Oleanolic acid and ursolic acid, extracted and purified from crude drugs, inhibited the insoluble glucan synthesis catalyzed by a crude glucosyltransferase (GTase) preparation from cariogenic Streptococcus mutans OMZ 176. Inhibition of the insoluble glucan synthesis was dependent on the concentration of oleanolic acid or ursolic acid, and was enhanced by pretreatment of crude GTase with these substances. In contrast, synthesis of soluble glucan was not significantly inhibited. These findings suggest that the substances inhibit mainly the synthesis of α-1,3-linkage but not that of α-1,6-linkages. These results indicate that oleanolic acid and ursolic acid are potential inhibitors of dental plaque formation and dental carie