Evidence indicating that multiple serotypes of Bacteroides forsythus participate in rapidly progressing periodontal infections has not been reported previously. Our aim was to develop an assay for detecting subsets of B. forsythus clinical isolates which differ in serogroup membership and subsets of patients with immunoglobulin G (IgG) responses which differ in serogroup recognition. A checkerboard enzyme-linked immunosorbent assay (ELISA) was used to assess variation in the IgG binding profiles of 22 clinical isolates in sera from 28 patients with early-onset rapidly progressive periodontitis. To accommodate the maximum number of isolates and sera in a given assay run, a multiplate assay grid with standard 96-well microtest plates was established. Single dilutions of individual sera were placed in rows crossing columns of isolate-coated wells, and antigen-specific IgG immobilized in the wells was measured as ELISA absorbance. Pooled sera and isolates were assayed in parallel to serve as negative controls for variation in IgG binding profiles. Correlation and hierarchical cluster analysis of the absorbance data matrix showed that the isolates could be sorted into at least four clusters based on variations in their IgG binding profiles across different sera. Furthermore, at least two patient clusters were defined by variations in their serum IgG antigen recognition profiles across different isolates. We conclude that multiple serogroups of B. forsythus exist and that different serogroups are dominant in the antibody response of different patients. The method applied here could be used to serologically classify clinical isolates of other species which evoke a serum antibody response in patients.