A summary of a series of bacteriological studies of endodontic infections is presented in this article. The bacteriology of 62 root canal infections was studied with special attention focused on the occurrence, role and taxonomy of Bacteroides spp. All infections except one were mixed infections dominated usually by anaerobic bacteria. Four to 6 different species were present in most canals. Species of the genus Bacteroides were found more frequently than species of any other genus. Seventy-eight Bacteroides strains were isolated from 45 canals. B. buccae, B. intermedius, B. denticola, B. oris, B. oralis, and B. gingivalis were the most common Bacteroides spp. At the beginning of the treatment 35 of 62 teeth caused acute symptoms. The results indicated that symptoms were a result of the synergistic action of the mixed anaerobic flora. The presence of B. gingivalis, B. endodontalis, and B. buccae was more often related to acute cases than other Bacteroides spp. Black-pigmented Bacteroides and a new Bacteroides-like organism, Mitsuokella dentalis, seemed to increase the probability that acute symptoms would persist one week after the beginning of the treatment. However, the treatment result assessed after 4 weeks and after 1 year was not affected by the composition of the mixed anaerobic flora. Calcium hydroxide was the only canal disinfectant used. Its efficacy was proved by a bacteriological sample at the second appointment in 10 cases. All teeth were asymptomatic at the third appointment. The susceptibility of the isolated Bacteroides strains to penicillin G was also studied. Only 2 B. buccae strains and 2 B. denticola strains were resistant at a concentration of 2.4 micrograms/ml. The patients were randomly divided into 3 groups which received 1) no antibiotics, 2) penicillin V (650 mg x 3) for 7 days, or 3) for 12 weeks. There was no difference between the 3 groups in the healing of the periapical lesion after one year. All patients attended the 1-year control. Fifty cases showed complete healing, partial healing was obtained in 11 cases and in 1 case no healing was observed.