Both mice and rabbits injected intravenously with viable Listeria monocytogenes and challenged with bacterial lipopolysaccharide release tumour necrosis factor (TNF) into the blood. Optimal conditions for production of murine TNF using Listeria were established. The cell-kill efficacy of Listeria-TNF and of Corynebacterium parvum-TNF are comparable. Also, the two sera have similar spectra of activity; a wide variety of murine and human tumour cell lines are vulnerable while others are not affected. Among the cell lines tested, Ehrlich ascites tumour (EAT) cell was found to be most susceptible to the cytotoxic effect of TNF. The advantages of using Listeria for producing TNF and using EAT cells in studying the mechanism of action of TNF are discussed.