Semen in espionage
When the British Secret Intelligence Service discovered that semen made a good invisible ink, Sir George Mansfield Smith-Cumming noted of his agents that "Every man (is) his own stylo".
The book Six: The Real James Bonds 1909-1939 by Michael Smith includes an excerpt about the semen ink method from a letter by one of Cumming's officers, Frank Stagg:
"Secret inks were our stock in trade and all were anxious to obtain some which came from a natural source of supply. I shall never forget [Captain Cumming's] delight when the Chief Censor [Frank] Worthington came one day with the announcement that one of his staff had found out that semen would not respond to iodine vapour and told the man that he had had to remove the discoverer from the office immediately as his colleagues were making life intolerable by accusations of masturbation. The Old Man at once asked Coney Hatch [lunatic asylum] to send female equivalent for testing and the slogan went round the office — every man his own stylo. We thought we had solved the problem. Then our man in Copenhagen, Major [Richard] Holme, evidently stocked it in a bottle, for his letters stank to high heaven and we had to tell him that a fresh operation was necessary for each letter."