Human and murine monocyte-macrophages kill actinomycin D (ActD)-treated WEHI 164 sarcoma cells in a 6-hr 51Cr-release assay (drug-dependent cellular cytotoxicity, DDCC). In this study, we have investigated the cytotoxic activity of human recombinant tumour necrosis factor (hrTNF) against untreated and ActD-treated WEHI 164 sarcoma cells. Human recombinant TNF when added to the 6-hr 51Cr-release assay killed ActD-treated targets at doses ranging from 33 to 0.33 ng/ml, whereas untreated targets were resistant to lysis. The kinetics of lysis of ActD-treated targets was similar for hrTNF and blood monocytes. The protease inhibitors phenyl-methyl-sulphonyl-fluoride (PMSF) and N-alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) reduced the DDCC activity of monocytes, monocyte supernatants and hrTNF. Killing of drug-sensitized target cells by monocyte supernatants was totally inhibited by a rabbit anti-TNF serum. These, as well as previous data on the physicochemical properties of the soluble cytotoxic factor released by monocytes, suggest that rapid monocyte-mediated killing of ActD-pretreated WEHI 164 sarcoma cells involves TNF or TNF-like molecules.