The Emerson String Quartet brought fire and grace to its program at the Renwick Gallery last night. Mixing the familiar and the rare, the ensemble added an unexpected treat, a newly discovered fragment of a Mozart string quartet, appropriately called a "Christmas bonus" by violinist Eugene Drucker.

Joined by flutist Sara Stern, the group began with Mozart's D-Major Flute Quartet, K.V.285. It is an uneven work, with an uninspired (for Mozart) first movement and a rollicking yet rather ordinary third movement rondo. In between, however, lies one of the most beautifully accompanied flute solos ever written, and Stern gave it an exquisitely shaded form. Her fresh and glowing style, matched by the light touch of her partners, made the other two movements come vividly alive.

Puccini's little-known "I Crisantemi" of 1890, a single-string quartet movement echoing the operas to come, also received special treatment. Its passages of throbbing melodic duets were given a dramatic reading by violinist Drucker and cellist David Finckel. The Mozart string quartet fragment proved equally moving, especially as the music halted in the midst of a pulsing forward motion.

Though there was plenty of passion in the closing String Quartet, Op. 59, No. 2, of Beethoven, it suffered from the uneven performance of first violinist Philip Setzer. Apparently a bit off his stride, Setzer had some distracting intonation problems and a few less-than-clean passages of finger work.

The program will be repeated tonight and Sunday at 7.