Acquired resistance to the facultative intracellular bacterium Listeria monocytogenes is thought to require immunologically activated macrophages. Using peritoneal exudate cells from nonimmunized mice in a suspension bactericidal assay, however, we found that peritoneal neutrophils obtained early during the inflammatory process (4 hr after elicitation) and macrophages obtained later during inflammation (maximal listericidal activity at 48 hr after elicitation) were able to kill Listeria in vitro. The kinetics of expression of bactericidal activity by inflammatory neutrophils and macrophages against both L monocytogenes and E coli were similar. Although intraperitoneal immunization or intravenous hyperimmunization markedly enhanced resistance of mice to Listeria in vivo, immunization did not increase the ability of inflammatory peritoneal phagocytes to kill Listeria in vitro. However, in response to intraperitoneal injection of proteose-peptone or dead Listeria, immunized mice mobilized more neutrophils and monocytes into the inflamed peritoneum. These data suggest that, rather than systemic activation of mononuclear phagocyte bactericidal activity, increased mobilization of neutrophils and mononuclear phagocytes into sites of infection may be of prime importance in resistance to listeriosis.