American Ballet Theatre's mixed repertory programs for its current series at Kennedy Center have been uniformly strong and well balanced. Last night's illustrated the point: Balanchine's "Theme and Variations," the revival of Tudor's "Undertow," and two facets of Jerome Robbins, in his "Other Dances" to the music of Chopin, and "Fancy Free," with its Bernstein score.

It would be hard to name a ballet of any era more compellingly conceived or structured than "Theme and Variations." The marriage, simultaneously in the score and choreography, of outline and ornament, of dynamism and repose, of feeling and form, never ceases to astonish.

Because it is so much of a touchstone, very few performances appear to do it full justice. It's a tribute to last night's cast that they approached the mark so closely, especially since the male lead, Patrick Bissell, is so young (20) and new to the part. Cynthia gregory, his partner, has all the right attributes for the ballet, as she so clearly demonstrated-a clean line, impregnable technique, musicality and self-possession. Bissell, so stalwart in figure and stance, has an apt look, but it was a surprise to discover how much authority he could muster in addition, in this extremely taxing role. Still, as fine as he was-tossing of double air turns and pirouettes like a veteran-neither he nor Gregory seemed able to get beyond the steps last night to the poetic grandeur of the ballet; perhaps the partnership merely needs time to evolve beyond the mechanics of collaboration.

Externals were splendidly observed, too, in "Other Dances," which found Natalia Makarova and Antony Dowell looking marvelously well, both singly and together. But some ultimate ingredient was missing here as well, the kind of kinesthetic combustion that can make this flavorsome pas de deux not just enticing but thrilling.

"Fancy Free," similarly, was almost there but not quite. Two of the men playing sailors in Robins' shore-leave capers-Danilo Radojevic and Charles Maple- are newcomers to the ballet. Their dancing was excellent and they've got the right Broadway zip, but the performance as a whole didn't connect; the joints were too visible.

The same cast which distinguished itself Sunday night in "Undertow," led by Fernando Bujones and Cynthia Gregory and including also Nanette Glushak, Hilda Morales, Stephen Hook and Chrisa Keramidas, was equally effective last night.