After the Cleveland Quartet concluded its absorbing Kennedy Center recital last night with the grave, touching final pages of Mendelssohn's lengthy A-major quartet, my companion remarked, "It must take guts to finish with something that hard."

Well, this is no suggestion that the unsurpassed musicians of the Cleveland lack guts, but I wonder if this ensemble hasn't now reached such an exalted level of music making that they don't even think of such things anymore.

Because the Cleveland plays here quite frequently, we have been able to plot the gradual evolution of the group from the status of young-and-extremely-gifted to its present level, that of a great quartet at the peak of its powers.

*All evening in the center's Concert Hall (as part of the Mostly Mozart series), we heard phrasing of great sensitivity, conceptions that had been honed quite beyond any uncertainty of what the players meant to say, and tone that was unfailingly full and blended. The tonal bloom was most remarkable in the Mendelssohn's quietest moment, the evanescent little intermezzo, done with a gossamer delicacy more exquisite than I had ever heard in the piece.

*There is no such thing as a musical group that does not constantly have more to learn, but at this point one of the Cleveland's greatest challenges is just to maintain the current level.

This Mendelssohn quartet, with all its soul-searching echoes of late Beethoven (who had just died), is amazing. Mendelssohn wrote it at 18 -- in other words, his artistic maturity.

Someday, if the Kennedy Center wants to do something really special, it might have a festival of music composed by Mendelssohn before age 25. The period is full of masterpieces -- like the Octet and much of "Midsummer Night's Dream." Such a festival could be a real joy.

There was one work last night from Mozart's early twenties, the flowing little Flute Quartet, K. 285. Carol Wincenc was a lovely flute player, but music so fragile suffers in the large hall's acoustics.

However, the other Mozart work of the evening, his G-minor piano quartet, K. 478, is made of such substantial stuff -- such intense, subtle sensibilities -- that a large-scale, deeply felt performance like last night's works well in the hall. Pianist Richard Goode played with passion and poise.