Apigenin, a nonmutagenic flavonoid, has been shown to inhibit ultraviolet light–induced skin tumorigenesis when topically applied to mouse skin. Our previous studies have shown that apigenin treatment of cultured mouse keratinocytes induces G2/M arrest accompanied by an increase in p53 protein stability and expression of p21waf1. In this study, we determined whether the G2/M arrest induced by apigenin was dependent upon the presence of the cyclin dependent kinase inhibitor p21waf1. We exposed WWT.8 (p21waf1 wild-type) and WKO.16 (p21waf1 null) mouse keratinocytes to various doses of apigenin for 24 h and observed G2/M arrest in both cell lines, thereby establishing that the apigenin-induced G2/M arrest was p21waf1 independent. A 4-h treatment with apigenin induced increases in p53 protein level by sixfold and tenfold in the WWT.8 p21waf1 wild-type cells and WKO.16 p21waf1 null cells, respectively. After 24 h in WWT.8 cells, p21waf1 protein also was induced in a dose-dependent manner, but it was not expressed in WKO.16 keratinocytes. We then measured the effect of apigenin treatment on the mammalian homologue of the yeast cdc2 gene (p34cdc2) cyclin-dependent kinase and cyclin B1 (cycB1), because these proteins complex to regulate G2/M progression. Apigenin treatment decreased the protein level of p34cdc2, and p34cdc2 kinase activity was inhibited in both p21waf1+/+ and p21waf1−/− cell lines by approximately 40%. The inhibition of p34cdc2 kinase activity by apigenin treatment correlated with increasing levels of p34cdc2 phosphorylation at Tyr15, a site in the p34cdc2 kinase that undergoes inhibitory phosphorylation by Wee1 kinase. Apigenin treatment also had no effect on the protein level or activity of the competing phosphatase, cdc25c, which dephosphorylates p34cdc2 kinase at Tyr15. Apigenin had little effect on the accumulation of cycB1 protein. These results supported the conclusion that G2/M arrest induced by apigenin was accompanied by inhibition of the p34cdc2 cyclin-dependent kinase protein level and activity in a p21waf1-independent manner. © 2002 Wiley-Liss, Inc.