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In page Abbott and Costello:

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The team's first known radio broadcast was on The Kate Smith Hour on February 3, 1938.[1] At first, the similarities between their voices made it difficult for radio listeners (as opposed to stage audiences) to tell them apart during their rapid-fire repartee. As a result, Costello affected a high-pitched, childish voice. "Who's on First?" was first performed for a national radio audience the following month.[1] They performed on the program as regulars for two years, while also landing roles in a Broadway revue, The Streets of Paris, in 1939.[2]