The presence and nature of surface antigens in BKV-induce tumor cells was studied by transplantation rejection experiments, immunofluorescent staining and by complement-mediated cytotoxicity assay. Immunization with an irradiated BKV-induced hamster tumor cell line protective syngeneic hamsters not only against a challenge of the same tumor cells but also against a SV40-induced hamster tumor cell line. A control hamster cell line (HDC-22) was also able to protect the hamsters against the later challenge with a BKV-transformed hamster cell line. In addition to species-specific reactions, cross-reactions between BKV-transformed hamster and rat tumor cells were detected by immunofluorescent staining of surface antigens. This reactivity was not due to the Forssman antigen, because the rat cell lines studied were negative by the same method using Forssman antiserum. By the complement-mediated cytolytic antibody assay using [51]Cr-labelled tumor cells as targets a "public" antigen was found again in BKV-, SV40- and non-virally transformed hamster and BKV-transformed rat cell lines.