--- title: "This is why World War I-era British spies used semen as invisible ink" description: "Some methods and tactics spies have come up with make you wonder how they even came up with it. This one makes you not want to ask." date: "2022-07-09" modified: "2022-07-09" authors: - name: "Blake Stilwell" job_title: "Editor-In-Chief, Air Force Veteran" link: "https://www.wearethemighty.com/authors/blake/" url: "https://www.wearethemighty.com/popular/this-is-why-world-war-i-era-british-spies-used-semen-as-invisible-ink/" categories: - "History" - "War History" - "World War I" --- # This is why World War I-era British spies used semen as invisible ink ![]() *A sex joke that was a little too on the nose even for the Bond series, apparently (MGM)* The first head of Britain's secret service — which would one day be called MI6 — carried a swordstick, drove a personal tank, and would sometimes stab his wooden leg with a pen just to see how people reacted. --- If that wasn't enough to make him eccentric, his department also discovered that [semen makes an excellent invisible ink](). ![]() *It's probably best not to ask why. Or how.* No one actually knows which British agent was the one who came up with the idea, but the book "[Six: The Real James Bonds 1909-1939]()" notes that his fellow spies made so much fun of him that he had to be transferred to another office. His name was — no joke — Captain Sir Mansfield Cumming and his agents [lived by the motto](), "Every man his own stylo." The truth was, British spies were searching for the perfect invisible ink during World War I and thought natural fluids were the ideal. The major issue with using semen to write letters? [The smell eventually becomes very distinctive](). Cumming ruled that agents abroad using this method of secret messaging ensure their ink was fresh for every letter. ![]() *Gloves, fellas?* The book details an agent in Copenhagen, a Maj. Richard Holme, who apparently kept a ready supply on hand. "...his letters stank to high heaven and we had to tell him that a fresh operation was necessary for each letter." In "[Prisoners, Lovers, and Spies: The Story of Invisible Ink]()," Kristie Macrakis writes that Cumming began inquiring about the use of bodily fluids as invisible ink as early as 1915 and told Walter Kirke, Deputy Head of Military Intelligence that he thought the best invisible ink was indeed semen. Semen does not react to the iodine vapor test, a method that then turned [all known invisible inks brown](). This was particularly attractive to the spy agency, but unfortunately (for spies — not for those concerned with hotel cleanliness) heat develops semen ink and it appears in ultraviolet light. ## Author Blake Stilwell is We Are The Mighty's Editor-in-Chief, covering military news, entertainment, and history, along with veteran education, employment and entrepreneurship. He is a former U.S. Air Force combat cameraman with degrees in Graphic Design, Television and Film, International Relations, Public Relations, Business Management and Middle Eastern Affairs. After leaving the military, he began working in film and television production in New York and Los Angeles, work that included ABC News, NBC Olympics, and HBO Sports. He then spent two years in Washington with White House Communications, before joining the Near East Foundation. At NEF he served as an International Development Fellow, then Communications Officer, and finally, Media Officer for the Middle East and Africa. Based in the Palestinian Territory, he coordinated strategic communications between eight country offices, international media, and government and NGO partners. He began a career focused on writing and editing in 2015 with We Are The Mighty. He left in 2019 for Military.com, but returned as Editor-in-Chief in 2025. Blake’s work has been seen on CBS News, Fox News, CBC, The Chicago Tribune, Task & Purpose, Military Times, Recoil, Skillset, American History, and was shockingly even used in a Supreme Court argument. He is an avid traveler and small business owner in Ohio, where he spends most of his energy fixing up a very old house. ### Author social links [Facebook]() [Twitter]() [Instagram]() [LinkedIn]()