How the result of the newly developed caries activity test (Cariostat) is affected by bacteriological factors and whether or not this test is valuable in determining the caries activity of children were investigated and the following results were obtained.1) When five different S. mutans strains (serotypes a, b, c, d and g) were inoculated into this test medium, the pH of each medium fell to the final pH of 4.8 ( +2, Yellow-green) after 48 hours of incubation, but when five strains of lactobacilli were inoculated, different patterns of the decrease in pH were observed. When streptococci other than S. mutans were inoculated, the pH fell to a final pH between 5.2 and 5.5 ( +1, Green).2) When 59,2-2.5-year-olds were used as subjects, a much higher correlation was attained between the number of def (def) and the results of Cariostat (C. A. )(p<0.001) than between def and the ratio of S. mutans to the total viable counts (p<0.05) or between def and the ratio of S. mutans to streptococcus counts (p<0.01).3) When 121,1 ½ year-olds were used as subjects, there was a correlation of p<0.01 between the C. A. and def at that age, and at the ages of 2 and 3. There was a correlation of p<0.05 between the C. A. at age of 1 ½ and the increment of def (Δdef) within 6 months and of p(0.01 between the C. A. at the age of 2 and the Δdef within 12 months.4) When 425,6-15-year-old children were used as subjects, a correlation of p<0.05was obtained between the C. A. and the Caries Severity Index (CSI) except for the 8-year-olds. Furthermore, the change of the average value of the C. A. according to age was similar to that of the mean CSI. Based on the above findings, it was found that Cariostat is valuable as a means in determining the caries activity in children.