Species of Actinomyces have been associated with endodontic treatment that failed to heal. In this study polymerase chain reaction was used with a pair of universal primers for Actinomyces and species-specific primers to evaluate the contents of infected root canals and aspirates from abscesses or cellulitis for the presence of Actinomyces israelii, A. naeslundii, and A. viscosus. DNA was extracted from 131 clinical samples. DNA from 2 of the original 131 samples was not available for polymerase chain reaction with the universal primer for Actinomyces and A. naeslundii. DNA reacting with the universal primer for Actinomyces was detected in 72 of 129 (55.8%) clinical samples. Of those 41 of 51 (80.4%) were from infected root canals, 22 of 48 (45.8%) were from abscesses, and 9 of 30 (30%) were associated with cellulitis. A. viscosus was detected in 42 of 131 (32.1%) clinical samples. Of those 31 of 52 (59.6%) were from infected root canals, 6 of 43 (14%) were from abscesses, and 5 of 36 (13.9%) were associated with cellulitis. A. israelii was detected in 31 of 131 (23.7%) clinical samples. Of those 14 of 52 (26.9%) were from infected root canals, 11 of 43 (25.6%) were from abscesses, and 6 of 36 (16.7%) were associated with cellulitis. A. naeslundii was detected in 11 of 131 (8.5%) clinical samples. Of those 7 of 51 (13.7%) were from infected root canals, 2 of 48 (4.2%) were from abscesses, and 2 of 30 (6.7%) were associated with cellulitis.