Preventive measures, which reduced the salivary level of mutans streptococci in mothers, delayed colonization by these organisms in their children. The preventive programme was discontinued when the children were 3 years old. These children, at the age of 7 years, and their mothers were now examined to determine the effect of the earlier caries-preventive approach. The control mothers had significantly higher levels of salivary mutans streptococci and lactobacilli than the test mothers (p < 0.05). The median level of salivary mutans streptococci was 0.6 x 10(6) c.f.u. (colony-forming units) per ml in the test mothers and 1.3 x 10(6) c.f.u. in the control mothers. The median level of salivary lactobacilli was 10 times higher in control than test mothers (80 x 10(3) and 7 x 10(3) c.f.u. per ml, respectively). Significantly more children of control than test mothers carried mutans streptococci (95 versus 46%) (z-test; p < 0.01). Children of test mothers had lower salivary levels of mutants streptococci and lactobacilli (p < 0.05). Twenty three % of the test children were caries-free, compared with 9% of the control children (z-test; p < 0.01). The mean caries experience of the test children was also significantly lower than that of the control children (defstot 5.2 and 8.6, respectively; p < 0.05). The results show that reduction of the mutans streptococci in the mother during the emergence of the primary teeth in her child has a long-term influence on colonization by these bacteria and the caries experience in the child.