Electron microscopic studies of laboratory-passaged and animal-passaged isolates of Bacteroides fragilis showed significantly more capsular material around the latter. This observation correlated with increased survival of animal-passaged bacteria in rabbit intraperitoneal chambers and increased resistance to phagocytosis and opsonophagocytic killing by neutrophils. With an initial inoculum of 2.5 x 10(6) colony-forming units/ml, the number of bacteria surviving after incubation for 2 hr with neutrophils and pooled normal human serum was significantly (P less than 0.01) greater for animal-passaged than for laboratory-passaged bacteria. Neutrophil uptake of 14C-labeled animal-passaged and laboratory-passaged bacteria after incubation for 20 min was 45% and 63%, respectively (P less than 0.05). No significant difference in survival was found between animal-passaged and laboratory-passaged isolates of Bacteroides thetaiotaomicron. These findings suggest that on mechanism whereby the capsule of B. fragilis contributes to virulence is by inhibiting opsonophagocytic killing.