The most gratifying feature of last night's mixed repertory program by American Ballet Theatre at the Kennedy Center was the return of Antony Tudor's "The leaves Are Fading," that aromatic lyrical abstraction created for the troupe in 1975, to littleknown but endearing string music by Dvorak.

In retrospect, the ballet seems more and more to belong right up there with, say, "Dark Elegies," among Tudor's deepest work. Some find it maudlin, but perhaps they are misled by surfaces. Cetainly it is sweet, but Tudor's taste is austere-he knows the difference between tenderness and mush and never oversteps the bounds.

"Leaves" is indisputably generous in choreographic invention. Yet there's not a false move in it. There's no gesture, no step that's not exact in poetic function and weight; no phrase out of proportion to its context. At the same time, within a relatively narrow range of autumnal expression, the ballet manages an amazingly ample, iridescent play of affections -- much like the music, which is so effusive yet so intimate at the same time.

It's also endlessly intriguing on the formal side -- the spill of its legato is truly like a tumble of leaves. But its patterns, so rich in interlocking circulations, are rigorously designed -- they follow the logic of rivers, of tributaries gathering ineluctably to a mainstream.

The ballet also seems almost dancer-proof; it's so "dancerly," it's hard to imagine a flat performance. This is not to slight last night's fine dancing by Kristine Elliott and Michael Owen as the principal couple, nor other impressive contributions, especially those of Elaine Kudo, Gregory Osborne, Christine Spizzo and Frank Smith.

The evening's other performances -- of Ashton's "Les Patineurs," de Mille's "Fall River Legend" and Neumeier's "Desir" -- were all cases of nearly, but not quite. Natalia Makarova and Anthony Dowell were almost, but not altogether, as commanding as they werelast Friday night in their arrestingly dour pas de deux. Warren Conover's Green Boy in "Patineurs" was admirably clean and stylish, but a bit short on oomph. And Jolinda Menendez' depiction of the wretched Lizzie Borden in "Fall River," though impassioned and pathetic in all the right ways and places, still seemed too externalized to be very affecting.