The sera and polymorphonuclear leukocytes (PMNs) of healthy human subjects were tested against several isolates of Bacteroides fragilis. All sera killed most of the fecal isolates, but were active aganist only one of the clinical isolates. The degree of killing was directly related to the serum concentration but was independent of the bacterial inoculum within the range of bacterial concentrations studied. The serum bactericidal activity was heat-labile. Engulfment and killing of B. fragilis by PMNs were demonstrated consistently. The opsonim involved in phagocytosis was also heat liable. Calculations involving log-transformed data permitted the quantitative study of the separate and combined effects of serum and PMNs on bacteria which were killed by serum alone. Whereas serum alone usually killed 0.5 log of serum-sensitive bacteria, the addition of PMNs was usually associated with one log further killing. The studies reported here demonstrate the presence of heat-labile serum factors in normal human sera which killed B. fragilis directly and which promote its phagocytosis and killing by PMNs. These observations provide a foundation for investigations into host defense mechanisms against anaerobes.