1. The potentialities that viruses have for the superinfection of virus tumors have not been recognized nor has the fact that a single cell can harbor more than one virus.
2. Rabbit papillomas, induced by the papilloma virus (Shope), were superinfected by B virus, myxoma virus, vaccinia virus, and probably, virus III. Similar attempts at superinfection by herpes virus were without success. The criteria for parasitization included the histopathological finding of specific inclusion bodies, the recovery of each virus by suitable animal passage, and the immunological identification of each virus.
3. Papillomas and probably the individual cells thereof were readily infected simultaneously by two viruses when the combination of B virus and myxoma virus was used.
4. Cells of the Shope papilloma have a selective affinity for certain extraneous viruses introduced at a site distant from the tumor growths. It was found that exceptionally few cells in the basal layers of the epidermis escaped when myxoma virus was used as the superinfecting agent.
5. The cells of an epidermoid carcinoma which terminated the rabbit papilloma-to-carcinoma sequence were readily infected by B virus with resultant multiple intranuclear inclusion bodies in single cells.
6. Attempts to establish a prolonged superinfection of cells of the Shope papilloma by virus III were unsuccessful. This rabbit tumor differs therein from the Brown-Pearce tumor, an epithelioma of rabbits, in its susceptibility to infection with virus III.