The inhibitory effect of flavonoids and phenolic acids, which could be transformed from flavonoids by human intestinal microflora, on the growth of Helicobocter pylori (HP) was investigated. Ponciretin, hesperetin, naringenin and diosmetin were active against HP. Among them, ponciretin was the most potent and its MIC was 10-20 µg/ml. However, these active compounds against HP did nearly not inhibit the urease activity of HP.