MI6 'used bodily fluids as invisible ink'

[David Morris]

http://www.telegraph.co.uk/news/uknews/8015180/MI6-used-bodily-fluids-as-invisible-ink.html

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.