Mystery man of the new-music scene offers. . . works for strings?

Years ago, mystery man Terry Riley helped pioneer the pulsing, repetitive style that marks the work of such colleagues as Philip Glass and Steve Reich. While those composers grew in popularity and fame, however, Riley retreated from the public eye.

Though he has released a few records over the years, his recent Town Hall recital marked his first New York appearance in eight years. It showed him in full control of his traditional style and reaching out to new areas that are more promising yet.

Heavily influenced by Indian music, Riley favors a richly polyphonic sound, weaving improvised melodies - several at a time - around a solid rhythmic and harmonic core. The effect is dense, intricate, and sometimes interminable. A good example is his latest record, ''Descending Moonshine Dervishes,'' on the imported Kuckuck label (UCK 047) from West Germany. If not available in local record stores - and Riley is no bestseller despite his talents - it can be obtained from the New Music Distribution Service at 500 Broadway, New York City.

Riley opened his Town Hall recital with an hour-long organ improvisation. Then came a somewhat shorter work for synthesizer and his own voice, both droning in Indian fashion. The words sounded trite when they were intelligible at all - something about a dusty road and flames - but they weren't the point. It was quintessential Riley: diverting, digressive, dull after a while, yet virtuosic all the way.

The evening caught fire in its second half, when the Kronos String Quartet played three new Riley works for conventional chamber instruments. The first two were revelations - crisp, tuneful, witty, and even concise! The last featured Riley's synthesizer and singing as well as the Kronos players, blending the old Riley and the new into an uneasy but not unattractive melange.

Like another old cohort of his, La Monte Young, the Indian-inspired Riley may be too exotic and individualistic to gather the kind of wide acclaim that has lately accrued to such fellow radicals as Glass and Reich. Certainly his long keyboard improvisations are not the kind of accessible stuff that broad popularity is made of.

But his recent string compositions are a different breed. It's hard to imagine them failing to attract a following, if talented performers like the Kronos quartet give them a healthy amount of exposure. From the looks of it, Riley can have a dual career for the asking - continuing his lonely experiments in polyphonic improvisation, while tossing out spectacular bouquets in the vein of his string pieces.