Expression of the cellular p53 tumour-suppressor protein was examined in 78 epidermal tumours, including basal and squamous cell carcinomas, keratoacanthomas, solar keratoses, Bowen's disease and viral warts. An immunohistochemical study was employed using the antibody CM-1, raised against recombinant human p53 protein. Positive staining for p53, not detectable in normal cells because wild-type p53 is rapidly degraded, reflects abnormal stabilization of p53 protein, and in many cases suggests p53 gene mutation. p53 immunoreactivity was not observed in normal skin or in viral warts. In contrast, positive staining for CM-1 was seen throughout the tumour in the majority of basal and squamous cell carcinomas and in Bowen's disease. Immunoreactivity to p53 was also observed in the majority of keratoacanthomas and solar keratoses, but was confirmed to areas of dysplastic basal epithelium. This study demonstrates that accumulation of p53 protein, suggestive in many cases of p53 gene mutation and hence loss of tumour-suppressor function, may occur as an important early step in the development of diverse epidermal cancers.