Human epidermal growth factor receptor 2 (HER2) is reported to be overexpressed in breast cancers and is associated with poor clinical outcome. Trastuzumab is a humanized anti-HER2 antibody that offers significant survival benefits to patients with HER2-overexpressing breast cancer. In this study, a novel anti-HER2 monoclonal antibody (mAb), H2Mab-19 (IgG2b, kappa) was developed. Antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and antitumor activity of H2Mab-19 were investigated using both breast cancer and oral cancer cell lines. H2Mab-19 demonstrated cytotoxicity in BT-474 (a human breast cancer cell line) and HSC-2 or SAS (human oral cancer cell lines). H2Mab-19 also possessed both ADCC and CDC activity against BT-474, HSC-2, and SAS cell lines. In comparison to control mouse IgG, H2Mab-19 significantly reduced tumor development in BT-474, HSC-2, and SAS xenografts. Collectively, these results suggest that treatment with H2Mab-19 may be a useful therapy for patients with HER2-expressing breast and oral cancers.