Seventy-five subjects were examined for the incidence of isolation of mycoplasmas from their oral cavities, identification of mycoplasma strains isolated, the number of viable mycoplasma organisms in oral cavities, and antibody response to M. salivarium and M. orale 1. The subjects were divided into 21 edentulous and 54 dentulous ones. Dentulous subjects were subdivided tentatively into normal, gingivitis, and periodontitis groups on the basis of their periodontal score. Mycoplasmas were isolated from 7 of 21 edentulous subjects, all of whom did not use complete dentures, 18 of 22 subjects of normal group, and all of the subjects in gingivitis and periodontitis groups. The number of viable mycoplasma organisms in oral cavities was significantly greater in gingivitis and periodontitis groups than edentulous and normal groups. There was a tendency for subjects in the periodontitis group to have a greater number of viable mycoplasma organisms than those in the gingivitis group, although there was no significant difference between these two groups. The number of M. salivarium and M. orale 1 strains isolated from edentulous, normal, gingivitis and periodontitis groups was in the ratio 1:1, 1:1, 2:1, and 4:1, respectively. On the other hand, as a result of titration of IHA antibodies to M. salivarium and M. orale 1 antigens in dentulous subjects, the incidence of detecting antibodies to M. salivarium antigen was lower in patients than in healthy persons, while to M. orale 1 antigen increased in the order of normal, gingivitis, and periodontitis groups