Semen used as invisible ink during WWI

There are other choices besides lemon juice:

Though British counterintelligence spent a considerable amount of time trying to detect enemy secret ink, MI6, British foreign intelligence, experimented with its own recipes... Officers were especially interested in experimenting with bodiily fluids like blood, saliva, urine, and semen - all readily available substances.  An advantage of using bodily fluids is that possession of these substances is not proof of guilt... [spies had been convicted and executed based on the discovery of lemon residue on writing instruments].

Mansfield Cumming, head of MI6... thought "the best invisible ink is semen."  MI6 investigators thought they had solved a great problem, and the men started gleefully experimenting with the new discovery.  Obviously, the main way to produce semen at the office was through masturbation.  The agent who had discovered the covert use of semen reportedly had to transfer to another department after he was teased so much by other staff members.  One officer in Copnhagen took the new discovery so seriously that he "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." (p. 151)

Found in Prisoners, Lovers, & Spies; The Story of Invisible Ink from Herodotus to al-Qaeda, by Kristie Macrakis.