Growth of the human epidermoid carcinoma cell line A431 in vitro is stimulated by low concentrations of epidermal growth factor (EGF; 0.1-10 pM) and inhibited by high concentrations (0.1-10 nM). This cell also grows as a solid tumor in female athymic mice. Sustained high levels of EGF in vivo can be achieved by the administration of testosterone to female mice via a cholesterol-based pellet inserted subcutaneously. This chronic elevation of EGF levels (serum concentration = 90 ng/ml), however, does not affect growth of the tumor. In contrast, low levels of the growth factor (0.5 micrograms/g body wt by injection 5 times/week; serum concentration = 8.25 ng/ml) stimulate growth of the tumor. These data suggest that the mechanism(s) involved in the inhibition of A431 cell growth by EGF in vitro does not function in vivo and the physiologically significant effect of EGF in vivo is growth promotion.