Acquired resistance to listeriosis is thought to require immunological activation of mononuclear phagocytes to an enhanced microbicidal state. In this study we found that both neutrophils and mononuclear phagocytes from nonimmunized human donors killed Listeria monocytogenes in vitro as well as they killed Salmonella typhimurium and Escherichia coli. Bactericidal activity was detectable using both adherent cell and cell suspension bactericidal assays; however, bactericidal activity was greater when the suspension assay was used. Perhaps more surprising, freshly-obtained monocytes were more bactericidal than were monocytes cultured in vitro for 5-7 days, even though monocytes cultured in vitro acquire many characteristics of mature macrophages. These data suggest that newly emigrated monocytes and neutrophils may be particularly effective cell types in resistance to listeriosis.