mouse tissue alloantigen detected by this procedure. An antigen of cross-reacting specificity was revealed in tissue extracts of many other animal species. We next wanted to learn more about the antigen responsible for induction of postoncolytic immunity. Extracts from virus-infected tumors were immunogenic, and both active and inactive fractions of such extracts were obtained. The growth of the virus in the tumor cells was studied with the electron microscope, in the hope that this might shed some light on the manner in which viral infection transforms a poorly immunogenic tumor into a...