There has been only a fragmentary knowledge concerning mycoplasmas in the dental plaques and, in particular, no quantitative studies have been made on them. For this reason, isolation and enumeration of the mycoplasmas in the dental plaques have been attempted in this present study. The incidence of mycoplasmas was significantly higher in the dental plaques accumulated on the molars (76.l%, 35/46 specimens) than on the incisors (37.2%, 16/43); on the healthy enamel surface (hereafter called normal plaques; 6 l.0%, 61/100) than on the superficially decayed enamel surface (hereafter called caries plaques; 14.8%, 4/27) and on the healthy cervical enamel surface but in contact with the inflamed gingival marginal area (hereafter called gingivitis plaques; 30.3%, 10/33).
The number of organisms (cfu) in the dental plaques (mg) was g1eater in the normal plaques (range, 0-1.57×106: mean, 6.31×104) than in the caries (0-4.62×102: 2.28×102) and in the gingivitis (0-4.14×104: 6.55×103). There was a significant difference only between the normal and caries plaques. In addition, the number of organisms in the 1-day and 3-day-old dental plaques suggested that the mycoplasmas were not one of the microorganism s which appeared at the early stages of dental plaque formation.
M. salivarium (311 strains) and M. orale (13) were isolated from 67 samples of dental plaques, but U. urealyticum was not. Of the 67 samples, 60 (89.5%), 1(l.5%) and 6 (9.0%) samples were positive for M. salivarium, M. orale and both of these two species, respectively.