Peter Cushing as Dr. Victor Stein

Francis Matthews as Dr. Hans Kleve

Eunice Gayson as Margaret Conrad

Michael Gwynn as Karl

John Welsh as Bergman

Lionel Jeffries as Fritz

Oscar Quitak as Dwarf

Richard Wordsworth as Up Patient

John Stuart as Inspector

Michael Ripper as Kurt

Charles Lloyd Pack as President

Arnold Diamond as Molke

Marjorie Gresley as Countess Barscynska

Anna Walmsley as Vera Barscynska

George Woodbridge as Janitor

Ian Whittaker as Boy

Avril Leslie as Girl

John Tatham as Manservant (uncredited)

Eugene Leahy as Kleine (uncredited)

Charles Gilliard as Policeman (uncredited)

Jimmy Charters as Patient (uncredited)

Julia Nelson as Inga (uncredited)

Robert Brooks Turner as Joseph the Groom (uncredited)