In July 1951 the overseas British Secret Intelligence Service (MI6) wrote to the internal British Security Service (MI5) seeking information about an American citizen living in London. The request was prefaced with a sentence that this essay seeks to explain: L. ADLER cannot return to the United States with any hope of earning a living there, owing to the allegations against him that he is communist sympathiser. We have no trace of him, and should be grateful if you would let [us] know if you have any record … or if anything is known about him in this country. This correspondence opens a personal MI5 file, first released in April 2011 through the United Kingdom National Archives. Using that file, this paper will throw new light on the Cold War years of one of America’s most celebrated musicians: the harmonica virtuoso, Larry Adler. Some details in the first section of the paper, on the ‘Red Libel Suit’ in Connecticut, USA, may once have been familiar to keen contemporary observers of American blacklisting during McCarthyism. But the second part of the story, when the paper shifts to Great Britain and differences between American and British intelligence agencies are revealed, has not previously been told.