Seventy-seven first-time mothers were selected on the basis of high salivary counts of Strep. mutans [greater than 10(6) c.f.u. (colony forming units) per ml saliva]; 40 mothers were in the control group and 37 in the test group. Their infants were 3-8 months of age at the start of the study. A prophylactic programme for the test mothers, aiming at a reduction of Strep. mutans, was repeated at intervals of 2-4 months as and when necessary until their children were 3 years old. The test mothers as a group showed approx. 10-fold fewer Strep. mutans during the test period. At the age of 3 years, 70 per cent of the children in the control group carried Strep. mutans, compared with 41 per cent in the test group (p less than 0.01). Fifty-two per cent of the children who carried Strep. mutans had caries at this age, compared to 3 per cent of the children without this organism. The time when Strep. mutans was first detected in the children seemed to influence subsequent development of caries because 77 per cent of the children who carried Strep. mutans at the age of 15 months had caries at the age of 3 years. Approximately 40 per cent of the children in both the control and the test group had detectable lactobacilli in their saliva at 3 years. In general, the children in the control group had more lactobacilli.(ABSTRACT TRUNCATED AT 250 WORDS)