Abstract
Adult mice and hamsters can be made resistant to an isologous transplantable polyoma tumor by an inapparent infection with polyoma virus. This resistance is cell-mediated and seems not to be related to anti-viral serum antibodies. The basis of the resistance appears to be a transplantation type of cellular immunity directed against a "foreign" antigen contained in the tumor cell. Evidence has been presented to demonstrate this tumor antigen. It is possible that this phenomenon may explain the lack of oncogenesis by polyoma virus infection of adult mice, and the rarity of naturally occurring polyoma tumors.