In the course of long-term infection with Porphyromonas gingivalis in adult periodontitis, a specific antibody response to this organism is generated. We describe a potential novel approach for identifying an immunodominant antigen in human periodontitis patients. First, various monoclonal antibodies (MAbs) were established from mice immunized with crude antigen preparations of P. gingivalis FDC 381. The antigen specificities of these MAbs were compared with those of serum antibodies of 10 periodontitis patients in a competitive enzyme-linked immunosorbent assay. The binding of one MAb (termed PF18) was readily inhibited by sera from all patients but not by sera from healthy volunteers. The antigen recognized by PF18 existed on the cell surface, presumably in the capsule layer, shown by immunoelectron microscopic analysis. Purification of the antigenic substance, termed PF18-Ag, was performed by immunoaffinity chromatography with the MAb. Characterization of PF18-Ag suggested that the epitope was composed of carbohydrates but not peptides and that the substance was different from lipopolysaccharide. Measurement of levels of serum antibody to PF18-Ag better discriminated periodontitis patients from healthy individuals than measurement of antibodies to crude antigen preparations of P. gingivalis. Immunoglobulin G2 was the predominant isotype among the antibodies to PF18-Ag in the patients' sera. These results suggest that PF18-Ag, which is possibly a novel substance, is an important antigenic substance and is potentially useful for the clinical diagnosis of adult periodontitis. The approach that was used would also be relevant to detecting immunodominant antigens of other infectious microorganisms.