As first violinist Robert Mann remarked after intermission, "There are certain nights that seem to be different from others." Last night's weekly concert by the Juilliard Quartet at the Library of Congress brought one surprise after another.

The expected opening Hadyn quartet had been replaced by a Beethoven quartet, duly noted in the program. The G Minor Quintet by Mozart that followed turned out to be the D Major Quintet, not noted in the program. And in the pause before the D Major Quintet's final movement, a loud voice from the back of the hall complained of a high, whining noise that was, apparently, interfering with enjoyment of the music. The mysterious noise did not cease, nor did the complaint, until Mann tactfully suggested that the quintet be finished and the interference checked at intermission.

The biggest, most significant surprise of the evening the the warmth and flexibility of the Juilliard's playing. Like old friends who can surprise by suddenly showing a new quality, the Juilliard seemed to be in an atypically relaxed and lyrical mood. Part of the credit may be due to the rounded phrasing and full tone of violinist John Graham, who joined the group for the Mozart and Beethoven string quintets. Not even a trace of the nervous edge that often colors the Juilliard's performances could be heard. Everyone was in glorious form, playing with forthright joy and uncommonly easy good humor.