An immunoconjugate containing a glycoengineered, humanized monoclonal antibody directed against the human epidermal growth factor receptor HER3 (ErbB3), with potential antineoplastic activity. Upon administration, lumretuzumab binds to the extracellular domain of HER3 and inhibits HER3 dimerization; thereby, preventing EGFR-dependent signaling. In addition, RO5479599 stimulates the immune system to exert antibody-dependent cellular cytotoxicity (ADCC). This may decrease proliferation of HER3-overexpressing tumor cells. HER3, a member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases, is frequently overexpressed in tumors; it has no active kinase domain but is activated through heterodimerization with other members of the EGFR receptor family, such as HER2.