In a randomized clinical trial, six applications of an I2-NaF or a placebo solution were administered to the mothers' dentition at the time of tooth emergence of her infant, to evaluate its effect on transmission of mutans streptococci (MS) to the infant. MS and other bacterial levels were periodically monitored in 48 mother-infant pairs until the child's third birthday. There were significant reductions in maternal salivary MS (P = 0.04), lactobacilli (P = 0.04), total streptococci (P = 0.002), and total cultivable organisms (P = 0.004) immediately following treatment. In children, 3-yr incidence of MS colonization and the time of acquisition of MS or the caries experience did not differ significantly between the two groups. We conclude that short-term application of I2-NaF to the maternal dentition of predentate infants significantly lowers MS, lactobacilli, total streptococci, and total cultivable bacteria in mothers' saliva, but does not influence the incidence and the time of acquisition of MS or the caries experience in children.