Bacteroides fragilis plays a key role in the pathogenesis of anaerobic infections and is often found mixed with aerobic organisms. We explored the interactions of this organism with phagocytes in an attempt to discern additional information about its virulence factors. We confirm an earlier report that killing of aerobic organisms by polymorphonuclear leukocytes (PMN) is decreased in the presence of high numbers of Bact. fragilis but this effect could also be demonstrated with Bact. distasonis or Staphylococcus aureus. Our data support the concept that this phenomenon may be due to competition for opsonins. Virulence of Bact. fragilis has been associated with a polysaccharide capsule. We were unable to demonstrate any deleterious effect of the purified capsular polysaccharide of Bact. fragilis on phagocytosis, killing, or chemotaxis by PMN. We were not able to demonstrate any effect of subinhibitory levels of clindamycin on the interactions of neutrophils and Bact. fragilis.