Great importance is placed on the participation of various immune responses in the pathogenesis and processes of periodontal disease. Long term sensitization with oral microflora may cause tissue destruction by an intricate mechanism.<BR>In this report, the chronic sensitization with human oral bacteria in guinea pigs was analyzed and the antibody to oral bacteria was assayed. The presense of antibody to oral bacteria or to the patient's own gingiva extract in sera of subjects with periodontal disease, and the total amount of IgG were also investigated.<BR>Results:<BR>1. The production of antibody to oral bacteria in guinea pigs reached a maximum (titer: 80μg N/ml) three months after sensitization, and thereafter decreased gradually.<BR>2. In unsensitized guinea pigs, there was no production of antibody to oral bacteria.<BR>3. The titer of antibody to oral bacteria in the sera of ten patients with periodontal disease (53μg N/ml) was lower than that in normal persons. However, two exceptional cases showed extraordinarlly high values.<BR>4. The sera concentration of IgG in patients (1, 487mg/dl) was higher than that in normal subjects.<BR>5. There were no autoantibodies to gingiva extract from either patients or normal subjects.<BR>These results suggest that chronic sensitization may not be induced in the course of periodontal disease.