Filmmaker Robert Altman's sudden - and auspicious - stage venture

A few months ago Robert Altman sold his production company - Lion's Gate Films - and hightailed it out of the movie business. This distressed the fans who have cheered such pictures as ''M*A*S*H,'' ''Three Women,'' ''Nashville,'' and ''A Wedding,'' and who regarded Altman as one of the few independent voices in Hollywood.

But it turns out Altman hasn't retired, just altered his course. Hoping to continue his career in the cable-TV and home-video markets, he has meanwhile set his sights on the stage. ''2 by South,'' presented recently at the Los Angeles Actors' Theater, has now arrived Off Broadway, marking Altman's New York stage-directing debut. It is an auspicious occasion.

The evening consists of two one-act dramas by a new playwright named Frank South. Both are cautionary tales, tracing the path of seemingly reasonable men to eruptions of violence. Perhaps more to the point, both provide ideal working material for Altman, who thrives less on plot than on dense characterization and unconventional structure. In this regard, he and author South are kindred spirits.

The first play, ''Precious Blood,'' is the strongest. The setting, designed by John Kavelin, looks like the interior of an ordinary house, though the downstage area has a hint of hospital to it. The two characters, sensitively played by Guy Boyd and Alfre Woodard, move through this ambiguous arena, passing each other - quite literally - like ships in the night, sharing the same space but obviously dwelling in different worlds. Taking turns, talking partly to themselves and partly to the audience, they unfold long monologues that seem separate at first, but gradually converge as the play proceeds.

Bit by bit, their situations become clear. He has suffered a great bereavement, she is an acquaintance who tried to help. Yet there is something peculiar about their relationship, which is reflected in their odd relationship on the stage: speaking, looking, and moving past each other, in a disquieting variation on conventional theatrical behavior.

Eventually the mystery unravels, and we are able to see exactly how the lives of these people have intertwined. This leads to the climax, a violent encounter between them - reenacted by the woman alone, however, stressing the play's message that the weak must often struggle to survive the failings of the strong.

The play has failings of its own. It takes too long to clarify the relationship of its characters, wasting audience energy on needless puzzles. And the writing gets preachy near the end. Yet it's a forceful and engaging work, darkly convincing in its somber quest for the long-buried roots of the man's destructive behavior. Altman has staged it powerfully, choreographing the excellent performances - Miss Woodard is particularly dazzling - into a slow dance that's as elegant as it is unsettling. The sensitive lighting by Barbara Ling shifts according to the moods of the piece, which range from bluntly realistic to enigmatically expressive. Altman and his actors handle all of them with skill and assurance.

The evening winds up with ''Rattlesnake in a Cooler,'' a one-character melodrama - bracketed with songs by Danny Darst - about a cowboy-style punk who has lopped off more than he can deal with and is about to meet a fate as nasty as his own career. Again, the play takes too long to hit its stride, as the lone character struts and frets and soliloquizes about things that turn out to be mere hors d'oeuvres, preparing us for the grim memory-monologues that are the main course. Once the drama hits its stride, though, it grips the imagination forcefully, dragging the audience by sheer verbal strength into the protagonist's wild and woolly world. Altman's contribution seems less bravura here, with Leo Burmester's explosive performance storming ahead under its own impressive power. It's a relentless show, with its portrait of a bright young man whose anger seethes too strongly for civilized life to contain it. But it's a fascinating experience, at least until the very end, which depends on a shock effect that's too tricky and abrupt for comfort.

On the evidence of ''2 By South,'' Altman can have a thriving theatrical career for the asking. On a small stage (at the St. Clement's Theater) and with a minimum number of actors, he has created two imaginary worlds that seem urgently alive, maintaining their visual power even when the quality of their plotting sags. Playwright South is clearly a newcomer to be watched. But the main discovery of the evening is the old-timer named Robert Altman, who has proved himself in a challenging new milieu.