The aim of this study was to compare the ability of selected plant extracts to inhibit acid production from starch gels by plaque in vivo and by mutans streptococci in vitro. Commercially available propolis, liquorice, green tea, grape fruit seed, black currant seed and cloudberry seed extracts were used in the study. The extracts were added to a starch gel, which consisted of 8% acid hydrolysed corn starch, 25% maltitol syrup and water (w/w). The pH of the gel was adjusted close to 6,5. For the in vivo acid production tests the subjects accumulated plaque for 2-3 days. After a single intake of 6 g of the test product, it was swished around in the mouth for 2 min. Changes of plaque pH in 2-3 interdental spaces were measured with a microelectrode (Beetrode) for 30 min. In the in vitro tests Streptococcus mutans NCTC 10449 was mixed with the gel suspensions and acid production was followed electrometrically for up to 10 min. Propolis inhibited acid production both in vitro and in vivo at ≥ 5%. Liquorice inhibited acid production at 1% in vitro and at 5.0 % in vivo. Grape fruit seed and green tea extracts prevented acid production completely: grape fruit seed in vitro at 0.5% and green tea in vitro at 0.1% and in vivo at 1.0 %. Berry seed oil extracts (1 %) had no effect on acid production. In conclusion, acid production was inhibited by several plant extracts, which makes them promising components of toothfriendly products. Financial support by the National Technology Agency (Tekes), Finland, is gratefully acknowledged.