We previously found that K vitamin analogues caused cell growth inhibition in Hep3B hepatoma cells in vitro, which was associated with their inhibitory effects on protein tyrosine-phosphatases. In this study, we show that Cdc25A, a protein phosphatase, was inactivated by novel arylating K vitamin analogues. The inactivation of Cdc25A correlated with their effects on cell growth inhibition. Cyclin-dependent kinase (Cdk) 4, an important regulator for G(1) progression, was found to be tyrosine-phosphorylated by the arylating analogues, and this phosphorylation was correlated with the inhibitory effects of the analogues on Cdc25A activity. Furthermore, Cdk4 dephosphorylation experiments showed that Compound (Cpd) 5, a prototype arylating analogue, inhibited Cdc25A-mediated Cdk4 dephosphorylation, whereas Cpd 26, a nonarylating vitamin K analogue, had no effect on this event. We also examined Cdk4 kinase activity using retinoblastoma protein as a substrate and found that Cpd 5 inhibited retinoblastoma protein phosphorylation in a concentration-dependent manner, indicating that Cdk4 activity was inhibited by Cpd 5 treatment. Moreover, the thiol-antioxidants glutathione and N-acetyl-L-cysteine antagonized the Cpd 5-induced Cdk4 tyrosine phosphorylation, whereas the nonthiol-antioxidants catalase and superoxide dismutase did not. These results suggest that Hep3B cell growth inhibition by these K vitamin analogues may be related in part to inactivation of Cdc25A activity and support the hypothesis that Cdc25A is an attractive target for drugs designed to inhibit cancer cell growth.