Extensive study of the p53 gene has established its role as a tumour-suppressor gene, and the involvement of mutant p53 in a wide spectrum of human malignancy. Many mutations of p53 result in a protein product that is abnormally stable, so that it becomes readily detectable by immunocytochemistry. In contrast, under normal conditions, it has been considered that levels of wild-type p53 were too low to be detectable. Although positive immunocytochemistry has been used as a marker of mutation, recent evidence suggests that this assumption may not always be valid. We have carried out both PCR-sequencing of exons 5-8 of the p53 gene in 20 basal cell carcinomas (BCC), and immunocytochemistry of these tumours with the anti-p53 antibody DO7. Twenty cases of Bowen's disease, in which we had previously documented mutations, were also immunostained. We report a low rate of p53 mutation in the BCCs we examined (2/20), and a discrepancy between tumours with positive immunostaining and those with mutation in both Bowen's disease and BCC. Of eight tumours in which we detected mutation, only four were immunopositive: of 19 immunopositive samples, only four showed detectable mutation. We discuss the implications of our results for the use of positive immunostaining in clinical diagnosis, and the involvement of p53 in skin carcinogenesis.