Flavonoids are distributed in many plants. We studied the antitumor effects of flavonoids isolated from Viticis Fructus, casticin, artemetin, quercetagetin and 5,3'-dihydroxy-6,7,4' -trimethoxyflavanone. Casticin inhibited the growth of KB cells markedly (IC(50)=0.23 microM), compared with the other flavonoids tested (IC(50)=15.3-18.6 microM). In contrast, casticin did not inhibit the proliferation of A431 cells similar to normal cell lines, 3T3 Swiss Albino and TIG-103. Flow cytometric analyses revealed that the exposure of KB cells to casticin led to significant arrest at G2-M. In immunostaining of KB cells, casticin disrupted mitotic spindles. These results suggest that G2-M arrest by casticin may be relevant to its antimitotic activity, although the mechanism of selective growth inhibition has been unknown. Further examinations are required to confirm that casticin is an antitumor drug for specific cancers with low toxicity.