To determine the underlining mechanism of the difference in innate susceptibility of mouse strains to infection by Salmonella typhimurium, the ingestion and in vitro intracellular killing of S. typhimurium by resident peritoneal macrophages of mouse strains that differ in natural resistance to this microorganism has been studied. The results revealed that the rate constants of in vitro phagocytosis (Kph) in the presence of inactivated rabbit immune serum did not differ between macrophages of susceptible C57BL/10 and resistant CBA mice (for both strains: Kph = 0.021 min-1). The rate constant of in vitro intracellular killing (Kk) was determined 1) after in vivo phagocytosis (CBA, Kk = 0.055 min-1; C57BL/10, Kk = 0.031 min-1), 2) after in vitro phagocytosis of preopsonized bacteria (CBA, Kk = 0.020 min-1; C57BL/10, Kk = 0.012 min-1), and 3) during continuous phagocytosis in vitro (CBA, Kk = 0.029 min-1; C57BL/10, Kk = 0.013 min-1). With all three approaches, the initial rate of intracellular killing by normal macrophages of Salmonella-resistant CBA mice amounted to about 1.7 times the value found for macrophages of susceptible C57BL/10 mice (p less than 0.01). This trait difference was independent of the previous way of ingestion of the bacteria, unaffected by the kind of opsonization, and specific for S. typhimurium, because Staphylococcus aureus and Listeria monocytogenes were killed by macrophages of these mouse strains with equal efficiency (p greater than 0.50). These findings indicate that a difference in genetic background expressed in the efficacy of intracellular killing by resident peritoneal macrophages immediately upon ingestion of S. typhimurium is relevant for the innate resistance of mice against S. typhimurium.