In this preliminary report there are presented some of the results obtained from crossing mice from tumor strains with males from other sources. The comparison of the tumor incidence in the inbred and backcross daughters, though the numbers given are small, supports the theory that the tendency to develop neoplasms is hereditary and the frequency with which tumors appear in the first filial generation of such crosses, indicates that the character is dominant. Additional experiments involving larger numbers are in progress.