"I can't remember the last time we played in Washington outside of the Library of Congress," said Earl Carlyiss.

"We're not supposed to play anywhere else in Washington," Samuel Rhodes explained to a fan. "Not for money, anyway, and not without special permission."

The members of the Juilliard Quartet were relaxing and sipping champagne at a black-tie reception in the Corcoran Gallery, after giving a robust performance of Mozart's Quartet in E-flat, K. 428, and a bright, lyric, superbly integrated reading of the Third Quartet of Brahms. Leonard Bernstein, who was unable to attend the benefit performance, sent a telegram: "My heart is with you all. . ."

As galas go in Washington these days, the event was unostentatious. Except for the champagne (which was American), the refreshments were all home-made: the little sandwiches that can be had at hundreds of grass-roots parties across the nation on any evening, vegetables with dip and bouquets of radishes lovingly carved to look like flowers. The Juilliard, one of the world's most prestigious organizations, had played for the benefit of the Urban Philharmonic Orchestra, a worthy group that has been struggling slowly uphill for 10 years and seems about to make a breakthrough with a summer program at Carter Barron and is negotiating for another in the Kennedy Center.

"Ted says the Carter Barron program is firm," said Darrold Hunt, music director of the Urban Philharmonic, "but we haven't ironed out the details yet." "We're planning four Sunday programs in July," confirmed Ted McCann of the Interior Department, which runs the Carter Barron. "We think that will be an ideal place for the kind of program the Urban Philharmonic offers."

The Urban, which has played at locations that range from the Kennedy Center to urban churches up and down the East Coast, emphasizes performances in less affluent neighborhoods and performs music rooted in various American ethnic traditions along with the standard classics. Its constituency does not include the kind of socialites seen at $500-a-ticket benefits, but last night's crowd had a sprinkling of congressmen and sub-Cabinet government officials, all obviously enjoying the concert and reception. The atmosphere was relaxed, and there was more elbow room than is usual at more expensive galas.

"We could have had more people," said ViCurtis Hinton who had arranged the evening, looking ruefully at leftover refreshments as the party broke up. "I started thinking we didn't have any more room and turning people away. The next time, we'll pack them in."