Herpes virus which ordinarily produces in Cebus olivaceus monkeys an acutely fatal encephalitis closely resembling in time, symptoms and pathology the acute, herpetic disease of rabbits may—in more resistant individual monkeys—lead to a more prolonged malady which, while unquestionably produced with herpes virus, simulates with considerable accuracy the human disease of acute encephalitis, in symptoms, in course and in pathological changes.