Bacteria belonging to the Bacteroides fragilis group (B. fragilis, B. ovatus, B. vulgatus, B. distasonis, B. thetaiotaomicron and B. uniformis) were cultured in quantities of less than or equal to 10(5) c.f.u./ml from the excised appendix from 30 patients. Twenty-two patients (group I) had an acute purulent appendicitis (three with perforation), four were classified as slightly inflamed cases (group 2) and four had a normal appendix (group 3). The B. fragilis group of bacteria dominated among the anaerobic isolates, but Bifidobacteria, Fusobacteri, Clostridium perfringens, Lactobacilli, Leptotrichia and Veillonella, in decreasing order, were isolated as well. Among isolates of aerobic bacteria, E. coli was most frequently isolated (26/30 patients). No other genus was isolated from more than four of the appendices. Serological investigations of the humoral antibody response to polysaccharide antigens from four of the Bacteroides species showed that a doubling of the titer, or more, in paired serum samples could be observed as follows: against B. fragilis 6/22 patients (group 1), 3/8 patients (groups 2 and 3). The corresponding figures for B. ovatus were 11/22 and 2/8, B. vulgatus 3/22 and 0/8 and B. distasonis 4/22 and 0/8. Sera from two patients with ruptured appendix, and B. fragilis cultivated, showed the highest titer increases against the B. fragilis antigen. The data from the bacteriological and immunological investigations make us conclude that (i) the B. fragilis group of bacteria is the most common anaerobic group of bacteria isolated from normal and diseased appendices, (ii) the mere isolation of B. fragilis, encapsulated or not, does not imply an etiological role in appendicitis since there was no obvious difference in titer increases against B. fragilis in sera from patients with or without an inflamed appendix, and (iii) high, and significant, titer increases against B. fragilis are seen in sera from patients with ruptured appendix.