13 patients with untreated adult periodontitis (AP) were compared to 8 subjects free of periodontal disease (H) with respect to plaque index (PlI), gingival index (GI), probing depth (PD) and differential counts of subgingival bacterial morphotypes from a pooled sample of 6 surfaces with the greatest probing depth. Serum antibody levels to T. vincentii and T. denticola strains were also determined in these subjects as well as in the sera from 5 subjects with localized juvenile periodontitis (LJP). Subjects with AP had significantly elevated proportions of spirochetes and motile rods and lower proportions of coccoid cells than H subjects. They also exhibited significantly higher PlI and GI scores and greater probing depths. Antibody levels were normalized against a standard serum and expressed as ELISA units (EU). IgA and IgG antibody levels to all tested spirochete strains were significantly elevated in AP subjects as compared to subjects in group H or subjects with LJP. No significant differences in antibody titers were detectable between the H and LJP groups with respect to any of the tested strains. No significant correlation could be demonstrated between serum antibody titers to any of the oral spirochete strains tested and the proportions of oral spirochetes determined microscopically.