The Vx2 and Vx7 rabbit carcinomas, both of Shope papilloma derivation, were cultivated in vitro and studied for immunofluorescence indicative of the presence of viral antigen. The cultured Vx7 cells were characteristically pleomorphic, round or polygonal cells, and immunofluorescence was demonstrable in 5-20% of them as counted on cover slip cultures after exposure to rabbit sera containing anti-Shope papilloma virus antibody. The Vx2 cells in contrast to this were of almost uniform size and character, small and approximately round, and the specific fluorescence was found in less than 0.1% of the total number. These findings indicate that some antigenic derivation from the Shope papilloma virus is still persisting at least in some fractions of the cells, not only of Vx7 tumors but also of the Vx2 tumors, even after maintenance of the growths by transplantation for more than 20 yr. During the course of the study, two lines of the Vx7 were kept continually growing while the culture of Vx2 was discontinued at its tenth passage after an accident due to mechanical failure of the incubator. The neoplastic capacity of the Vx7 cells cultured in vitro was demonstrated by implanting these intradermally on the ears of domestic rabbits. The result was destructive carcinomas of the Vx7 sort.