Cultures of rat palatal epithelium grown on collagen rafts were treated with different doses of the potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Sections from biopsies taken 1, 6, 24, and 48 hr after the addition of TPA were examined for the localization of staining by blood group antigen H antibody and antikeratin antibody AE1. In contrast to control cultures, where antigen H was seen exclusively at the cell membranes of the second and third cell layer, several antigen H-positive cells, some appearing in groups, were found in the basal cell layer of TPA-treated specimens. Staining for keratins with the AE1 antikeratin antibody showed no staining of basal cells but only suprabasal cells in controls, whereas several cells of the basal cell layer of TPA-treated cultures stained positively with this antibody. The results support the theory that TPA, by forcing a part of the basal cell population to terminal differentiation, strongly affects the composition of the basal cell population.