Fifteen years of Mostly Mozart is a modest period of time to explore the delights of that most elegant and diverse body of music, and undoubtedly there are many more years of delights to come. Meanwhile the celebration of the first 15 continues at the Kennedy Center where, last night, the Tokyo String Quartet held forth, joined by pianist Richard Goode and clarinetist Richard Stoltzman.

As in previous concerts of this series, which began Tuesday and continues through tomorrow, the program was preceded by a mini-recital. In this one, the Tokyo played the Mozart D Major Quartet K. 575 with a seamless refinement that seemed, at times, unworldly.

The main program brought the Haydn D Major String Quartet Opus 76 No. 5, the second of the two Mozart piano quartets, and the Clarinet Quintet.

Both Goode and Stoltzman outdid themselves in their respective roles. Goode, a master of the pianistic legato, projected Mozartean lyricism with an almost vocal sense of line, moving with just the shifting of weight, just the shaping of line needed to make perfect sense of the music.

Stoltzman, on the other hand, imparted to his familiar music a sweetness of tone and a feeling of grace that left the audience breathless.

The Tokyo ensemble was not quite up to its accustomed standards of excellence. Perhaps its new first violinist, Peter Oundjian, has not been completely assimilated into the group. His playing was oddly uneven throughout the evening and, undoubtedly, this had an effect on the other three. t