Five strains of enterobacteria (three of Escherichia coli and two of Proteus mirabilis) were studied to assess and compare their phagocytic uptake and intracellular killing by mouse macrophages. Each strain was injected intraperitoneally into separate groups of mice and peritoneal exudate cels were harvested after 3 min for phagocytosis to occur in vivo. Acridine orange staining showed that there were approximately 10-fold fewer intracellular P. mirabilis than E. coli cells. The average numbers of viable intracellular bacteria per leucocyte were 0.03 and 0.02 for P. mirabilis strains M13 and H1, respectively, and 0.48, 0.45, and 0.28 for E. coli strains M14, A-D M5 and H40. Thus, both P. mirabilis strains were ingested less readily than any of the three E. coli strains (p < 0.01). The rates of in-vitro intracellular killing were similar for all five strains of bacteria. The intracellular killing constants (K(k)) for the three mouse isolates were 0.017, 0.016 and 0.020 min for E. coli M14 and A-D M5, and P. mirabilis M13, respectively; the K(k)s for the two human isolates were 0.026 and 0.029/min for E. coli H40 and P. mirabilis H1, respectively. The K(k)s for all five strains were not significantly different. Assuming that the numbers of viable intracellular bacteria at the beginning of the assay represented 100% viability, 6-17% of the intracellular bacteria remained viable after 2 h, reflecting log10 3.9-5.6 bacteria (6-8) x 106 peritoneal exudate cells. Intravenous injection of these five strains into separate groups of mice demonstrated that the P. mirabilis strains were more virulent than the E. coli strains. Injection of each P. mirabilis strains were more virulent than the E. coli strains. Injection of each P. mirabilis strain was associated with ruffled fur and death, whereas mice given any of the three E. coli strains remained visibly healthy and none died. Consistent with these observations, quantitation of viable bacteria in the liver and spleen showed that greater numbers of P. mirabilis M13 than of E. coli M14 or A-D M5 persisted in these organs; similarly greater numbers of P. mirabilis H1 than of E. coli H40 persisted in the liver and spleen. Because the rates of intracellular killing of these five strains were similar, the relative virulence of both strains of P. mirabilis appeared to be associated with decreased phagocytic uptake rather than difference in intracellular survival.