For an unfettered grasp of a great composer's growth, you can hardly do better than the Cleveland Quartet's recording of Brahms' two string sextets.

Joined by Bernard Greenhouse playing cello and Pinchas Zukerman exchanging his fiddle for a viola, this world-class chamber group tackles Opus 18 in B-flat Major and Opus 36 in G Major.

Brahms, while a budding composer in Vienna and Detmold, finished writing the first work in the mid-19th century, and the second 10 years later. They remain his only works for two violins, two violas and two cellos: in Brahms' day, an all but untested bit of voicing, and still one that's seldom used. Taken together, the sextets reflect, in a contrast of rare clarity, a great leap in his mastery of thematic development.

Both sextets, in four movements, feature the composer's talent for elegant instrumentation, rich and colorful sound and startling opposition. In the earlier work, though, the movements are more or less self-contained -- the material in one seemingly separate from that in another.

By the later work, it's clear that Brahms has taught himself how to take a simple gesture or melody and expand it to the hilt -- making this sextet a deftly rounded whole. The third movement of Opus 36, Adagio, contains subtle echoes of the opening theme in the first, Allegro non troppo; the second and fourth movements both reply after their fashion to the half-note trill that underpins the first. What a difference a decade makes.

The performances on this two-record set are technically superb and musically careful. But they're not at all academic: Witness the players' heavy breathing and, in the sparkling finale of Opus 36, that muffled grunt -- or is it a moan of pleasure?

ON RECORD, ON STAGE

THE ALBUM

CLEVELAND QUARTET -- Brahms Sextets Opus 18 & 36 (RCA Records ARL2-4054).

THE SHOWS

CLEVELAND QUARTET -- Friday night at 8:30 at the Corcoran Gallery of Art; Saturday at 8:30 at Tawes Theater, University of Maryland.