The Juilliard String Quartet -- having unburdened itself in recent years of its third complete recording of the 16 Beethoven quartets, no less -- could no doubt play this mother lode of chamber music in its sleep. So it is in the irrepressibility of the Juilliard's very passion and rigor in Beethoven -- especially in the lofty profundities of late Beethoven -- that one gets perhaps the truest measure of what makes the Juilliard the Juilliard.

That was what made last night's all-Beethoven program, opening the ensemble's spring season at the Library of Congress' Coolidge Auditorium, so special. It drew a crowd that overflowed into the adjacent Whittall Pavilion, where dozens of people stayed around to monitor the concert on an in-house TV.

The program bridged the whole range of the quartets -- from the one believed to have been the earliest, Op. 18, No. 3, with its sometimes stiff formalities, to the questing metaphysical immensities of the one in B-flat major, Op. 130.

sk,3 The latter is one of those late Beethoven works so staggeringly ahead of its time both expressively and conceptually that it would be the end of the century, about 75 years later, before its full import emerged. And that came not in the quartet, but in the symphony -- specifically the epic symphonic dramas of Mahler. There the startling qualities of this quartet finally took root -- in the profusion of movements almost equally eloquent and enigmatic, in the juxtaposition within the same movements of themes that are a conflict of opposites and, above all, in the great grave adagios that in their deeply felt intensity constitute veritable artistic credos.

The Juilliard has never been the most polished ensemble around; nor was it last night. From the group's beginning back in the '40s, it has taken Beethoven's kind of depth and substance -- or Barto'k's -- to bring out the full measure of the Juilliard's character.

No other ensemble that I have heard can balance the seeming irreconcilables of the opening movement's themes so seamlessly, giving the composer his full emotional and intellectual force. And, as the enormous work proceeded, the ambivalence in the multitudinous elements Beethoven threw into motion was observed with remarkable steadiness and imagination.

This work was so problematic even for the composer that he wrote alternate, and utterly different, finales. Last night the Juilliard played the good-natured one, that sunny, dancing, witty rondo that was the composer's last work -- the only really straightforward movement in the whole quartet.

Also on the program was another emotionally troubled Beethoven quartet, the one in F minor, Op. 95, intensely played.