We have examined 6 human renal cell carcinoma (RCC) cell lines for their sensitivity and resistance to the cytolytic effect of tumor necrosis factor (TNF) and adriamycin (ADR), alone or in combination. The results of cytotoxicity mediated by TNF and ADR showed no direct correlation as TNF resistant lines were sensitive to ADR while the TNF sensitive lines were resistant. The combination of TNF and ADR resulted in enhanced cytotoxicity against the tumor lines. Induction of TNF mRNA and protein has been suggested as a mechanism of resistance to TNF in certain tumors. Resistant and sensitive lines were capable of upregulating TNF mRNA after treatment with TNF or PMA+ionophore for periods as short as 1 hour, but only the resistant lines were able to secrete detectable levels of TNF protein. Therefore, a positive correlation existed between resistance to TNF and production and secretion of TNF by the cell lines. In the presence of the protein synthesis inhibitor cycloheximide (CHX), the TNF mRNA level in the TNF resistant lines was increased while the sensitive lines required an additional signal, such as exogenous TNF, to upregulate the mRNA. Due to the enhanced cytotoxicity seen with the combination of TNF and ADR, we determined the effect of this combination on the levels of TNF mRNA. As examined in a constitutively TNF expressing line, ADR alone reduced the constitutive mRNA level and, in combination with TNF, reduced the level of induction produced by TNF. This downregulation of TNF mRNA by ADR may play a role in the enhanced cytotoxicity seen with combined TNF and ADR treatment. The present study demonstrates that RCC cell lines differ in their sensitivity and/or resistance to TNF. Further, ADR and/or TNF resistant RCC lines can be rendered sensitive by combining TNF and ADR. The constitutive and/or inductive secretion of TNF by certain lines and its relationship to tumor pathogenesis as well as overcoming resistance are discussed.