The Sequoia String Quartet, a fine California ensemble, came to the Library of Congress last night and brought along a new, and forbidding, work by Milton Babbitt, Mr. Rigor himself of contemporary composition.

Babbitt has taken serial music just about as far in the direction of complexity as it can go. This quartet is no exception. It is hard to imagine that anyone could come away from a first hearing confident that Babbitt's expressive meaning has been fully grasped.

The basic idea of the quartet is that the four instruments play against each other, as if in strenuous conversation. There is no effort at playing in union for a common expressive purpose, as in, say, the Debussy quartet that followed.

The Babbitt starts with a little introduction by the viola and the cello and then the violins assert an esoteric tone row. It's not exactly catchy, but is repeated sufficiently that it becomes unmistakable. The precise logic of the compositional intricacies that follow remains obscure. But the 20-minute work does reach a climax of sorts, and then has a quiet denouement, in which the cello bows out and the violins and viola play out the work in high, whining sonorities. One cannot say if the Babbitt quartet, his fifth, is good for the soul; it certainly provides, though, a workout for the brain.

The performance by the Sequoia, which commissioned the work last year, seemed utterly committed.

If the Babbitt seemed short on charm, there was much of that in the rest of the program. The first Beethoven quartet in F major was light, lyrical and sweet. Only in the anguished interior sections of the slow movement, anticipating the emotional depth of late Beethoven, was this approach insufficient.

The Debussy had no such flaws. It was tender, lush, a little sad and beautifully played.