The level of systemic antibodies to B. gingivalis was assessed longitudinally in a group of 11 adult patients with chronic periodontitis and in 9 periodontally healthy subjects. Immunoglobulin G (IgG), IgA and IgM-specific antibodies were measured by an enzyme-linked immunosorbent assay in serum samples obtained during the natural course of the disease and following scaling and root planing. A serological dichotomy was observed throughout the study in the chronic periodontitis patients. A first subgroup was characterized by a virtual absence of IgA antibody and by IgG and IgM levels similar to those of healthy individuals, suggesting a low level of colonization by B. gingivalis. The patients of the second subgroup had detectable IgA and a significantly higher IgG antibody level that probably reflected the presence of a B. gingivalis infection. There was no statistical difference between the two subgroups for Plaque Index, Gingival Index, probeable pocket depth and age. No cyclic pattern of the antibody levels was observed over the monitoring interval. No peak level of antibody was observed in the immediate posttreatment period, suggesting that scaling and root planing may not provoke active immunization with B. gingivalis. Antibody titres reduced by half 1 year following scaling and root planing were observed in patients presumably infected with B. gingivalis, suggesting that the procedure effectively reduced the immune challenge.