The present study deals with the ability of 11 bacterial strains, in various combinations, to induce periapical reactions. The indigenous oral bacteria were originally isolated from an experimental apical periodontitis in monkey. Eight of the strains were complete collection isolated from one root canal. These strains were inoculated together, in equal proportions, into 12 root canals. In addition, 63 canals were inoculated with other combination or separate strains. At the end of the experimental period it was found that in the mixed infections the Bacteroides oralis strain predominated in most root canals. In contrast, this Bacteroides strain was not reisolated in any of nine root canals when inoculated in a pure culture. Enterococci, however, survived as pure cultures in all canals. The mixed infections showed the greatest capacity of inducing apical periodontitis, as revealed by radiography, and most pronounced was the "eight-strain collection". The facultatively anaerobic streptococci induced only weak periapical reactions.