Cellular subclones of high and low tumorigenicity obtained from a mouse c-Ha-ras-transformed clone, were examined for their sensitivity to tumornecrosis-factor (TNF)-mediated cytotoxicity. Cells of the highly tumorigenic subclones showed a significantly enhanced resistance to the cytotoxic effect of TNF plus cyclohexamide (CHI) as compared to cells of the lowtumorigenic subclones. The enhanced resistance to TNF+CHI was not due to a lower expression of TNF receptors on the cells. The c-Ha-ras-transfected cells were transformed and maintained in culture only (C cells). In vivo passage of cells of the initially low-tumorigenic c-Ha-ras subclones through the mouse significantly enhanced the tumorigenic potential of these CTC cells (culture/tumor/culture). In correlation with their enhanced tumorigenicity, the CTC cells were highly resistant to TNF-mediated cytotoxicity as compared to C cells of the same subclone. Furthermore, the involvement of TNF in determining the tumorigenic phenotype of the c-Ha-ras-transformed cells was demonstrated in a more direct manner. Cells of a c-Ha-ras-transformed low-tumorigenic, highly TNF-sensitive subclone were selected by repeated cycles of in vitro exposure to TNFα. As a result, a stable, highly TNF-resistant population of cells emerged. These TNF-resistant cells caused more tumors in mice as compared to their original TNF-sensitive cells. These results show that the resistance to the cytotoxic effect of TNF plus cyclohexamide may be involved, at least partially, in the tumorigenic potential of c-Ha-ras-transformed cells and suggest a possible role for TNF in the enhancement of the tumorigenic potential of these cells in mice.