The transferrin receptor binds the major serum iron-transport protein, transferrin, and mediates cellular iron uptake. The receptor is a major immunodominant cell surface glycoprotein of cultured cells and its expression on the cell surface is co-ordinately regulated with cell growth. Recent structural and functional studies of the transferrin receptor are reviewed. The properties of monoclonal antibodies against the transferrin receptor that inhibit transferrin-mediated iron uptake are described. Studies with these antibodies establish that the transferrin receptor plays an important role in cell growth and suggest that monoclonal antibodies that interfere with the function of growth-related receptors may be useful in regulating tumour cell growth.