Internalization of superoxide dismutase-gold complex by isolated liver cells has been shown to occur via receptor-mediated endocytosis. As colloidal gold may act as a carrier in this process, carrier-free human Cu, Zn superoxide dismutase was incubated with cultured rat hepatocytes. Light and electron microscopy immunocytochemistry revealed the binding and internalization of free native human Cu, Zn superoxide dismutase (hSOD) by cultured rat hepatocytes. Immunocytochemical demonstration of binding to the cell surface (hepatocytes were incubated with hSOD for 15 min. at 4 degrees C) and internalization (hepatocytes were incubated with hSOD for 15, 30 and 60 min. at 37 degrees C) of the carrier-free superoxide dismutase was achieved by using a monoclonal antibody selectively reacting with the human protein. The results obtained indicate that carrier-free superoxide dismutase is bound and internalized by rat hepatocytes in primary cultures and that the enzyme can enter the cells via a receptor-mediated endocytosis pathway. We followed the binding and the internalization process of hSOD thus validating the use of the native enzyme in the therapy of free radical-related diseases.