Swiss 3T3 and C3H-M2 cells have a greater mitogenic response to epidermal growth factor (EGF) than do C3H10T 1/2 cells. The latter cell line, however, has a number of EGF receptors per cell intermediate between the two cell lines that have a more vigorous response to EGF. Scatchard analysis of binding data indicate that all three cell lines have one class of EGF receptor, with indistinguishable affinity for the ligand. When exposed to 10-nM EGF all three cell lines "down-regulate" their EGF receptors with the same time course, and to the same percentage of initial receptors.