Innate resistance to infection by Listeria monocytogenes is genetically controlled and is critically dependent on prompt macrophage recruitment to the sites of infection. Experiments reported here were designed to examine whether there was an additional, qualitative difference between the intrinsic bactericidal activity of the inflammatory macrophages of genetically resistant (C57BL/6J) and susceptible (A/J) hosts. To critically evaluate the bactericidal (rather than bacteriostatic) function of the macrophage, a temperature-sensitive (ts) mutant of L. monocytogenes was developed. Mutagenesis was induced with nitrosoguanidine, and the ts mutants were isolated following enrichment with penicillin-gentamicin combinations. The ts mutants were found to carry the cell surface and biochemical characteristics of the original wild-type strain of L. monocytogenes. Inflammatory peritoneal macrophages from resistant C57BL/6J mice were found to have enhanced listericidal activity when compared with inflammatory macrophages from susceptible A/J mice. However, further analysis of the macrophage populations revealed that this seemingly qualitative advantage was due to the relatively greater proportion of inflammatory macrophages present in the inflammatory exudates of resistant C57BL/6J mice. When homogeneous populations of pure inflammatory macrophages were compared, no interstrain differences in their listericidal activity in vitro were seen. These results suggest that the susceptibility of A/J strain mice to L. monocytogenes is not due to an intrinsic deficiency of the listericidal activity of the inflammatory macrophage. The slight increase in bactericidal activity of macrophages from resistant mice that was reported by others (C. J. Czuprynski, B. P. Canono, P. M. Henson, and P. A. Campbell, Immunology 55:511-518, 1985) is caused by the difference in the relative percentage of resident cells present in the peritoneal exudates from resistant and susceptible mice.