Quercetin is a flavonoid ubiquitously found in nature. The therapeutic effect of quercetin on human hepatoma cell line (HepG2) was evaluated in this study. Various groups were incubated with different doses of quercetin for 12-, 24-, 48- and 72-h time duration and compared with control groups. Dose- and time-dependent inhibition in HepG2 proliferation was found with quercetin treatment. At 48 h of incubation, 61.78% of the cells were arrested at G(1) phase with 25 microM/l quercetin while 89.62% were arrested at G(1) phase with 50 microM/l quercetin. Furthermore, the results indicate that quercetin increased the content of Cdk inhibitor p21 protein, which was correlated with the elevation in p53 levels during 12 h of incubation. In addition, quercetin also increased the level of Cdk inhibitor p27 protein during 24 h of incubation. From our results it can be concluded that quercetin blocks cell cycle progression at G(1) phase and exerts its growth-inhibitory effect through the increase of Cdk inhibitors p21 and p27 and tumor suppressor p53 in HepG2.