We have studied the effects of lipopolysaccharide (LPS) on tumoricidal activity of human monocytes freshly isolated from peripheral blood. Actinomycin D-treated WEHI-164 cells were used as targets because they are NK insensitive and are lysed rapidly by monocytes in 6-hr 51Cr-release assays. Monocytes exhibited significant spontaneous activity without endotoxin. Monocytes either pretreated for 1 hr with LPS or assayed in the presence of LPS exhibited 100- to 1000-fold increased cytolytic activity. A half-maximal response was observed with 100 pg/ml LPS. Lipid A was as effective as intact LPS but required slightly higher doses. Monophosphoryl lipid A had no effect. Supernatants of monocytes cultured 5 hr contained sufficient cytolytic activity to account for levels of cytolysis mediated by monocytes directly. Doses of LPS from 10 pg/ml to 10 micrograms/ml produced parallel increases in cell-mediated and supernatant-mediated lysis. Lymphocytes did not produce cytolytic supernatants. Cytolytic activity appeared in monocyte supernatants after 30 min and peaked after 4 to 7 hr regardless of the LPS concentration; longer incubation led to a loss of activity. Cytolytic activity was heat labile and trypsin sensitive, and was recovered from Sepharose S-200 columns in a single peak with an apparent m.w. between 25,000 and 40,000. Actinomycin D or cycloheximide treatment of monocytes before the addition of LPS inhibited cytolytic monokine production. Cytolytic monokine activity was partially neutralized by specific rabbit antisera to human tumor necrosis factor (TNF). We conclude that, although fresh human monocytes exhibit spontaneous tumoricidal activity, LPS is a potent activating agent. Its stimulatory effects depend on new transcription and translation and are mediated by enhanced secretion of a cytolytic monokine similar to TNF.