Activating mutations in the kinase domain of the EGF receptor have been reported in non-small cell lung cancer. The majority of tumors expressing these mutants are sensitive to ATP mimetics that inhibit the EGFR tyrosine kinase. The effect of antibodies that bind to the ectodomain of the receptor is less clear. We report herein the effects and mechanisms of action of the antibody cetuximab in lung cancer cells that naturally express receptor mutations and in ErbB-null 32D hematopoietic cells transfected with mutant EGFR. Treatment with cetuximab down-regulated EGFR levels and inhibited cell growth both in vitro and in vivo. This was associated with inhibition of ligand-independent EGFR signaling. These effects were seen in 32D cells arguing the growth inhibitory action was not because of the blockade of autocrine ligand action. Both antibody-induced EGFR down-regulation and inhibition of growth required receptor dimerization as monovalent Fab fragments only eliminated receptor levels or reduced cell proliferation in the presence of antihuman IgG. Finally, cetuximab inhibited growth of H1975 lung cancer cells and xenografts, which expressed L858R/T790M EGFR and were resistant to EGFR tyrosine kinase inhibitors. These data suggest that cetuximab is an effective therapy against mutant EGFR-expressing cancer cells and thus can be considered in combination with other anti-EGFR molecules.