Rudolf Serkin divided the completely familiar with the almost unknown yesterday afternoon in his Kennedy Center recital.

The afternoon began with the Italian Concerto of Bach in a reading that was a model of unhurried clarity. The long melodic line in the slow movement sang in a beautiful legato while the outer movements were ideally detached without being clipped. There was a remarkable beauty of sound throughout.

Bach was followed with one of his lineal descendants, Max Reger, as Serkin brought out the monumental Variations and Fugue on a Theme of Bach. The towering work in B Minor is one of the largest conceptions in all piano literature. Serkin, who over the years has given unusual but appropriate attention to Reger, played it with a technical mastery that was astounding in view of the fiendish demands.

Variations that was filled with huge, massed chords in rapid succession emerged in clear power, with the theme always prominent. In contrast, quiet, lyric pages, especially one in contrary motion, were played with a poetic grace that made you wish to hear them more often. The Reger is long, but Serkin sustained the attention throughout in a grand display of musicianship.

His closing work was the Waldstein Sonata of Beethoven, in which everything was in his familiar vein: a rapid opening that was not rushed; a slow movement of complete repose, and a finale that had ideal harmonic clouding, created by impeccable pedaling, yielding to the right kind of impetuosity at the close. The encore was the C Major Intermezzo, Op. 119, of Brahms.