From the minute Shirley Mann began to sing "I'm Checking Home Now" till the ensemble's final triumphant warning that "The Cradle Will Rock," Marc Blitzstein's music drama had a sympathetic Sanders Theatre audience. Saturday night. If the test of a good play is its grip on the listeners, then "The Cradle" was a success.

But the Student Union production had more than merely a welcome reception. It had brilliant melodies, clever lyrics, enough humor, and excellent acting by a sincere cast--the whole combining to make palatable a message of steel unionism and proletarian action. On a stage bare except for a few chairs and a piano, Big Business, the corrupt press, and a hypocritical clergy were treated to tuneful rapping.

Not everything was bright and sunny, of course. After the first four or five "frame-ups" and "sell-outs", the effect of the play's message began to wear off, simply because Mr. Blitzstein had cried "wolf" too often. The music was occasionally too loud, and the articulation not always clear. But these were only minor defects in a well-molded whole for which Directors Bernstein and Szathmary deserve considerable credit Miss. Mann's singing of "Nickel under the Foot" was delightful, the acting of Donald Davidson and Kendall Smith quite professional. By and large "The Cradle Will Rock" was an impressive example of modern folk opera. The special performance this week deserves to be as weld-received.