It is hard to define precisely what makes a program interesting. There are too many variables. But the Washington Quartet put together enough of the right ingredients at the Bradley Hills Presbyterian Church yesterday to produce a fascinating kind of "thinking man's" program.

Among the fortunate choice of variables were soprano Phyllis Bryn-Julson, who can be counted on to add distinction to any musical undertaking, and organist Donald Sutherland. If this sounds like an unlikely combination of forces, you're right. But there is fine music for organ and strings, and, of course lots of music for voice and strings, and the Bach, Purcell, Soler and Schoenberg that were played complemented each other splendidly.

In his arrangement of 14 of Bach's Canons, Olivier Alain used the organ as part of the string ensemble with considerable success. Soler, on the other hand, in his quintet, chose to contrast the organ's reedy stops with the strings in a concerto grosso style. Except for the fact that the organ's delayed speaking kept the rhythms a little at odds, the performances were delightful.

The concluding Schoenberg quartet (plus singer) was given a powerful and dramatic reading by Bryn-Julson et al.