Cultures of skin fibroblasts (SF) from individuals with hereditary adenomatosis of the colon and rectum (ACR) and from normal individuals were exposed to 12-O-tetradecanoyl phorbol-13-acetate (TPA) (0.5-100 ng/ml), and epidermal growth factor (EGF) (0.01-10.0 ng/ml) at both low and high cell densities.