Antimicrobial activities of tea extracts on the cariogenic bacterium, Streptococcus mutans, were studied. Extracts of green and black tea leaves effectively suppressed the growth of S. mutans as compared with those of oolong and Pu-erh tea leaves. Ethyl acetate and n-butanol extracts of both green and black tea leaves and the non-dialyzable fraction of the water-soluble fraction of black tea leaves showed strong antimicrobial activities on S. mutans. Catechins were found in the ethyl acetate and n-butanol extracts of green tea leaves, while catechins, theaflavins, and gallic acid were found in these extracts of black tea leaves. Polyphenols and esterified gallic acid were found in the nondialyzable fraction of black tea leaves.