The effect of intensive treatment with chlorhexidine on the number of S. mutans in saliva was studied in 24 schoolchildren highly infected with this organism. Chlorhexidine gel in individual dental trays was applied under supervision on two consecutive days, four times the first day and three times the second day. The number of S. mutans in saliva was monitored for 6 months. Immediately after treatment, the number was greatly reduced in all of the children. Thirty days after treatment only three children had more than 2 X 10(5) S. mutans/ml saliva, and after 4 months, 11 children still had less than 2 X 10(5) S. mutans/ml saliva. After 6 months the effect of the treatment was obvious in 20% of the children. The results indicate that intensive treatment with chlorhexidine can be used to reduce the S. mutans infection. The antimicrobial treatment, however, should be individually controlled because of variations in the response of the subjects.