Actinobacillus actinomycetemcomitans(Aa), elaborating a multiplicity of virulence factor and tissue-damaging products, is considered an etiological agent in periodontal disease. Serotype b is the most frequently isolated serotype in localized juvenile periodontitis patients, suggesting a particularly high periodontopathic potential for serotype b strains. Interleukin-6(IL-6) plays an important role in the mediation of inflammatory and immune responses as well as in the osteoclastic bone resorption. However, there is little information regarding the effect of the different serotypes of Aa on IL-6 production by human gingival fibroblasts (HGF). Therefore, the purpose of this study was to compare the ability of the three serotypes (a, b, and c) of Aa sonicates to induce the production of IL-6 by HGF. In fibroblast cultures, confluent monolayers of HGF were incubated with sonic extracts of Aa-511 (serotype a), Aa-Y4 (serotype b), and Aa-652 (serotype c) at various concentrations for 48 h at 37 degrees C in 5% CO2 and air. At the end of the culture period, supernatants were collected and analysed for IL-6 content by using EIA and bioassay. In order to compare the effects of non-lipopolysaccharide (LPS) activation of Aa sonicates on IL-6 production by HGF, we added polymyxin B in cultures with Aa sonicates to bind LPS. The results were summarized as follows. (1) All three serotypes of Aa sonicates had similar dose-dependent stimulant effects on IL-6 production by HGF, and the biological activities of IL-6 correlated with their immunoreactivities. (2) The maximum releases of IL-6 by HGF were achieved at concentrations of 10 to 100 micrograms protein/mL of Aa sonicates, and the ability of Aa-Y4 to induce the release of IL-6 was higher than that of Aa-511 and Aa-652 at these concentrations. (3) Polymyxin B (50 micrograms/mL) effectively decreased the amounts of IL-6 produced by stimulation of the HGF with 10 micrograms protein/mL of Aa sonicates. However, the polymyxin B-treated Aa-Y4 sonicate showed a higher ability to induce the release of IL-6 than the other two strains. These results indicate that Aa-Y4 (serotype b) has a higher potency to induce HGF secretion of IL-6; thus contributing to a comparatively stronger efficacy to the destruction of periodontal tissue in periodontitis.