Elevated serum IGG antibody levels against B. gingivalis have been found in patients with periodontitis. In this study, we determined the antigenic specificity of the serum antibodies directed towards antigens of B. gingivalis. The serum samples collected from 19 control subjects with clinically healthy gingiva, 26 adult periodontitis (AP), and 21 rapidly progressive periodontitis (RPP) patients were analyzed by using SDS-PAGE and Western blots. The sonicated cell extract of B. gingivalis 381 was solubilized in sodium dodecyl sulfate solution by heating at 100 degrees C for 5 minutes. After SDS-PAGE, the proteins were transferred to nitrocellulose membrane, and then were probed with serum samples. The strong reaction observed at the apparent molecular weight of 44 kDa protein suggested that it might be an important component which was specific to the antibody of patients (P less than 0.01). A clear difference in the antibody binding between the serum samples from AP and RPP was also recognized. The antibodies from AP frequently reacted with high molecular weight proteins (82, 57, and 44 kDa) while those from RPP frequently reacted with lower molecular weight proteins (44, 27, 25, and 18 kDa). The results indicate that the antigenic components detected in B. gingivalis are in sufficient amounts and specificities to be immunogenic to the host, particularly in patients with periodontitis.