The levels of salivary lactobacilli and yeasts were determined annually in half of the subjects of the xylitol (X) and fluoride (F) groups of a 3-year caries study. These measurements were carried out with Dentocult (DC) and Oricult (OC) dip-slide methods. A high level of lactobacilli prevailed throughout the study in these subjects. Significant changes were registered in some subgroups--that is, improvement in one of the X institutions and worsening in one of the F and one of the X institutions. At base line the counts of OC were worse in the X than in the F group. In the total material some improvement occurred during the course of the study, mainly in the X group. The changes associated with the preventive program should also be considered as related to the improvement in oral hygiene, restorative treatment, and the mere study effect.