We previously reported that the bulk of Listeria monocytogenes injected intravenously into mice is taken up in the liver and replicates within the parenchymal cells (hepatocytes). Although IFN-gamma is known to play an important role in host defenses to listerial infections of the liver, the mechanism(s) that underlies this role remains to be fully delineated. In the initial experiments presented here, we demonstrated the elevated expression of IFN-gamma message in the livers of mice during primary listerial infections. Subsequent experiments showed that the listerial burden of the hepatocyte population was increased significantly in mice administered monoclonal anti-IFN-gamma. Conversely, the administration of murine rIFN-gamma resulted in a marked (2 log10) decrease in the number of hepatocyte-associated Listeria. In vitro, IFN-gamma stimulated the listericidal activity of purified hepatocytes. Infected hepatocytes incubated in the presence of > 0.1 U/ml murine rIFN-gamma exhibited a significant reduction in intracellular Listeria. The elevated antilisterial activity of IFN-gamma-treated hepatocytes in culture was abrogated by the presence of compounds that scavenged or inhibited the production of reactive oxygen intermediates. Taken together, these findings suggest that activation of the oxygen-dependent, antimicrobial activity of hepatocytes may constitute a principal effector function of IFN-gamma in host defenses to listerial infections of the liver.