Theater thrives on the West Coast from San Diego to Seattle. Though my "El Dorado" is purely mythical, it suggests what could be California's most effective contribution to American theater since the days when David Belasco and the DeMille brothers were putting California stages on the map.

For "Zoot Suit," a collaboration of the Chicano El Teatro Campesino and the Mark Taper Forum, is bound for New York's Winter Garden Theater tonight. With that city's large Hispanic audience potential, "Zoot Suit" could be to Hispanics what "The Wiz" has been to blacks.

Above all, "Zoot Suit" is a swell show in every sense of the term--colorful, novel, passionately conceived, theatrically staged.

The zoot suits of the early '40s were the garb of this area's "pachucos," urban cowboys who strutted the streets in jackets that reached below the knees, shoulders padded and trouser cuffs cut tightly at the ankles. The pachuco wore key chains two yards long and, likely as not, a cross on his neck chain.

The pachuco was, like El Dorado itself, a mythic figure, unreal but understood, feared yet admired. Through the 1942 Sleepy Lagoon murder case, the pachuco became the image for 17 young Chicanos accused and convicted of murder. There were riots between servicemen and the Los Angeles Zoot Suiters, but two years later the courts freed them, a historic victor for Mexican-Americans.

The son of migrant workers, 38-year-old Luis Valdez was raised with this episode in his cultural fiber. A member of the San Francisco Mime Troupe and an ally of Cesar Chavez during his early farm-workers' strikes, Valdez founded El Teatro Campesino, for which he created agitprop plays.

Gordon Davidson, director of the Los Angeles Theater Group's Mark Taper Forum, commissioned a work from Valdez as both author and director. With pop songs of the period and brilliant choreography by Patricia Birch, it was such a smash on the Taper's forceful thrust stage that it was transferred to Sunset Boulevard's Aquarius Theater (once Earl Carroll's art deco showplace), where an identical and vital thrust stage was built.

One of the work's pleasures is it's mixture of American customs and language with Chicano counterparts. Family relationships are strong in the Valdex story. So are such terms as "abusado" (shut up), "bato" (dude), "huisa" (girlfriend) and "ya pues" (that's enough). There is imagination in the character of "El Pachuco," whose smart, mocking appearances etch more deeply the central role of an accused, Henry Reyna--parts to be acted in New York by their creators, Edward James Olmos and Daniel Valdez.

"Zoot Suit" marks the first broad acclaim for any of the area's Chicano theater groups. It serves both the pride of Chicanos and the ignorance of Anglos to whom the surrounding culture (now approaching 50 percent of the population) has appeared, at best, shadowy.

The major leads of the 28-member cast, replaced for the continuing Aquarius run, state that the Winter Garden production is planned to be "more extravagant and costlier." That may be helpful, but one recalls other regional creations fatally tarted up for Broadway inspection. The style indeed is inherently exaggeration, but one hopes that the hype will not falsify the honesty and that the Chicano words will be left as they were.

"Zoot Suit" could well provoke the most excitement of New York's financially booming but curiously unexciting season.

The Los Angeles stage, though lively enough, remains strongly New York-oriented. Nine major theaters are lighted.

The Shubert's "Annie" has a featured part for Georgetown University's one-time star, Frank O'Brien, as Rooster. Hume Cronyn and Jessica Tandy have been selling out with "The Gin Game" and are at last headed for the Kennedy Center April 149

Eva Marie Saint, her performance always a shimmering treasure, is realizing a long-frustrated dream at the Huntington Hartford, where she co-stars with Henry Fonda in "First Monday in October," the Supreme Court play which bowed last winter at the Eisenhower.

"One of my first theater jobs," Saint recalls, "was as Jocelyn Brando's standby for that short role of the nurse, almost a walk-on, in Fonda's original 'Mister Roberts' company. In all these years Hank and I never have acted together, and when I was chosen for the Jane Alexander role in this company, he came to my home every afternoon and we went over the script as though it were an entirely new play to him. What a remarkably, thoughtful, understanding thing to do."

With Larry Gates and Tom Stech Schulte in their original roles, the Fonda-Saint company is into a sold-out Los Angeles month, to be followed by a six-week Chicago run and a wind-up in Denver.After a summer off, Fonda hopes to return to what has become the favorite role of his maturity, Justice Daniel Snow of the Supreme Court.

