The role of Bacteroides fragilis in the pathogenesis of acute appendicitis was studied in 135 patients in four patient groups: normal (17); phlegmonous appendicitis (17); gangrenous appendicitis (75); and septic complications of appendicitis (26). Aerobic and anaerobic bacteria were isolated from all groups and members of the 'B. fragilis group' were the most common anaerobic isolates. The rate of isolation of B. fragilis was similar from normal and inflamed appendices but was significantly higher from those with septic complications (p less than 0.01). Antibodies against B. fragilis were demonstrated in patients of all groups and occurred with similar frequencies in patients with normal and inflamed appendices but at a significantly higher rate in those with septic complications (p less than 0.01). Whereas patients in this latter group showed IgM-antibody responses to B. fragilis only, those with acute appendicitis had IgM antibodies against a wide range of organisms of the 'B. fragilis group' which suggests that B. fragilis does not play a significant role in acute appendicitis but may be a major cause of its septic complications.