Induction of differentiation is today a useful strategy in cancer therapy but the clinical practice is insufficient in squamous cell carcinomas. We examined the effect of vesnarinone, a differentiation-inducing agent, on the cell cycle and cellular differentiation in four cell lines established from oral squamous cell carcinomas possessing a wild-type or mutated p53. Vesnarinone dose-dependently inhibited cell growth and induced G1 phase accumulation regardless of p53 gene mutation. The expression of involucrin and transglutaminase was increased by 4 days treatment with 60 microg/ml vesnarinone in all cell lines. Although p21 promoter activity was suppressed by vesnarinone, p21-mRNA was stabilized by the agent and expression of p21-mRNA was maintained for a long time. Corresponding to the prolonged p21-mRNA expression, p21 protein was induced by cell treatment with 60 microg/ml vesnarinone for 12 h and longer. The induced p21 protein bound cyclin E and suppressed cyclin E/Cdk2 kinase activity suppressing the phosphorylation of retinoblastoma (Rb) protein. These results suggest that vesnarinone possesses activity to induce p21 protein by stabilizing its mRNA with induction of differentiation of squamous cell carcinoma cells in a p53-independent manner.