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