Human HT-29 colon carcinoma and HeLa D98/AH2 and SK-MEL-109 melanoma cells were sensitive to synergistic growth inhibition by concentrations of recombinant human tumor necrosis factor (rTNF) and interferon-gamma (rIFN-gamma) which individually were only slightly inhibitory. We investigated whether this synergism could be explained by the presence of an increased number of TNF receptors in cells treated with rIFN-gamma. These receptors were measured by incubating cells resuspended from monolayers with 125I-rTNF. HT-29 cells treated for a few hours with rIFN-gamma could bind more 125I-rTNF than control untreated cells, but this binding returned to the level of control cells after 24 hr. The treatment with rIFN-gamma did not change the binding affinity of TNF receptors, but increased their number to 1800 per cell from a basal level of about 800 per cell. Inhibitors of RNA synthesis prevented this increase. HT-29 cells were significantly more growth-inhibited when treated first for 6 to 12 hr with rIFN-gamma and then with rTNF, than when treated first with rTNF and then with rIFN-gamma. Untreated HeLa D98/AH2 and SK-MEL-109 cells had 2400 and 9000 receptors per cell, with a KD similar to that of HT-29 cells (approximately 2 X 10(-10)M). A significant increase in TNF receptors after treatment with rIFN-gamma was observed in HeLa D98/AH2, but not in SK-MEL-109 cells. No increase in TNF receptors was detected in cells treated with rIFN-alpha 2. These results indicate that the synergism between rTNF and rIFN-gamma may be due, at least in part, to a transient induction of the synthesis of TNF receptors by rIFN-gamma in cells with a relatively low number of these receptors.