Different strains of Bacteroides fragilis exhibit great differences in sensitivity towards serum from healthy volunteers. In the presence of 10% autologous serum, neutrophilic granulocytes and monocytes (macrophages) caused significant killing of B. fragilis. The measured phagocytic and killing activity of the cells is comparable to their activity against aerobic bacteria (S. aureus). In four patients with chronic granulomatous disease of childhood, phagocytosis was normal but killing of B. fragilis and S. aureus in granulocytes or monocytes (macrophages) was appreciably lowered. This malfunction of the cells was accompanied by a disturbance in oxidative metabolism and inadequate iodination after phagocytosis of B. fragilis. The results suggest that granulocytes and monocytes play an important role in host defense against endogenous infections with anaerobes.