To determine the role of Bacteroides fragilis in the development of infectious complications following hysterectomy, serum antibody to the capsular polysaccharide was measured with a radioactive antigen-binding assay in 53 women before and several days after surgery. Patients with postoperative abscess formation had a significantly greater mean increase in antibody concentration (4.91 μ/ml) than those without complications (0.62 μg/ml), those with wound infection (1.90 μg/ml), those with pelvic cellulitis (0.63 μ/ml), and those with febrile morbidity (0.95 μg/ml). There were no significant differences among the latter 4 groups. The authors conclude that B fragilis may play an important role in abscess formation following hysterectomy, but is infrequently involved in other infecious complications of this procedure.