The down-regulation and internalization of the epidermal growth factor (EGF) receptors induced by two separate anti-EGF monoclonal antibodies (mAbs), IgG1 mAb-225 and -455, and by EGF was examined. mAb-225 competitively inhibits EGF binding and it is internalized to an extent comparable to EGF. The antibody down-regulates surface EGF receptors in a dose-dependent manner. In contrast, mAb-455 does not competitively inhibit the binding of EGF or mAb-225, but it specifically immunoprecipitates the EGF receptor. mAb-455 also down-regulates the EGF receptor. Unlike EGF, which elicits phosphorylation of the receptor at tyrosine, threonine, and serine residues, neither of these antibodies elicits phosphorylation of the EGF receptor in intact A431 cells or in KB cells. Our studies suggest that EGF-stimulated phosphorylation of the receptor is not required for the internalization of the ligand-receptor complex.