Newborn and adult Syrian hamsters were injected with wild-type SV40 and its temperature-sensitive (ts) mutants A30, A209, A239, B201 and BC210. In contrast to wild-type SV40, ts A30, ts A 239 and ts BC210 were oncogenic in adult hamsters inducing tumors after almost the same latent period as wild type SV40 in newborns but in lower number of animals. Study of TSTA in some tumors (1 generation) induced in newborn and adult hamsters by wild type SV40 and its ts mutants (with the use of immunogenicity and immunosensitivity tests) revealed no significant difference among compared tumors: most of them were immunogenic and immunosensitive. In contrast hamster embryo cells in vitro transformed by SV40 ts A30, ts A239 and ts A209 mutants, studied at different passage levels were all immunoresistant during about 30 in vitro passages and in most cases 10--100 times less immunogenic than hamster embryo cells in vitro transformed by wild type SV40. At higher passage level in some of these lines expression of TSTA improved. The data obtained are discussed in connection with the recently demonstrated [5] significant quantitative difference in tumor specific transplantation antigen activity in hamster cells in vivo and vitro infected, or transformed by wild type SV40 and its ts A mutants.