Abnormalities of the p53 gene are frequently observed in human tumors, including urinary bladder carcinoma, suggesting that p53 plays an important role in human carcinogenesis. However, its role in rat bladder carcinogenesis is unclear. We investigated p53 gene mutations and expression in rat urinary bladder carcinogenesis in vivo and in vitro. Fifteen urothelial cell lines, including six untransformed (nontumorigenic) ones, six transformed (tumorigenic) in vitro, and three derived from tumors induced in vivo, were examined for p53 expression by immunochemical analysis and for p53 mutations; in addition, 81 rat bladders were analyzed immunohistochemically for p53 expression, and 23 rat bladder tumors were analyzed for p53 mutations. Four cell lines had mutations in the p53 gene. Two of these were missense point mutations, and the other two were splicing mutations. On the other hand, no mutations were found in the bladder tumors induced in rats. By immunoprecipitation with PAb240, which is supposed to be specific for mutant p53, we detected mutations in three of the cell lines; PAb240 did not react with wild-type p53. However, in all cell lines and in growing populations of primary cultured bladder urothelial cells, p53 expression was detected immunohistochemically or by western blotting using PAb240 or PAb 421 monoclonal antibodies. In a high percentage of transitional cell carcinomas, wild-type p53 expression was detected by immunohistochemical analysis with PAb240. These results suggest that p53 gene mutations may not occur frequently in rat bladder carcinogenesis in vivo but may occur in vitro and that p53 overexpression detected immunohistochemically is common and may be related to cell proliferation rather than to the presence of mutations in rat bladder carcinogenesis.