Retinoids elicit many biological and biochemical responses from cells in vitro1-5. One widely used criterion for the responsiveness of cells to retinoids is inhibition of growth; retinoids reduce the saturation density and/or growth rate of many normal and tumorigenic cell lines2,6,7. Propagation of eukaryotic cells has been demonstrated to be dependent on the presence of macromolecular growth factors such as epidermal growth factor (EGF)8-10, which can stimulate proliferation of epithelial and fibroblastic cell lines. We now describe the effect of retinoids on the binding of EGF to its receptor. Retinoic acid enhances binding of 125I- labelled EGF to various fibroblastic and epidermal cell lines. It has no marked effect on the affinity of this growth factor for its receptor, but increases the number of EGF receptor sites. Retinoic acid has little effect on the binding of concanavalin A (Con A) and insulin, indicating the specific nature of the action of retinoids on cell-surface glycoproteins. Treatment of cells with the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) and retinoic acid shows poor antagonism between these compounds on EGF binding. It has been previously shown that retinoids induce or stimulate differentiation of embryonal carcinoma cells11-13. EGF binding can be used as a marker to monitor differentiation of these cells. © 1980 Nature Publishing Group.