The tumor suppressor gene p53 is involved in controlling cell cycle checkpoint or triggering apoptosis. p53 may accomplish these roles by acting as a sequence-specific transcription factor. One of the downstream targets of p53 transcription control is the WAF1/CIP1 gene, whose gene product p21 interacts with several cyclins and cyclin-dependent kinases, resulting in inhibition of these kinases. In our previous studies, we have shown that the p53 protein level in mouse keratinocytes was elevated following UV-B/A irradiation. In this paper we further investigated the consequences of increased p53 protein level by characterizing p53 DNA-binding level and WAF1/CIP1 gene expression in UV-B/A-irradiated mouse keratinocytes. Consistent with the increased level of p53 protein, both p53 DNA-binding level and steady-state level of WAF1/CIP1 mRNA were elevated. We have demonstrated that the induction of WAF1/CIP1 mRNA was mediated by p53, since no WAF1/CIP1 induction was observed in p53-deficient cells upon UV-B exposure. These observations suggest an important role for the tumor suppressor gene p53 in the response of keratinocytes to the biologically relevant UV-B/A irradiation and in suppressing UV-induced skin cancer.