The purpose of this study was to investigate the aerobic and anaerobic flora of periapical abscesses and evaluate their susceptibility to various antibiotics. In 52 patients, with a diagnosis of periapical abscesses, pus cultures were taken. Forty-two aerobes and 122 anaerobes were revealed, with 2 or more than 2 anaerobic strains isolated in 36 patients. Infections were purely aerobic in 6%, purely anaerobic in 17% and mixed in 75%, while in 2% of the specimens there was no growth of microorganisms. Among anaerobes, microorganisms from the Bacteroides group (38.5%), Peptostreptococcus spp. (24.6%), Peptococcus spp. (13.9%), and Fusobacterium spp. (4.1%), predominated in all cultures. Among aerobes the most prevalent bacteria were streptococci (47.6%) followed by staphylococci (35.6%) while Enterobacteriaceae were isolated in 4.8% of specimens. Selected susceptibility tests performed on several anaerobic species revealed that nitroimidazole derivatives, chloramphenicol and clindamycin retain their broad spectrum killing activity against anaerobes, followed by cefoxitin and moxalactam. Of the newer quinolones, ofloxacin was the most effective. It is evident that the high isolation rate of anaerobic bacteria should influence empiric therapeutic decisions.