Reprinted from yesterday's late editions.

Last-minute concert program changes dictated by the performers' flu viruses are generally bad news for audiences. But when the the Juilliard String Quartet on Thursday night had to substitute different works for the two that were to precede the intermission, there need have been no such concern.

In fact, it led to a bit of a news event: the performance of an exciting virtuoso unaccompanied cello work by Roger Sessions that surely few in the Library of Congress audience could have heard before.

Callld "Six Pieces for Unaccompanied Cello," it presents an unlikely combination of elements that strikes one on first hearing as a great success. Sessions is known for his cerebral, atonal sympho ies and other works in larger forms. Here he composes, instead, in short, less cerebral forms.

It seems to me that the dialogue that is movement No. 2 comes closest to directly showing what his special esthetic - impenetrable to some people - is all about.

Here we have, on a single cello mind you, a conscious imitation of human speech. Time and again Sessions' melodic lines make better sense analyzed as poetry instead of as song.

Written for his cellist son John in 1966, the pieces are enormously taxing with their contant leaps from range to range and their wide intervals. Juilliard Quartet cellist Joel Krosnick tossed it off with stunning skill and was rewarded with prolonged applause.

Another substitute was the Mozart G major quartet, K. 387, which is one of the great ones. The second movement begins with one of those haunting arias in the violin, whose implied pathos is explored in the breathtaking series of modulations later on.

With the esteemed Rudolf Firkusny at the piano, the program closed with that landmark of the repertory, the Schumann Piano Quintet, in as passionate, lyric and, may one say, virile a performance as one is ever likely to near.