So long as the money is there to be endowed, commissions for new classical music continue apace even if, as in the last few decades, the public appetite for such works is at one of its lowest ebbs. But all too often the scenario is as follows: The score is delivered, the payment is made, and the music is granted its token hearing before sinking into oblivion.

It was all the more important, then, that something different happen when the Library of Congress commissioned a work to honor the 20th anniversary there of its resident quartet, the Juilliard. No ensemble is more celebrated for its mastery of contemporary chamber music, and the occasion called for something worthy of that mastery.

From its premiere here by the Juilliard in 1983, it was clear that the product of the commission, Donald Martino's String Quartet, was of genuine substance. A four-movement work of Brahmsian proportions and currents, it was also an atonal creation of enormous harmonic, rhythmic and developmental intricacy.

Arresting as the work was, however, the real issue was whether its composer had really brought off the full challenge, in both expressive and technical senses. It never seemed clearer that the answer is yes than in Thursday night's repeat of the quartet at the library.

Recent hearings of a tape of the premiere performances show that from the beginning the work has been a tour de force for the Juilliard, but Thursday night the work took on greater specificity of mood and sustained sense of drama than the players brought the first time around. There was a surer sense of play in the lavish pizzicatos of the second movement, and a deeper gravity in the slow movement.

This is one of those works that grow on the listener with repeats. Last fall's performance by the Kronos Quartet that led to the work's first-place tie in the Friedheim Competition was another step in that direction. Now comes this. It would be a shame if the Juilliard did not go ahead and record it.

Also on the program were a delectable playing of Mozart's lustrous and daring Quartet in G, K. 387; and the broad, sumptuous Ravel Quartet.