The title was taken from a friend of Buñuel, José Bergamin, who was writing a play with that title but never finished it. When Buñuel wanted to title his film, he asked for the rights of the title from his friend, but he answered that there was no trouble, because it was taken from the Bible, the Book of Revelations.
In his autobiography Luis Buñuel claims he was asked by Warner Brothers to work on a story that eventually was filmed as The Beast with Five Fingers (1946). "The Exterminating Angel" contains many of the elements of the earlier film including the large mansion, piano recital, and stabbing of a disembodied hand.
An early working title of the film was _The Castaways of Providence Street._ Luis Buñuel took the title _El ángel exterminador_ from his friend José Bergamin, the Spanish poet, who had mentioned it the year before for a play he wanted to write. Buñuel told Bergamin, "If I saw _The Exterminating Angel_ on a marquee, I'd go see it on the spot."