Newborn BALB/c, C57BL/6, and (BALB/c × C57BL/6) F₁ (CBF) mice were equally susceptible to tumor induction by Moloney sarcoma virus (MSV). A significant incidence of spontaneous regressions of primary tumors was observed— a phenomenon unique to this tumor system. Of tumors induced in newborn BALB/c, C57BL/6, and CBF mice, 4/151, 20/43, and 12/49, respectively, regressed. Serum from mice whose primary tumors regressed neutralized the oncogenicity of MSV. The three strains of adult mice were equally susceptible to tumor induction by MSV. X-irradiation of adult mice with 350 R one day prior to MSV inoculation did not affect susceptibility to oncogenesis, but affected the fate of the tumor. Whereas all tumors induced in unirradiated adult mice completely regressed, many tumors induced in X-irradiated mice grew progressively and killed the hosts. It is therefore suggested that in this tumor system, with its remarkably short latency period to tumor formation, the immunologic competence of the host may not be a prime factor in tumor induction, but may determine whether a primary tumor, once formed, will regress or kill the host. All the adult mice were bled 9, 21, and 50 days after MSV inoculation, and their sera tested against Moloney lymphoma cells by the indirect fluorescent antibody test. No relationship between antibody production and tumor regression was evident.