ONCE A THRIVING cultural mainstay on New York's Lower East Side, Yiddish theater spawned such actors as Paul Muni, Molly Picon, Herschel Bernardi and Walter Matthau. At one point there were 23 Yiddish theaters in New York. From 1918 to 1950, the Yiddish Art Theater alone launched nearly 150 productions.

It is this Golden Age that the B'nai B'rith Klutznick Museum is celebrating with "Hooray for Yiddish Theater in America!", an exhibit of more than 200 items -- posters and playbills, costumes and costume jewelry. Don't miss the 65-minute documentary film showing here daily at 10:30, noon, 1:30 and 3. It's a fascinating array of reminiscences and Yiddish film clips with English subtitles.

The cast of characters in Yiddish theater included Boris Thomashevsky, who appeared in 1882 in the first Yiddish production in this country and, proclaiming himself "America's darling," became a matinee idol.

There was Abraham Goldfaden, "the Shakespeare of the East Side"; and Picon, who debuted in "Yonkele," an early version of "Yentl." And Maurice Schwartz, who played "Tevye the Dairyman," Sholom Aleichem's creation that became "Fiddler on the Roof." Schwartz also founded the Yiddish Art Theater and started a Yiddish company in Israel.

But Yiddish has been dying out, and with it the Yiddish theater, partly because of assimilation, and partly because of stricter American immigration laws in the '40s. Also, as noted in the documentary film, parents wouldn't teach their children to speak Yiddish, but rather spoke it to each other when they didn't want to be understood.

The Cafe Royal, which was the Stage Delicatessen for Second Avenue, is now a dry cleaning shop. The Hebrew Actors Union is still alive, but more as a place to play chess. And although a Yiddish musical with English subtitles is currently running on Broadway ("A Match Made in Heaven"), one hesitates to say a revival is taking place.

HOORAY FOR YIDDISH THEATER IN AMERICA! -- At the B'nai B'rith Klutznick Museum, through August 1986; film, through March. Hours: Sunday to Friday, 10 to 5.