Recombinant human tumor Necrosis Factor (rHuTNF) produced dose-dependent cytotoxicity against human ovarian cancer cells, OSC and OMC, obtained from fresh ascites. A combination of rHuTNF and the topoisomerase II inhibitor, Mitoxantrone, produced dose-dependent synergistic cytotoxicity on OSC and OMC cells. When OMC cells were incubated simultaneously for one hour with rHuTNF and Mitoxantrone, increased numbers of DNA single-strands breaks were produced. rHuTNF alone did not induce DNA single-strands breaks. These data are consistent with a role for topoisomerase-linked DNA lesions in the rHuTNF mediated potentiation of killing cells by Mitoxantrone.