We have estimated by ELISA the levels of IgG subclass antibodies against P. gingivalis strain W83 whole cells in 35 cases of adult periodontitis and 35 age, sex, ethnic origin and plaque index-matched healthy controls. The mean levels of IgG1 (p < 0.0003) and IgG2 (p < 0.0416) were significantly elevated in the cases. Many sera had no detectable IgG4 antibodies but by categorising IgG4 responses into high, low and absent, this subclass was more often present (p < 0.002) in the cases. Analysis of the paired differences between cases and controls showed that only IgG1 (p < 0.0005) and IgG4 (p < 0.003) levels were significantly greater in the cases. The data support the role of P. gingivalis as a key pathogen in adult periodontitis and high levels of IgG4 antibodies might possibly provide a marker of patients with active disease.