The importance of the viruses of western1and of eastern equine encephalomyelitis2in the causation of human disease is now well established. Meyer,3shortly after the discovery of the western equine virus in 1930, described the occurrence of encephalitis in 3 persons who had cared for sick horses and he voiced the suspicion that human infections with this virus may occur. In 1938 Eklund and Blumstein,4investigating the occurrence of 6 human cases of encephalitis in Minnesota, found neutralizing antibodies to the western virus in the blood serum in 1 of 3 cases tested, and in the same year Howitt5isolated the virus from the brain in a human case of encephalitis. In 1941 the largest epidemic of encephalitis ever recorded occurred in and around North Dakota,6which alone had 1,080 cases with ninety-six deaths, and the causative agent was shown to be the