To examine the role of germinal mutation in transformation by phorbol esters, we studied the induction of anchorage-independent variants of mutant human diploid fibroblasts derived from normal-appearing skin of individuals with hereditary adenomatosis of the colon and rectum (ACR). Liquid cultures were chronically exposed to 12-0-tetradecanoyl phorbol-13-acetate (TPA), then plated in agar and injected subcutaneously into athymic mice. Cultured ACR cells showed an unusual biphasic dose response to TPA. Colony-forming cells in agar were obtained at a frequency of about 5 x 10(-5). They did not, however, seem to increase in frequency during subsequent passages in liquid cultures continuously exposed to TPA. The isolated anchorage-transformed clones showed an altered clonal morphology and a considerable increase in cloning efficiency in liquid cultures and agar. The results suggest that ACR cells may be used to screen for potential tumor promoters in our environment.