Serum antibody titers from patients with periodontitis were compared with those from periodontally healthy subjects. With the micro-enzyme-linked immunosorbent assay, immunoglobulin G (IgG), IgA, and IgM antibody titers to isolates of Streptococcus sanguis, Actinomyces viscosus, Bacteroides gingivalis, Bacteroides melaninogenicus subsp. intermedius, Bacteroides ochraceus, and Fusobacterium nucleatum were determined. Antibody titers of the IgG and IgA classes to B. melaninogenicus, B. ochraceus, F. nucleatum, and S. sanguis were found to be significantly higher in the controls than in the patients. No correlations were found with serum IgM titers. These findings indicate that periodontitis may be associated with depressed antibacterial serum antibody titers of the IgG and IgA classes.