In its brief history, the Smithson String Quartet has been known mainly for its practice of playing chamber music from the Classic period on original instruments--gut strings, short necks and all that.

The ultimate consideration, though, is not the instrument but the way the music is phrased. And last night at the Smithsonian, with wind player Lawrence McDonald, the quartet captured the sublime equipoise of the Mozart Clarinet Quintet to such utter perfection that this listener would have been perfectly happy to have had them play it on kazoos if they could bend the instruments to their purposes.

There is no greater piece of Mozart chamber music than this glowing, infinitely refined masterpiece. From the beginning the performance was exactly on the right track, with the lyric sweetness of the strings juxtaposed with the assertive sweep of the clarinet (it is not unlike the role of the piano in some of the late concertos). The slow movement was all impassioned dignity. The minuet was deliciously pointed, its elegance spiced with a touch of the feisty. The variations in the finale were superbly gauged.

In the use of authentic instruments, polish is not the object. The period clarinet, an 1806 instrument in this case, is harder to play than the present-day clarinet; despite all of McDonald's art, a few notes were lost along the way. But the playing had such character and vitality that one hardly cared. Likewise, the string playing was not the last word in polish, but it approached the last word in expressiveness.

To hear such a performance casts the rest of the program in an almost inevitable shade. First violinist Jaap Schroeder wisely counter-programmed with more modest works. There was an early Schubert quartet, in E-flat major, D. 87. It has an opening movement that is full of nervous energy, and the slow movement is one of those great hymn-like Schubert creations. The performance was lovely.

There also was a Haydn quartet, in G major, Op. 17, No. 5. It is a powerful work with rhetorical lines in the slow movement that startlingly resemble the recitative material that opens the last movement of Beethoven's Ninth Symphony.