The word for American Ballet Theatre's "Romeo and Juliet," unveiled at the Kennedy Center Opera House last night, is big. This is a production that's mammoth in scale, in conception, in its technical demands on dancers, in its histrionic flourish, and, from the look of it, in cost as well.

What ABT presented last night was the first American staging of a ballet choreographed by Sir Kenneth MacMillan in 1965 to Serge Prokofiev's justly celebrated score, and originally performed by England's Royal Ballet. It has long since been regarded as one of the Royal's "classics," and one of the mainstays of the 20th century repertory of full-length story ballets. Generations of British dancers and guest artists from abroad have made or prodigiously enhanced their reputations performing it.

Perhaps MacMillan's "Romeo" seems all the more massive in aspect to those of us who less than a month ago witnessed the Joffrey Ballet's account of John Cranko's 1962 version, also at Kennedy Center. Set side by side this way, the Cranko seems like a watercolor next to MacMillan's thickly encrusted oils. The Cranko appeared to speed the tragedy along like an arrow in flight, the swift passage to destiny accentuated somehow by the youth of the Joffrey cast. The MacMillan, in contrast, appeared not only larger and longer (it actually uses more of Prokofiev's score), but weightier and more slow-moving. The sense of gravity was underlined not only by Nicholas Georgiadis's colossal decor but also by the contributions of more mature dancers within the dramatic ensemble.

A salvo of bravos greeted the final curtain, and the remainder of the run of six performances (with two alternate casts of principals yet to come) is very close to a sell-out. Given the ingredients -- Shakespeare, Prokofiev, the doomed lovers, MacMillan's already tested, masterly choreography and the formidable artistic resources of ABT -- it was predictable that the production would be a popular success almost before it was out of the starting gate.

But big as it is in dimension and overall impression, the ballet seemed no more than halfway realized last night, in view of its potential performance impact.

The problem was one of imbalance in the casting. Leslie Browne's Juliet, though perhaps overdrawn in some aspects, was a full-blown, urgent, compelling and superbly danced portrait. Robert La Fosse's Romeo, on the other head, was dramatically callow and often half-baked as dancing. If La Fosse has it in him to achieve the heroic stature, convincingly romantic demeanor and technical finish required of Romeo, it wasn't often in evidence.

Browne seemed to start with her characterization as the essential underpinning from which all else flowed -- her way of moving, her phrasing, her line, all this evolved from her vision of Juliet as an impassioned, brave, defiant woman, extravagant in desire, dream and gesture. La Fosse never did establish a character. Instead, he seemed to smooth on and wipe off a discrete series of emotional poses like so many layers of makeup. He's shown himself a fine young dancer in a broad repertoire of major roles, but even his virtuosity seemed unequal to MacMillan's demands a good deal of the time last night.

Similar disparities marked other portrayals. Ron Tice's excellent Tybalt gave us, first of all, a strong character -- not the personification of evil, as in some interpretations of the role, but a proud, burly, tough customer. Beside him, Romeo seemed a puppy, and diminutive Danilo Radojevic looked not only hopelessly outclassed physically but puny as a character. Radojevic fired off some effective rounds of bravura, but he too was not infrequently discomfited by the difficulties of MacMillan's complex choreography for the males in this ballet.

The production drew sustenance from some of the older members of the cast, such as Susan Jones as the roly-poly, fussbudget Nurse, and John Taras (ABT's new associate director) as a serenly devout Friar Laurence. As Lady Capulet, Georgina Parkinson -- now an ABT ballet mistress and MacMillan's original Rosaline (and later a Juliet) for the Royal -- was gloriously melodramatic in her ravings over Tybalt's death, but the characterization looked almost excessive in last night's context. Most lesser roles were filled with distinction, and the company as a whole gave thrust and coherence to the sumptuous crowd scenes. Half the effect of the ballet comes from Georgiadis' lavishly styled Renaissance set and impeccably detailed costumes, splendidly reproduced for ABT. Alan Barker led an aptly sonorous rendition of the Prokofiev.