Like a beautiful woman with good bone structure, the Tokyo String Quartet is growing more interesting as it matures. As last night's concert at American University revealed, the group still possesses the silken ensemble style that has always been a special feature of its playing. New, however, is the depth of its sound--one could almost call it a gutsiness--and the bite of its interpretations. Both add up to a much more penetrating performance.

Though the changes may simply be a part of the quartet's natural evolution, some of the credit must go to first violinist Peter Oundjian. In his relatively brief period with the ensemble he has inspired continued probing beneath the surface to uncover the fibrous strength in the notes. Yet the evening did not lack for beauty. The entire ensemble seemed to be in top form, producing elegantly sustained lines and graceful agility in Mendelssohn's E-Flat Major Quartet, Op. 12.

Berg's string quartet, Op. 3, of 1910 proved particularly noteworthy. His last effort under Schoenberg's tutelage, the seldom-heard work usually takes a back seat to his "Lyric Suite" for string quartet written in 1926. The earlier work shows Berg well on his lyrical way to the atonal alternative of his later works. Filled with imaginative explorations in timbre and ingenious development of material, the piece received dynamic treatment from the Tokyo String Quartet.