The aim of the present study was to examine the Actinobacillus actinomycetemcomitans carrier rate in Chinese subjects, and to determine serotype distribution, presence of the leukotoxin gene lktA and the structure of the lktA-promoter region. Subgingival microbiological samples were obtained from 31 Chinese subjects with moderate to advanced adult periodontitis, 73 young factory workers, and 81 adult residents of a rural area. Bacterial isolates phenotypically identified as A. actinomycetemcomitans were found in 116 of the 185 subjects (detection frequency over-all: 63%). Presence of the leukotoxin gene lktA was demonstrated for all 115 isolates that could be subcultured. The PCR analysis of the lktA-promoter region showed that none of these strains had the deletion in the promoter region known to enhance expression of lktA. No significant difference in the frequency of A. actinomycetemcomitans could be observed between the subjects of the 3 study groups. Analysis by logistic multiple regression indicated a homogeneous distribution of A. actinomycetemcomitans in the 3 cohorts and a lack of significant influence of subject gender or age. Serotype a was found in 21 subjects, serotype b was found in 9, serotype c in 67 and serotype e in 11 individuals. Serotype d was not detected in any subject. Nontypeable isolates, lacking serotype a, b, c, d, or e antigens, were found in 9 individuals. A high prevalence irrespective of gender, age, and cohort suggests that A. actinomycetemcomitans is a common constituent of the normal flora in the Chinese subjects of this study and suggests differences in the microbiological composition of subgingival plaque may exist for this population group as compared to north American and European populations.