The objective of this study was to establish whether human recombinant tumor necrosis factor (TNF) can significantly stimulate the proliferation of some tumor cells. Treatment with TNF had little or no effect on the growth of human tumor cells and murine NIH/3T3 cells cultured in medium with high serum concentration. Two tumor lines, SK-MEL-109 melanoma and HOS osteosarcoma cells, were adapted to grow in medium supplemented with 0.5% serum. The growth of these SK-MEL-109 cells was inhibited by TNF, but that of the HOS cells was greatly stimulated by TNF in a dose-dependent way. Treatment with 10 ng/ml of TNF resulted in a two-fold increase in the rate of cell division. This effect of TNF was also shown by measuring DNA and protein synthesis. The continuous presence of TNF was not required for its mitogenic activity on HOS cells cultured with 0.5% serum, since treatment for only one day with TNF resulted in prolonged growth stimulation. The failure of TNF to promote division of cells cultured in medium with 10% serum may possibly be explained by the presence of saturating amounts of growth factors in serum. Interferons abolished the mitogenic activity of TNF on HOS cells. Furthermore, TNF did not show synergism with insulin or epidermal growth factor in stimulating growth of these cells. The level of c-myc mRNA was increased five-fold after 30 minutes of treatment with TNF. This shows that TNF is a growth factor for HOS cells and that it induces accumulation of c-myc mRNA.