Dental plaque has many varieties and properties. Its properties are deeply involved with dental cavities. The dental plaque, detected from children's mouths is usually white, but we also recognize yellow-brown colored plaque in some cases. In this study, we research the differences between yellow-brown colored plaque and white plaque from the clinical and molecular biological view points.<BR>Plaque samples were collected in 30 children who had dental treatment at Pediatric Dentistry of Hiroshima University from erupted teeth, and those sites in the subjects. Differences of the composition among bacterial types, production power of acidity, caries experiences, and the tolerance power of bacteria, in regard to S. mutans and S. sobrinus, which were separated from yellow-brown and white dental plaque, were investigated. In this study, we examined 15 samples of the yellow-brown colored plaque, and the same 15 samples of the white plaque. The results of this study were as follows:<BR>1. Among children with white plaque and yellow-brown colored plaque, it was observed that the children with yellow-brown colored plaque experienced less dental caries than the children with white plaque, and the rate score was significant.<BR>2. In comparison to the productive power of acidity by using cryostat (CAT21), the yellow-brown colored plaque group showed a tendency of low scores (under 1.5, which showed lower risk) and the average score of CAT was also lower against the white plaque group.<BR>3. PCR assay was conducted to identify the bacterial species which were included in each plaque. In the yellow-brown colored plaque group, the detection rate of cariogenic species (S. mutans, S. sobrinus)was low. Mixed infections of these cariogenic species were not identified. The high detection rate of non-cariogenic species (S. sanguinis, S. mitis) was noted. Whereas, in the white plaque group, the detection rate of cariogenic species was high, and both infections were noted. The rate of noncariogenic species was lower in the white plaque group, compared with the yellow-brown colored plaque group.<BR>4. The survival assay of the two species (S. mutans and S. sobrinus) to examine the acidity tolerance power of those clinical isolates showed that the tolerance score of the white plaque group was slightly higher than that of the yellow-brown colored plaque group.<BR>The above suggests that the yellow-brown plaque has a lower risk than the white plaque, in the light of the cariology, because of the different distribution of the species and inducing power of dental caries.