An antibody mixture composed of six humanized, immunoglobulin G1 (IgG1) monoclonal antibodies directed against three members of the human epidermal growth factor receptor (EGFR; HER) family: EGFR (HER1; ErbB1), HER2 (ErbB2) and HER3 (ErbB3), with potential antineoplastic activity. Upon administration of anti-EGFR/HER2/HER3 monoclonal antibody mixture Sym013, the six antibodies bind to non-overlapping epitopes on EGFR, HER2 and HER3, which prevents both ligand binding and receptor activation, and induce simultaneous down-modulation of EGFR, HER2 and HER3. This inhibits the activation of HER-dependent signaling pathways and HER-dependent tumor cell proliferation. Overexpression of the HER family plays a key role in many cancers; targeting multiple HER family members simultaneously may increase therapeutic efficacy.