A bispecific monoclonal antibody directed against the human epidermal growth factor receptors ErbB2 (Her2) and ErbB3 (Her3) with potential antineoplastic activity. The anti-ErB2 targeting arm of anti-ErbB2/anti-ErbB3 bispecific monoclonal antibody MM-111 binds to ErbB2 on tumor cells with high affinity while the anti-Erb3 therapeutic arm binds to ErbB3, which may result in the inhibition of cellular proliferation and differentiation in ErbB2-overexpressing tumor cells via inhibition of ErbB3-dependent signal transduction pathways. ErbB2 and ErB3 are members of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases and are frequently overexpressed in solid tumors.