The effects of green tea extract on caries inhibition of hamsters and on acid resistance of human tooth enamel have been investigated. Both in vivo and in vitro experiments showed that original extract of green tea had the significant effects on these points. The dialyzed tea solution in which the fluoride was removed almost completely, also showed the remarkable effects both in vivo and in vitro experiment as is similar to the original tea extract. The results obtained from this study suggested that fluoride in green tea may play a role to increase the cariostatic action in cooperation with other components in tea. However, the action of fluoride does not seem to be so important, because its concentration is very low. The effect of green tea on caries inhibition as well as on the increment of acid resistance appears to be more correlative with the non-dialysable substances in tea.