. A smaller show of "Masterpieces of Korean Art" was shown at the National Gallery 22 years ago.

The Asian Art Museum of San Francisco had organized the exhibitions in cooperation with the National Museum of Korea in Seoul, and with the help of a grant from the National Endowment for the Humanities.

Included will be 5th and 6th-century gold crowns, belts, jade pendants and other jewelry uncovered in recent tomb excavations, along with 11th to 13th century Koryo celadon porcelains, iron and stone sculpture, gilt bronzes, paintings, tiles, ceramics and other treasures from 50 centuries never before seen in the West.

Thomas Lawton, director of the Freer Gallery in Washington, said that this "lavish, wide-ranging show is one we'd like to be able to accommodate if we had our new building." Donor Charles Lang Freer's will forbids the exhibition of objects not part of the Freer's own collections in the present building. The Smithsonian is considering building two new structures behind the Renwick castle, one of which would house the Museum of African Art, and the other a gallery for temporary exhibitions of oriental art.