Tumor colony-forming cells were grown from fresh biopsy specimens from 102 patients with a variety of nonhematologic malignant neoplasms and exposed in vitro to pharmacologically achievable doses of recombinant human tumor necrosis factor (rTNF). In 68 instances, the tumor specimens were also tested against recombinant human gamma-interferon (rIFN-gamma), as well as the combination of rTNF and rIFN-gamma. rTNF exhibited dose-dependent and tumor-type-dependent antitumor effects. Sensitivity to rTNF at doses of less than 100 U was observed in 28% of the tumors tested. A higher than average frequency of sensitivity was observed in colorectal and lung cancer. Resistance to rTNF was observed in 42% of the tumors, including 52% of the ovarian cancer specimens tested. In paired experiments, exposure of tumor specimens to rTNF and rIFN-gamma in combination often resulted in a greater antitumor effect than was observed with either agent alone, with at least subadditive effects seen in 62% of the specimens tested against the combination. Antagonism between rTNF and rIFN-gamma was observed in 18% of the studies. Overall, exposure to the combination of rTNF and rIFN-gamma reduced the dose of rTNF required for significant antitumor activity by about threefold. Normal bone marrow granulocyte-macrophage colony-forming cells were also tested against both rTNF and rIFN-gamma and the combination. The bone marrow progenitors were more sensitive to rTNF and the combination with rIFN-gamma than were the tumor cells; however, the significance of this comparison between two different in vitro assay systems is indeterminate. Based on our observations, rTNF warrants phase II clinical trials in selected solid tumors with definite emphasis on colorectal and lung cancer. Additionally, studies of the combination of rTNF and rIFN-gamma are indicated and will be of particular interest in endometrial and breast cancer.