First-time mothers who had a high salivary number of Strep. mutans [greater than or equal to 10(6) colony-forming-units (c.f.u.) per ml] were selected. Every second mother was given a special preventive programme to reduce her salivary level below 3 x 10(5) c.f.u. per ml. Where a reduction of Strep. mutans was achieved in the mother, the establishment of Strep. mutans in her infant was prevented or delayed. Thus, 28 mothers were successfully treated until their infants were 23 months old and only 3 of their infants (11 per cent) were infected with Strep. mutans, compared with 17 out of 38 infants in the control group (45 per cent). In both groups, the percentage of infected infants increased with increasing age, although at all ages fewer infants were infected with Strep. mutans in the test group than in the control group. Sixteen infants of successfully treated mothers had reached the age of 36 months. Three were infected (19 per cent) compared with 17 out of 27 in the control group (63 per cent). These findings show that the spread of Strep. mutans can be delayed or prevented by measures directed against the main source of infection, an approach which is successful in the prevention of other infectious diseases.