Tests for transplantation immunity and for the occurrence of virus-neutralizing serum antibodies were performed on mice, inoculated when newborn with the Schmidt-Ruppin strain of Rous sarcoma virus (RSV-SR). Mice developing no palpable primary sarcomas showed a clear-cut resistance against the isografting of established specifically antigenic Rous tumors. Transplantation tests performed on primary tumor hosts after extirpation of the tumors revealed neither any clear-cut immunity nor tolerance to the specific transplantation antigen(s). Serial pretreatment of operated primary tumor animals with irradiated autologous or syngeneic tumor cells resulted in a clear-cut transplantation immunity. Virus-neutralizing activity was only found in a few sera from newborn infected mice, and in these cases there was no positive correlation with the transplantation immunity.

It seems probable that a successful immunization against the RSV-SR specific transplantation antigen(s) prevents the development of primary tumors. There is no indication of any tolerance to this antigen in connection with the induction of primary tumors.

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