The tumor supressor protein, p53, is proposed to have a critical role In maintaining the integrity of the genetic material. It has been established that p53 induces a cell cycle block in the G1 phase upon cellular DNA damage. Recent evidence also indicates the involvement of p53 directly and Indirectly in nucleotide excision repair (NER). We have examined the role of p53 with respect to UV- induced mutagenesis. By gene transfer, we established a mouse fibroblast cell line overexpressing the val135 temper ature-sensitive p53 allele. In this line, p53 activity can be modulated through temperature shift, as confirmed by Western blot and by cell cycle analysis. This cell line was also constructed to contain a recoverable lambda phage shuttle vector carrying the supF mutation reporter gene. Induction of p53 was found to enhance the donogenic survival of the cells following UV-irradiation compared to the p53-deficient parental mouse cell line. The transfectant line also displayed a 4-fold reduction in the frequency of UV-induced mutations as measured in the chromosomally integrated supF reporter gene. Our results are consistent with a p53-induced cell cycle block at G1 allowing cells to repair chromosomal damage before DNA replication. However, our data may also reflect a more direct role of p53 in the repair of UV-induced lesions as suggested by studies showing that p53 can interact directly with repair factors.