We investigated the cytotoxic effects of recombinant tumor necrosis factor (TNF) alone and in combination with interferon-gamma (IFN-gamma) and/or cytotoxic drugs on a variety of human tumor cell lines (U937, IGROV-1, HT29, LoVo, MCF7 and U20S), including cell lines with in vitro acquired resistance (LoVo/DX and MCF7/DX selected for resistance to doxorubicin (DX) and characterized by pleiotropic drug resistance; U20S selected for resistance to cisplatin (CDDP], using MTT assay. U937 and MCF7 were sensitive to the cytotoxic effect of TNF, whereas all the other cells were insensitive up to 1000 U/ml (the maximum tested dose). Surprisingly, TNF was cytotoxic (30-40% cytotoxicity) against two resistant lines (LoVo/DX and U20S/Pt) but not against the parent sensitive lines. Treatment with increasing doses of TNF after 6 h incubation with a subtoxic concentration of IFN-gamma produced a synergistic effect in four cell lines (U937, HT29, LoVo/DX and MCF7), whereas in the other five the cell killing of the combination was comparable with that achieved by TNF alone. The combination of subtoxic doses of TNF and increasing doses of drugs targeted at DNA topoisomerase II (i.e. DX, actinomycin D and VP16) produced an additive cytotoxic effect in all cell lines. The same results were obtained combining TNF and CDDP, except in U20S/Pt cells in which TNF synergistically increased CDDP cytotoxicity. The combination of TNF and IFN-gamma enhanced cytotoxicity about 20-fold for DX and 6-fold for CDDP, evaluated in terms of the modification index, against LoVo/DX and U20S/Pt cells respectively.(ABSTRACT TRUNCATED AT 250 WORDS)