On April 29, 1947, Markle launched the 60-minute CBS radio series with an adaptation of Malcolm Lowry's Under the Volcano. The radio series continued until July 27, 1948.

Of those appearing on the show, the most notable were Marlene Dietrich, Jan Merlin, Anne Burr, Everett Sloane, Joe DeSantis, Hedley Rennie, Juano Hernandez, Robert Dryden, Dan Ocko, Paquita Anderson, Don Alberto, Leo Badilla, Ralph Camargo, Ivor Francis, Hester Sondergaard, Sebastian Cabot, Harald Dyrenforth, Ian MacAllaster, Maver Moore, Paul McGrath, Frank Behrens, Ruth Gilbert, Howard Smith, William Woodson, Evelyn Varden, John Sylvester, Leon Janney, Joan Alexander, Cathleen Cordell, Sarah Burton, Abby Lewis, Miriam Wolfe, Gregory Morton, Lou Merrill, Sydney Smith, Stefan Schnabel, Clarence Derwent, Martin Wilson, Lamont Johnson, Mercedes McCambridge, Michael Dreyfus, Rosemary Rice, Edwin Bruce, Joyce Van Patten, Rosemary Rice, Frank Readick, Betty Garde, Bud Collyer, Myron McCormick, Gary Merrill, John McGovern, Elspeth Eric, Raymond Edward Johnson, Robert Mitchum, Charles Laughton, Geraldine Fitzgerald, Frank Richards, Anne Seymour, Gene Kely, Ralph Bellamy, James Mason, Eileen Farrell, Joan Blondell, Gertrude Warner, Edna Ferber, Glenn Anders, Horace Braham, Alan Devitt, John Merlin, Bonita Granville, Donald Hastings, Richard Arlen, Michael Artist, Ronald Liss, Brainard Duffield, Louis Quinn, Paul Muni, Charles Irving, Claire Niesen, Walter Slezak, Luise Rainer, Beverly Roberts, Madeleine Carroll, Robert Young, Kathy MacGregor, Michael Redgrave, Judith Evelyn, Brenda Forbes, Burgess Meredith, Susan Peters, Glenn Ford, John Stanley, Walter Huston, Alan Baxter, Elissa Landi, Michael Fitzmaurice, Connie Lembcke, Bartlett Robinson, Betty Field, Ann Blyth, Margaret Philips, Franchot Tone, Rosemary De Camp, Michael Redgrave, Rene Ray, and George Breen.