From The Telegraph:
A diary entry belonging to a senior member of the Secret Intelligence Service (MI6) has revealed that during the First World War it was discovered that the bodily fluid could act as an effective invisible ink.
In June 1915, Walter Kirke, deputy head of military intelligence at GHQ France, wrote in his diary that Mansfield Cumming, the first chief (or C) of the SIS was “making enquiries for invisible inks at the London University”.In October he noted that he “heard from C that the best invisible ink is semen“, which did not react to the main methods of detection. Furthermore it had the advantage of being readily available.
However, the discovery also led to some further problems, with the agent who had identified the novel use having to be moved from his department after becoming the butt of jokes.
In addition, at least one agent had to be reminded to use only fresh supplies of the ‘ink’ when correspondents began noticing an unusual smell.