The Theater Chamber Players of Kennedy Center are five Japanese musicians who play a Mozart quintet as though they had been born in Vienna. Or rather, the Theater Chamber Players are an unusual pair of singers -- countertenor Derek Ragin and tenor James McDonald, who blended their voices superbly, without other accompaniment in a highly specialized and delicate kind of music: the two-part madrigals of 14th-century Italy. Finally, the Theater Chamber Players are a highly skilled and very heterogeneous group with a special expertise in contemporary music.

All three of these groups fit comfortably into the conglomerate of 30 performers under the musical direction of Leon Fleisher that is one of Washington's most extraordinary musical organizations. On Saturday night, 11 members of the group joined Fleisher in a program that spanned six centuries.

Ragin and McDonald opened with the beautifully elaborate harmonies and counterpoints of Jacopo da Bologna and Giovanni da Cascia preserved in the Squarcialupi Codex of 14th-century Florence. This is music that looks ahead directly to the Renaissance and hides elaborate art under an appearance of sweet simplicity. Both singers were excellent, but Ragin made an especially strong impression simply because a good countertenor is so much rarer than a good tenor -- and Ragin is a very good countertenor indeed.

The modern segment of the program was "Ave Maris Stella" by Peter Maxwell Davies -- a serene but vivid and sometimes mysterious series of nine musical landscapes, perhaps landscapes of the soul as well as of the British seacoast of the Orkneys where the composer had gone to live before composing this music. It contrasts sharply with some of his earlier work, which dealt very colorfully with disturbed psychological states, but it is no less striking in its quiet way. It requires playing of the utmost delicacy, and that is what Fleisher's baton evoked from such players as Albert Merz on marimba, Loren Kitt on clarinet, Dina Koston on piano and Hakuro Mori on cello.

Mori was one of the five players who concluded the program with Mozart's Quintet in G Minor, one of the most powerful pieces of chamber music composed before the late quartets of Beethoven, deeply emotional in a way that 18th-century music rarely is. The quintet, headed by violinist Koichiro Harada, who was the founding leader of the Tokyo Quartet, performed not only at the high technical level one expects when Harada is present but also with a total identification with the music's emotional richness.

Any one of these performances could have been the climax of a more specialized program by an ensemble with less versatility. The fact that all were presented under the auspices of one group is a sign of the musical richness we enjoy in this unusual organization.