The nuclear tumour antigen p53 is expressed by a gene localized on the p-arm of human chromosome 17, a region frequently deleted in colon carcinomas. Using a monoclonal antibody to p53 antigen, immunohistochemical analysis of carcinomas and dysplastic tubular adenomas of the colon has been performed to study the relation between p53 expression and dysplasia or malignancy. With this methods p53 was detectable in 55 per cent of colon carcinomas (n = 29). In 8 per cent of adenomas (n = 74), focal nuclear p53 expression was found in dysplastic epithelial cells. In general, these p53-positive regions of the polyps were histologically indistinguishable from the neighbouring tubuli. Sometimes the p53-positive nuclei were found in a focus of more highly dysplastic epithelium. The results suggest that expression of the p53 gene may be part of the process of malignant transformation of dysplastic colon polyps.