An anti-human epidermal growth factor receptor 2 (HER2)/anti-CD3 bispecific monoclonal antibody with potential immunostimulatory and antineoplastic activities. Anti-HER2/Anti-CD3 bispecific monoclonal antibody GBR 1302 possesses two antigen recognition sites, one for HER2, a tyrosine kinase receptor overexpressed by many cancer cell types, and one for the CD3 complex, a group of T-cell surface glycoproteins that interact with the T-cell receptor (TCR). Upon administration of GBR 1302, this bispecific monoclonal antibody simultaneously binds to both CD3-expressing T-cells and HER2-expressing cancer cells, thereby crosslinking HER2-expressing tumor cells and cytotoxic T-lymphocytes (CTLs). This may result in potent CTL-mediated lysis of HER2-expressing tumor cells. HER2 plays a key role in tumor cell proliferation and tumor vascularization.