Skin fibroblasts (SFs) from patients with adenomatosis of the colon and rectum (ACR) were shown, in general, to express elevated amounts of the p53 antigen as determined by immunoprecipitation with the monoclonal antibody PAb421. Infection with simian virus 40 (SV40) induced expression of the p53 antigen in SFs from normal individuals and further amplified its expression in ACR cells. The half-lives of the p53 antigen in mock-infected ACR cells and in SV40-transformed normal or ACR cells were about 2 and 15 hours, respectively. No immediate differences were detected in the coding segment of the human p53 gene between normal and ACR cells by either Southern or Northern blot analysis. These results suggest that over-expression of the p53 antigen is due, in part, to the increased stability of the p53 antigen in ACR cells. Over-expression represents an early event that is possibly associated with initiation and promotion of inherited colon cancer.