During the fall of 1987 one hundred and fifty-five 15 to 16-year-olds were examined with regard to caries prevalence, saliva secretion rate and salivary numbers of mutans streptococci and lactobacilli. In 15% of the subjects no mutans streptococci were detected, while 17.7% carried > 10(6) colony forming units (cfu) per ml saliva. The median value was 1.8 x 10(5) cfu per ml saliva. No lactobacilli were detected in 19% of the subjects and 14.3% had > 10(5) cfu per ml saliva. The mean caries prevalence was 21.0, of these 14.1 were incipient lesions. Increased numbers of mutans streptococci and lactobacilli were associated with increasing caries prevalence. Subjects with high numbers of both microorganisms showed about 4 times higher mean caries prevalence than those with low numbers. Streptococcus sobrinus carriers (15%) had both higher mean caries prevalence and a higher proportion of subjects with > 10(6) cfu mutans streptococci per ml saliva than was found in the whole group.