We analyzed the relationship of genetic factors determining the expression of endogenous type-C RNA tumor viruses and other host-gene markers to tumorigenesis. A hybridization experiment was performed with mice of strains AKR/J and C57L, the first filial (F(1)) generation hybrids, the second filial (F(2)) generation hybrids, and the backcrosses to the two parental strains. The results demonstrated a highly significant and predictable association between the expression of complete infectious virus or the viral group-specific (gs) antigen in spleens of young mice and tumorigenesis later in life. Most of the tumors were thymic leukemia and reticulum sarcoma, but other mesenchymal, as well as epithelial, tumors were also observed. Tumors occurred preferentially in gs-antigen- or virus-positive mice of all crosses; in the C57L-backcross and F(2) mice segregating for gs-antigen and virus expression, a few gs-antigen-negative mice developed reticulum cell sarcomas. At the time of their occurrence, the mice were all gs-antigen-positive, and most had virus as well.A minor effect of the major histocompatibility locus, H-2, on leukemogenesis was found in the F(2) mice. Several tumor types were also found that we have never observed in the two parental strains. Our data provide the most direct biological evidence in favor of the viral oncogene theory. Thus, from the presence or absence of expression in early life of splenic gs antigen or virus, we can predict whether or not a tumor is likely to develop later in life. These findings suggest that the genome of endogenous type-C RNA viruses is the major determinant for tumorigenesis although they provide no clues about the factors responsible for the various histological types.