A concert by the Concerto Soloists of Philadelphia is a special concert indeed. The audience last night at the University of Maryland saw an ensemble of 15 string players and a harpsichordist enter the hall informally -- as though they were just getting together for a chat. They stood during their performance; they changed positions; they demonstrated a concept of intonation and bowing so precise and so keen that they forced the audience to listen with the same intent.

The conductor, Marc Mostovoy, did a neat and businesslike job. One knows he is demanding but wisely does the real work in rehearsals. There must be many of them, judging from the sparkling and exact sound the Soloists make. It is efficient music-making.

Joined by five winds, the Soloists did their best playing of the evening with the best music, the Haydn "Trauersymphonie" in E Minor, No. 44. It is a somber work by a serious composer. The second movement, a minuet, is a marvel of counterpoint that conceals its great craft in shadow, and the slow third movement shows why it was one of Haydn's favorites. The Soloists' clean approach to bow and pitch created harmonies so lucid they seemed transparent; one could hear into them, even beyond them. The rhythmic phrases were well shaped, never losing their sense of direction. They told the listener much about the musicianship of the performers and their conductor, and spoke volumes about the quality of Haydn as a musical thinker.

The guest soloist was pianist and Mozart scholar Paul Badura-Skoda. He joined the Soloists for the Mozart E-flat Concerto, K. 271, a highly original, daring piece of music. It contains brilliant innovation, a hidden canon in the dark slow movement, and an elegant slow minuet in the middle of the final rondo.

Some of this came across, but the performance was oddly skittish: a tentative first movement, a rather academic second and a final rondo that went so fast it ran into problems. It seemed glib instead of substantial. Badura-Skoda is capable of beautiful, lyric sound. In this concert, he only hinted at his true capacity.

It was a good program, well performed by a fine group of young musicians. But it raised a big question: why the music never got off the ground.