Mozart referred to his six string quartets dedicated to Haydn as "the fruits of long and laborious endeavor." The performance of the Juilliard String Quartet--which included Mozart's first "Haydn" Quartet in G Major, K. 387, on the program at the Library of Congress last evening--warranted a similar description: Their fruits were of a long and seemingly effortless endeavor.

The Juilliard String Quartet makes the most difficult passages appear as child's play; yet, it unveils fresh insights into familiar themes. Concentrating on pieces that place equal importance on each of the string voices, the quartet proved once again why it is the preeminent chamber group in this country.

In the opening Allegro of the Mozart, the players maintained a closely monitored four-way dialogue, which carried over into the Menuetto. They approached the intrusive unison G minor melody of the Trio with caution, a pleasant change from the aggressive attack usually applied.

With American composer Roger Sessions' Quartet No. 2, they brandished their bows in a work whose blurred tonalities, quartal harmonies and intensely expressive melodies at times drag on like a quadraphonic filibuster. In five movements without pause, the players surmounted all of the musical obstacles.

Violinists Robert Mann and Earl Carlyss subtly emphasized the plaintive themes in Brahms' Quartet in A Minor, Op. 51, No. 2, before breaking into a gallop with violist Samuel Rhodes and cellist Joel Krosnick for the csa'rda's-like Finale.