Abbott and Costello's first known radio appearance was on The Kate Smith Hour in February 1938. Initially, the similarities between their voices made it difficult for listeners (as opposed to stage audiences) to tell them apart due to their rapid-fire repartee. The problem was solved by having Costello affect a high-pitched childish voice. "Who's on First?" was first performed for a national radio audience the following month. They stayed on the program as regulars for two years, while landing roles in a Broadway revue, The Streets of Paris, in 1939.
In 1940 Universal Studios signed them for the film One Night in the Tropics. Cast in supporting roles, they stole the show with several classic routines, including "Who's on First?" The same year they were a summer replacement on radio for Fred Allen. Two years later, they had their own NBC show.
The Abbott and Costello Show mixed comedy with musical interludes (by vocalists such as Connie Haines, Ashley Eustis, the Delta Rhythm Boys, Skinnay Ennis, Marilyn Maxwell, and the Les Baxter Singers). Regulars and semi-regulars on the show included Artie Auerbach ("Mr. Kitzel"), Elvia Allman, Iris Adrian, Mel Blanc, Wally Brown, Sharon Douglas, Verna Felton, Sidney Fields, Frank Nelson, Martha Wentworth, and Benay Venuta. Ken Niles was the show's longtime announcer, doubling as an exasperated foil to Abbott and Costello's mishaps (and often fuming in character as Costello routinely insulted his on-air wife). Niles was succeeded by Michael Roy, with announcing chores also handled over the years by Frank Bingman and Jim Doyle. The show went through several orchestras during its radio life, including those of Ennis, Charles Hoff, Matty Matlock, Matty Malneck, Jack Meakin, Will Osborne, Fred Rich, Leith Stevens, and Peter van Steeden. The show's writers included Howard Harris, Hal Fimberg, Parke Levy, Don Prindle, Eddie Cherkose (later known as Eddie Maxwell), Leonard B. Stern, Martin Ragaway, Paul Conlan, and Eddie Forman, as well as producer Martin Gosch. Sound effects were handled primarily by Floyd Caton. Guest stars were plentiful, including Frank Sinatra, The Andrews Sisters, and Lucille Ball. (Source: wikipedia.org)