The technique of peripheral lymphocyte stimulation in response to antigen was used to demonstrate the existence of cell-mediated anti-tumor immunity in chickens bearing tumors induced by avian sarcoma viruses. The expression of this anti-tumor response against virus-containing transformed cell culture supernatant fluids and cell extracts varied among animals in terms of time after inoculation with the oncogenic agent. Animals whose tumors had completely regressed rapidly lost the ability to mount such continuing immunity. In addition, we found evidence that some normal animals express endogenous levels of natural immunity against neoplasms induced by these viruses and/or against the viruses themselves.