The inheritance of suceptibility of mice to primary sarcomas induced by neonatal injection of RSV-induced chicken tumor material was studied in segregating populations derived from F1 hybrids between the resistant strain A.SW (H-28) and the susceptible strains A/WySn, B10.A (both H-2a), CBA/J (H-2k), and C57BL/10 (H-2b). In F1 hybrids between A.SW and each of the four susceptible strains, resistance is apparently dominant. In (A/WySn × A.SW) F2, (A.SW × B10.A) F2, and (A.SW × CBA/J) F2 generations tumor incidence was significantly associated with H-2 type. H-2aa and H-2kk homozygotes were as susceptible as A/WySn, B10.A, and CBA/J parental strain mice; and H-2ss homozygotes were as resistant as A.SW parental mice. H-2as and H-2ks heterozygotes were of intermediate susceptibility. No evidence was seen for association of susceptibility with three coat color genes or with sex. There was no maternal influence on inheritance of suceptibility. In a (B10.LP × A.SW) × C57BL/10 backcross generation H-2sb segregants were significantly more resistant to Rous tumorigenesis than were H-2bb segregants. No influence of genes linked to agouti or of X-linked genes was seen in this backcross generation. When mice of high and low primary tumor incidence strains were immunized with the same allogeneic Rous sarcomas, resistant strain mice developed stronger cellular anti-Rous tumor immunity (as measured by the Winn lymphocyte-tumor cell cotransfer assay) than did susceptible strain mice.