The aim of this study was to determine the optimal concentration of Korean propolis against clinical isolates of mutans streptococci (MS) from Koreans. The antimicrobial activity was evaluated using the minimum inhibitory concentration (MIC) and time-kill curves against mutans streptococci. The $MIC_{90}$ values of propolis for MS were 35 ${mu}g/ml$. Propolis had a bacteriostatic effect on Streptococcus mutans ATCC $25175^T$ and bactericidal effects on Streptococcus sobrinus ATCC $33478^T$ at > $2{ imes}MIC$ (70 ${mu}g/ml$). These results suggest that the propolis can be used in the development of oral hygiene products for the prevention of dental caries.