Tumor necrosis factors (TNFs) are a class of cytokines secreted by activated effector cells involved in host defense against tumor progression. Epidermal growth factor (EGF) and recombinant human transforming growth factor-α (rHuTGF-α) were shown to interfere with the in vitro antiproliferative effects of recombinant human tumor necrosis factor-α (rHuTNF-α) and -β on a human cervical carcinoma cell line, ME-180. The inhibitory effect could be observed at EGF or rHuTGF-α concentrations of 100 pg/ml, and was maximal between 1 and 10 ng/ml. This response was not due to down regulation of the TNF receptor or to alteration of the affinity of TNF-α for its receptor. Since the antiproliferative effect of recombinant human interferon-γ was not significantly affected by the presence of EGF or rHuTGF-α, the inhibition was specific for recombinant TNFs and was not due solely to enhanced proliferation induced by the growth factors. Neither growth factor had a substantial protective effect on the synergistic cytotoxicity observed when tumor cells were exposed simultaneously to rHuTNF-α and recombinant human interferon-γ. TGF-β can also interfere with the antiproliferative effects of rHuTNF-α in vitro. At concentrations of less than 1 ng/ml, TGF-β significantly antagonized the cytotoxic effects of rHuTNF-α on NIH 3T3 fibroblasts. Since EGF, platelet-derived growth factor, and TGF-β all enhanced NIH 3T3 cell proliferation, but only TGF-β interfered with rHuTNF-α cytotoxicity, the protective effects of TGF-β were not related in a simple manner to enhanced cell proliferation. rHuTGF-α and TGF-β did not have a significant protective effect against rHuTNF-α-mediated cytotoxicity on two other tumor cell lines, BT-20 and L-929 cells. Based upon these observations we suggest that growth factors might enhance tumor growth in vivo by a combination of distinct mechanisms: (a) by autocrine stimulation of tumor cell growth; and/or (b) by interfering with normal effector mechanisms of host defense.