Peter Cushing (Philip Grayson) tried to get out of his contract so that he was not away from his sick wife Helen, but he had to carry on. Helen Cushing died in January of 1971, a month or so before this film was released.
Jon Pertwee replaced the American horror film star Vincent Price, who was first offered the part of Paul Henderson but had to pass because he was under contract with American International Pictures to make horror films at the time.
Christopher Lee (John Reid) can be seen reading his favorite book, "The Lord of the Rings" by J.R.R. Tolkien, thirty years before he played Saruman in the three movies based upon it: "The Lord of the Rings: The Fellowship of the Ring (2001)," "The Lord of the Rings: The Two Towers (2002)," and "The Lord of the Rings: The Return of the King (2003)." Lee read the book once a year.
Despite the title, not one drop of blood is seen in the entire movie.
Among the photographs in the frame of Paul Henderson's mirror is one of Jon Pertwee driving "Bessie", the car he drove as the Third Doctor in Doctor Who (1963) (from the 1970 story "Inferno"). When Pertwee made this movie, he was playing the Doctor.