The function of the asialo-glycoprotein receptor was studied in hepatocytes from carcinogen-treated rats. The cells were isolated at an early stage of carcinogenesis, when histochemically demonstrable enzyme-altered foci but no neoplastic nodules could be observed. The rate of uptake of a trace concentration of [125I]asialo-orosomucoid in hepatocytes from carcinogen-treated rats was significantly below the rate in normal hepatocytes. In contrast, the rate of degradation of endocytosed protein was not changed. The slower rate of uptake can be ascribed to an approximately 70% lower binding capacity in treated hepatocytes, whereas the association constant of the asialo-glycoprotein receptor is not changed. Hepatocytes from carcinogen-treated rats attached to a substratum of adsorbed asialofetuin at a slower rate than did normal cells, probably because of a lower density of asialofetuin receptors on the cell surface.