p53 expression in rat tongue epithelium was investigated immunohistochemically following experimental carcinogenic induction by 4-nitroquinoline 1-oxide (4NQO). p53 assays were performed using PAb240 anti-p53 monoclonal antibody during various stages of experimental carcinogenesis. I found that p53-positive epithelial cells increased in direct proportion to development of the tumor. Significant increases were observed during the periods from control to 4 weeks and from 8 to 12 weeks after 4NQO administration. Although no histological changes were observed in the earlier period, hyperplasia and dysplasia were observed in later stages. Mutant type p53 was detected by immunoprecipitation at 16 and 20 weeks after administration of 4NQO. Results suggested that epithelial p53 detected prior to onset of histological changes was wild-type, while that in the hyperplastic or dysplastic epithelium was mutant.