Listeria monocytogenes is a facultative intracellular pathogen and survives within phagocytic cells by escaping from phagosomes into the cytoplasm. It has been reported that, in vivo, L. monocytogenes is effectively eliminated through cell-mediated immunity, especially by macrophages which have been immunologically activated by cytokines such as gamma interferon (IFN-gamma). However, this killing mechanism for L. monocytogenes and the role of macrophage activation in this bacterial killing are unclear. We demonstrated the listericidal effect of oxidative radicals induced by lipopolysaccharide (LPS) and IFN-gamma, using a macrophage-like cell line, J774.1, and a mutant cell line, LPS1916. LPS1916 cells do not exhibit normal generation of O2- and H2O2 after treatment with 0.1 microgram of LPS per ml, although J774.1 cells generate 100 times the normal level of oxidative radicals with the same LPS treatment. The growth of L. monocytogenes was strongly inhibited in J774.1 cells pretreated with 0.1 microgram of LPS per ml or the combination of 0.1 microgram of LPS per ml and 10 U of IFN-gamma per ml. On the other hand, in LPS1916 cells, the growth of L. monocytogenes was not inhibited by treatment with LPS only, although LPS1916 cells pretreated with the combination of LPS and IFN-gamma showed moderate inhibition of listerial growth. This killing was not influenced by treatment with NG-monomethyl-L-arginine, which is a strong inhibitor of nitrite oxide generation. Interestingly, J774.1 cells treated with LPS did not show enhanced intraphagosomal killing of a nonhemolytic strain of avirulent L. monocytogenes that lacks the ability to escape from phagosomes, and this killing was not influenced by treatment with NG-monomethyl-L-arginine either. These results suggest that the reactive oxygen radicals are more important than nitric oxide in the mechanism underlying the intracellular killing of virulent L. monocytogenes and that there seem to be different killing mechanisms for virulent and avirulent strains of L. monocytogenes in activated-macrophage cell lines.