F1 and F2 and first backcross hybrids and second backcross families of the high mammary tumor incidence strain GR and the low incidence strain C57BL were examined for the segregation of mouse mammary tumor viral (MMTV) expression. Although GR has been reported to transmit MMTV as a single dominant gene, several lines of evidence suggest there are multiple genetic factors that influence MMTV expression, MMTV expression as measured by double antibody radioimmunoassay for MMTV p14 segregated in 106 first backcross progeny at a 60:40 ratio, intermediate between what would be expected for either a single or two gene hypothesis. In female second backcross progeny of either male or female first backcross, a heterogeneous pattern of expression was noted that does not fit any simple Mendalian pattern. From an analysis of serial lactations of first backcross and second backcross families, it appears that all hybrid females contain MMTV proviral information that may be expressed either at late lactations or in a variable proportion of progeny mice. These combined results are most consistent with a vertically transmitted genome regulated by multiple factors in these crosses.