Acute dentoalveolar abscess is a common infection of man which originates at the root apex of a tooth with a necrotic pulp and spreads locally into the surrounding facial tissues. Recent studies have shown that these abscesses are usually polymicrobial with anaerobic gram-positive cocci or anaerobic gram-negative bacilli being the predominant species. Although the literature contains qualitative information about the types of microorganisms involved, little quantitative data is available on the overall microbial load and the relative proportions of individual bacterial strains. The present study was performed to characterise and quantify the microbial species present in each of 50 acute dentoalveolar abscesses using aspiration sampling and culture techniques capable of isolating strictly anaerobic bacteria. Not only did the results confirm that acute dentoalveolar abscesses usually contain a mixture of bacterial species (mean number of species per abscess, 3.3) but also revealed that organisms were present in high concentrations (mean bacterial load, 7.9 x 10e6 cfu/ml). Forty per cent of the abscesses studied had a purely anaerobic flora whereas 6 per cent yielded facultative bacteria only. Although a mixture of facultative and strictly anaerobic bacterial species was cultured from 27 abscesses, strict anaerobes predominated in all but 5 of these cases. A wide range of bacteria was isolated but Strep. milleri, Peptococcus species, Peptostreptococcus species, B. oralis, B. gingivalis and B. melaninogenicus were the most frequent. Not only were strict anaerobes isolated more often than facultative bacteria but they were also recovered in higher concentrations. The results of this study confirmed the polymicrobial nature of acute dentoalveolar abscess and provided scientific data to support the current general impresssion that strict anaerobes predominate in these lesions.