Introduction. Following Jenner's demonstration that the inoculation of cowpox virus protected humans against subsequent infection with smallpox, numerous vaccine lymph laboratories were established throughout the civilized world. In 1886 Buist (11) described the elementary bodies of vaccinia and variola, but it was not until 1906 that Paschen (80) demonstrated their etiological role. During the ensuing thirty-three years, especially in the period 1920–1940, vaccinia virus has been intensively studied. These investigations were greatly simplified by the wide range of animal hosts susceptible to vaccinia virus infection, and by the resistance of the virus to heat, so that it could be easily handled in the laboratory. As a result there probably is a larger body of data about vaccinia virus, its host range, antigenic constitution, and chemical structure, than about any other animal virus. Contrasting with this wealth of knowledge about laboratory aspects of vaccinia virus is the paucity of reliable information of the epidemiology and inter-relationships of the natural pox diseases of animals.