Data presented here demonstrate that recombinant gamma interferon (rIFN-gamma) activated a single population of 10% fetal calf serum-elicited mouse peritoneal exudate cells to express tumoricidal activity but not bactericidal activity for the facultative intracellular bacterium Listeria monocytogenes. Fetal calf serum-elicited cells incubated with rIFN-gamma phagocytosed listeriae normally, suggesting that their inability to kill this bacterium is not because they cannot phagocytose it. Data also show that proteose peptone-elicited peritoneal exudate cells, which are bactericidal but not tumoricidal, acquired tumoricidal activity but lost bactericidal activity following incubation overnight with rIFN-gamma. These experiments show that under conditions sufficient for rIFN-gamma to induce macrophages to express tumoricidal activity, the same cell population does not express bactericidal activity for the facultative intracellular bacterium L. monocytogenes. This suggests that mechanisms responsible for these two biological activities may be different.