Authors Jerome Lawrence and Robert E. Lee, by the way, will be turning to a musical for their next venture. Shaw's "Major Barbara" is the inspiration for music by Heny Mancini and lyrics by Leslie Bricusse. If Peter Ustinov and Coral Brown can settle down, they'd be an ideal Lord and Lady Undershaft. What a grand Barbara Julie Andrews might make, but taxes preclude her living here more than six months a year.

I've never stayed in San Francisco long enough to see all the plays in the ACT (American Conservatory Theater) repertory, but what a quality company and what a creamy choice of plays!

This year William Ball's lineup is: Turgenev's "A Month in the Country," O'Neill's "Ah, Wilderness!," Maugham's "The Circle," Dickens' "A Christmas Carol," Shaw's "Heartbreak House," Durenmatt's "The Visit," Coward's "Hay Fever," Feydeau's "Hotel Paradiso" and Lanford Wilson's newest and finest, "5th of july."

"Of course I want to do new plays," remarks Ball, who once staged "A Month in the Country" at Arena. "But our premise is built on the finest of the world's dramatic literature and that, in turn, places new American plays in a very tough spot with the critics. They compare our developing playwrights with history's finest, usually unfavorably. But we alwasy do new plays, if not on our main stage here at the Geary, then in our 'Play in Progress' series across the street."

There are daily, evening and summer courses for adult, youth, blacks and Asians, with Allen Fletcher as conservatory director and a faculty drawn from the leading players.

"5th of July" dramatizes the same shift of young people from the late '60s into the early '70s, as does Michael Weller's Arena Stage premier, "Loose Ends." Wilson's play is infinitely superior, richer in characters, variety and credibility. Seen too briefly in New York last season (but respected enough to merit condensation in Dodd, Mead's new "Best Plays" volume,) "5th of July" merits a Washington production.

May it be as well attended as ACT's. Director Edward Hasting's entire cast for this story of Lebanon, Mo., expresses ACT's ideal of physical vitality and subtleties. Making her bow in a major role is a product of the conservatory training, Danette Pachtner, and she is splendidly at home with such skilled players as Mark Murphy, Barbara Dirickson, Joy Carlin, Susan E. Pellegrino, Peter Davies, Daniel Kern and Isaid Whitlock Jr.

A particular ACT favorite is Sydney Walker, an Arena Stage veteran whose Scrooge has become a Yule tradition. Ray Reinhardt is another Arena Stager in the company which further includes one of the finest of all young American actresses, Elizabeth Huddle.

There's more to this city, but once I meet up with ADT's plays and players, it's time for the airport.

Seattle, to the surprise of all Easterners, is an extraordinarily lively theater center. There are half a dozen Equity companies, with much moving about between productions, as well as other respected, enduring companies.

The Seattle Rep has just opened a revival of "The Glass Menagerie" after a run of "Side by Side by Sondheim." This company's role seems to be top-draw productions of classics, much the style of the nearby Opera House with music and dance. Verdi's "Macbeth," the Royal Winnipeg Ballet and the Seattle Symphony (when not on strike) share its stage.

Another respected Seattle company is another ACT, "A Contemporary Theater," headed by Gregory A. Falls, It has just presented "The Odyssey" in the Kennedy Center's Children's Art series and on its home stage is doing "The Rivals."

Seattle Post-Intelligencer critic Magie Hawthorn directs me to an unfamiliar play, Thomas Babe's "A Prayer for My Daughter," which had a short New York run last year and also is in the new "Best Plays" volume.

So I discover The Empty Space Theater, a third-floor walkup under the direction of M. Burke Walker. Seating of a hundred-odd is rearranged for each play, in the manner of the New Playwrights' Theater off Dupont Circle.

Babe's gradually unfolding story of two policemen and two prisoners in a New York police squad room has been staged by Walker with stunning confidence in playwright and actors. He lets the play's internal rhythms assert themselves, and as the play builds so do these splendid actors. John Procaccino, Allen Nause and Brian Thompson are noted area players and young Michael Longfield wil be.

Babe, a discoverer of the O'Neill Center's playwrights' conference and Joseph Papp's Public Theater, seemed to me overpraised for his sprawling "Rebel Women" and "Kid Chapion," but "A Prayer for My Daughter" reveals a welcome firm construction and discipline of narrative detail. This is another new play Washington should see.

Clearly, people are searching for more than dreary TV provides and the burgeoning audiences from Southern California to Seattle reflect a revolt that is quickening theater everywhere. CAPTION: Picture, Broadway-bound "Zoot Suit": "It could be to Hispanics what 'The Whiz' has been to blacks."