Hare Sitting Up (Sir John Appleby #18) by Michael Innes

A top-level scientist has disappeared. . . with a deadly bacteriological culture.
Kidnapped? A nervous breakdown? Defected to the enemy? 

Or is he quietly plotting the destruction of the whole human race?

            HARE SITTING UP
Juniper stopped in his tracks, and Appleby saw that he had quite suddenly gone deadly pale. 'You can't mean--'

   'Yes. You yourself came quite near to mentioning it, if only as a sort of joke. Your brother may have taken something with him. Something that ought never to have left a locked refrigerator in his laboratory.'

   'It can't be true!' Juniper's voice had risen in pitch again. 'Howard would never--Why, it's madness!'

   Appleby turned and looked at him steadily, 'Isn't madness,' he asked, 'one of the possibilities we've been talking about?'

Genres: Mystery - Crime - 20th Century - Scottish Author