Nothing in music is more doleful than an aged string quartet whose sense of ensemble is in tatters and whose individual members have grown bored with the sounds of their own voices. Except perhaps a quartet cutting out in its prime.

The Mendelssohn String Quartet Wednesday night played what was billed as its last-ever concert in Philadelphia. The group was first hosted locally by the Philadelphia Chamber Music Society in 1988, and now, three decades after its start, the players have decided to hang up their bows. The quartet plans to disband after a concert in Buffalo next month.

Revival is always implicit in such declarations - we can hope, anyway - but whatever the Mendelssohn's future might be, it has established an unusual and firmly expressed philosophy. Technical polish isn't where the core identity is found, though there's plenty of it. At various critical moments Wednesday in the Kimmel's Perelman Theater, you could hear an individual member taking an expressive risk at the potential expense of accuracy, and it almost always paid off in a revelation.

The spiritual heart of the group is violinist Miriam Fried. In both Mendelssohn's String Quartet in A minor (Op. 13) and Beethoven's String Quartet in A minor (Op. 132), the first violin has what can be heard as recitatives - that is, free solo lines momentarily divorced from the general tempo of the piece. It was in these spots you could best sense what a special player Fried is. She fills a phrase wall to wall with sound, her intensity applied, as a vocal line is, with dramatic shape. She has a deep glow, full of overtones. She is above all human and expressive.

Her closest ally was cellist Marcy Rosen, with whom ideas were passed back and forth, each time with a glimmer of shared nuance, though the entire group had the kind of inner pulse in the Beethoven that doesn't happen automatically. That haunting three-quarter-time material near the end? It was tight and paced to stunning effect.

Bartók's String Quartet No. 2 (1915-17) has its share of dissonance, yet it was easier to notice the Mendelssohn's great warmth, precision, and lyricism. Second violinist Nicholas Mann and violist Daniel Panner were close partners, and cellist Rosen strummed like a guitarist with resonance.

But the evening's most impressive stretch came in the emotionally disparate last three or four minutes of the Mendelssohn Op. 13. It builds, it dissolves, it becomes a different kind of a piece at one point. The players not only pulled it together coherently, they also made it mean something. When it ended, you were so busy thinking about what happened it seemed almost impertinent to applaud. But what else could you do, especially after three decades?