MI6's Invisible Ink

Besides not reacting to iodine fumes, it likely had the advantage of raising morale, and, as the article says, "it had the advantage of being readily available."

Mansfield Cumming, the first chief of the Secret Intelligence Service (MI6) during the Great War apparently was delighted. James Bond, MI6's most famous, if fictional, member, would have been pleased. The invisible ink: fresh semen.

For more on the early days, see 'MI6: The History of the Secret Intelligence Service 1909-1949' by Professor Keith Jeffery.