Saturday night's performance by the Stuttgart Ballet, repeating Thursday evening's mixed repetory program that I had been obliged to miss, afforded me my first contact with Kenneth MacMillian's "Requiem." I found it a shattering experience.

There are new ballets and new ballets, some good, some not; but very few are the ones which seem to extend the very borders of choreographic art, opening up new and unsuspected realms of Vance vision. Critics belong out on a limb in a while - let this be one of mine.

I would wager that as time passes this extraordinary opus, now receiving its first performances in this country, will be regarded as a milestone, not only in MacMillian's career but in the evolution of ballet during our centur's latter half.

Until now, MacMillan had seemed an expert craftsman ("Solitaire," for example,) capable of some superficial, flawed or facile work ("Manon," "Concerto," RRutuals," "Elite Syncopations"), as well as some genuinely distinguished achievement ("Romeo and Juliet"). But his "Song of the Earth" (1965) gave hints of other dimensions to his talent, and now "Requiem" confirms a creative originality that gives a whole new prespective on the man and his potential.

"Requiem," set to Gabriel Faure's Latin Mass, establishes a new kind of physical rhetoric for the choreographic expression of interior feeling. At a first fleeting look, it may resemble little more than a new infusion of "modern dance" into the ballet vocabulary. But I think the language of solo and ensemble movement MacMillan has devised for "Requiem" goes beyond such labels - it is a real, personal discovery, prompted by the unique character of Faure's score and the latent gestural possibilities MacMillan had already begun to explore in "Song of the Earth."

If the novel way MacMillan uses the dancers' bodies in "Requiem" has any antecedents, they are to be found not so much in modern dance as a Isador Duncan's notions of movement as a plastic analogue of music, in the case of the solos; and in the kind of architectural poetry Bronislava Nijinska made from human masses in her "Les Noces," in the case of the ensemble patterns.

But MacMillan relates these tastics to the specific emotional texture of Faure's elegaic polyphony, and the result is something distinctly new. He has found poingnantly apt dance images for the anguished supplication of the Offertorium and Libera Me; for the cloying, beatific ache of the Pie Jesu and Agnus Dei (so exquisitely realized by Marcia Haydee and Birgit Keil); and for the celestial radiance of the Sanctus and In Paradisum.

And the totality to reveal new facets with each repeated viewing.