Pus samples from twenty-three dentoalveolar abscesses were collected by needle aspiration and examined by direct inoculation technique using 6 different aerobic and anaerobic agar plates. For aerobic culture, sheep blood agar and chocolate agar (Kyokuto Pharmaceutical Co., Tokyo, Japan) were used. For anaerobic culture, four different media, 1) brucella HK agar with hemolyzed rabbit blood and defibrillated sheep blood, 2) paramomycin-vancomycin brucella HK agar with hemolyzed rabbit blood, 3) phenylethyl alcohol brucella HK agar with hemolyzed rabbit blood, 4) bacteroides bile esculin agar (Kyokuto Pharmaceutical Co., Tokyo, Japan) were prepared in an anaerobic jar prior to inoculation. The aerobic agar plates were incubated for 24 h at 37 degrees C, and the anaerobic plates at least 48 h at 37 degrees C in anaerobic jars. From 23 closed odontogenic abscess samples, a total of 112 bacterial strains were isolated; 81 strains (72.3%) were strict anaerobes, and 31 strains (27.7%) were aerobes. The mean number of bacterial strains per positive sample was 4.86. Oral Streptococci, Prevotella, Fusobacterium, Peptostreptococcus and Veillonella were common isolates. The combination of Oral Streptococci and Prevotella was found in 11 patients (47.8%), and that of Prevotella and Peptostreptococcus in 12 patients (52.2%). The present study demonstrated that closed odontogenic abscesses are polymicrobial infections by aerobes and anaerobes. Application of a direct inoculation technique for bacterial culture made it possible to isolate more anaerobes than our commonly used technique using transport medium and to delineate the semiquantitative bacteriology of closed odontogenic abscess.