Basil Rathbone
Sherlock Holmes

Nigel Bruce
Doctor Watson

Marjorie Lord
Nancy Partridge

Henry Daniell
William Easter

George Zucco
Heinrich Hinckel aka Richard Stanley

John Archer
Naval Lt. Pete Merriam

Gavin Muir
Mr. Lang, government agent

Edmund MacDonald
Detective Lt. Grogan

Don Terry
Howe

Mary Gordon
Mrs. Hudson

Bradley Page
Cady

Clarence Muse
George

Mary Forbes
Mrs. Pettibone

Holmes Herbert
Mr. Ahrens

Thurston Hall
Sen. Henry Babcock

Ian Wolfe
Antiques Store Sales Clerk

Gerald Hamer
Alfred Pettibone

Gilbert Emery
Sir Henry Marchmont (uncredited)

Margaret Seddon
Miss Pringle (uncredited)

Mary Field
Party Pianist (uncredited)

Colin Kenny
Hotel Doorman (uncredited)

Caroline Frances Cooke
Mrs. Ruxton