Interspecific hybrid cells, derived from fusion of normal and leukemic (CML) human leukocytes with tumorigenic P19 mouse or a3 Chinese hamster cells, were tested for their tumor-forming capacity in congenitally athymic nude mice. Partial suppression of tumorigenicity was observed in several hybrid clones derived from both normal and leukemic leukocytes. Chromosome analysis of the hybrid cells before inoculation in nude mice and of the derived tumors did not reveal a human chromosome bearing factor(s) which singly appeared responsible for suppression. The presence of the Philadelphia translocation in the leukemic cells does not seem to have deprived these cells of their tumor-suppressing ability.