American Ballet Theatre took the occasion of its Gala benefit at the Kennedy Center Opera House last night, opening a second week of performances, to present the Washington premiere of Kenneth MacMillan's "Requiem," with a musical score by Andrew Lloyd Webber.

MacMillan is assuredly among the most gifted, prolific and accomplished choreographers of the neoclassical tradition. He's also, in my estimation, subject to wide fluctuations of inspirational quality. At his best, in works like "Song of the Earth," "Gloria" and an earlier (1976) "Requiem" to music by Gabriel Faure' -- all on the dark side, emotionally -- he achieves considerable depth, at the same time extending the technical language of ballet in markedly original ways.

The new "Requiem" also deals with somber subject matter, but this time rather superficially. MacMillan's craftsmanship never deserts him, and the new ballet is theatrically striking in numbers of ways. But it seems, at first viewing, perilously thin in choreographic and expressive substance.

Perhaps a primary reason for this is Lloyd Webber's fashionably gushy and melodramatic score, calling for soprano, tenor and treble soloists, chorus, organ and large orchestra. The music divides into nine sections and lasts 47 minutes. Lloyd Webber, the composer of "Evita," "Cats" and other hit musicals, has skills of his own. The effect, though, is one of ersatz solemnity, a thin veneer of pious attitudinizing over a crassly sentimental core.

On the plus side there are Yolanda Sonnabend's scenery and costumes, and the ingenious ways they are lit by Thomas Skelton to suggest a multitude of moods and settings. In the beginning one sees a stained, splotchy backdrop in reds and blacks, overhung with masses of netting -- it looks much like the fetid, ominous jungle of "Apocalypse Now." Lloyd Webber's score is said to have been prompted both by the death of his father and a newspaper account of a Cambodian boy who was given his choice of deaths -- himself or his sister. In a program note, MacMillan attests that the boy's conflict is "the underlying theme of the choreographed version of the score ."

The prevailing movement style might be termed gymnastic classicism. Images are compounded from a classical base overlaid with wraparound lifts, upendings, slidings between legs, and floor splits. At one startling moment in the Agnus Dei, Alessandra Ferri is stretched precariously along one side of the reclining Gil Boggs. There's also much suggestion of the wiping of tears, and death marches. The one section that comes closest to the ethereal poignancy of MacMillan's earlier "Requiem" is a tender duet for Ferri and Boggs in the Pie Jesu.

Boggs (who has a particularly treacherous, twisty-legged solo in the Hosanna movement), Ferri, the six demi-soloists and the ensemble of 20 danced with all the conviction and dexterity one could ask. Praise belongs as well to singers Cyndia Sieden, Joseph Ravenell and Denes Striny, the vocal chorus prepared by Paul Hill, and conductor Jack Everly, who kept the demanding score in splendid focus.

The most extraordinary dancing of the evening came in the opening "Swan Lake, Act II," featuring Bonnie Moore as Odette and Mikhail Baryshnikov as Siegfried. To those in the audience who have followed Moore's rise from her days with the Washington Ballet, the sight of her blossoming last night could not have been a surprise. She joined ABT in the fall of 1984 and was promoted to soloist only a couple of months ago, but she's dancing Clara, Juliet and Odette in the Kennedy Center repertory and elsewhere on the company tour.

The dancing began, unconventionally, with Siegfried's Act I solo -- a model, in Baryshnikov's account, of musically sensitive mime and pensive lyricism such as he alone can provide. Moore's Odette was far from polished -- her solo variation had a number of tentative moments. Still, this was an order of classical dancing and interpretation that one seldom sees anywhere. The shivers of her head and arms weren't bird imitations, but the tremors of a tragic spirit. The urgent inwardness of her portrayal was matched in outward form by exquisitely arched feet and a perfect line in arabesque. Moore is just 21 and has much to learn about projection and other things, but hers is plainly a future to watch.

Completing the program was an indifferent performance of Balanchine's "Bourre'e Fantasque."