ErbB-2, a member of the epidermal growth factor(EGF) receptor tyrosine kinase family, is often overexpressed and/or amplified in breast, ovarian and gastric cancers, and other malignancies. ErbB-2 is a candidate as one of the best target molecules for cancer therapy. Many anti-ErbB-2 monoclonal antibodies(MoAbs) have been developed. An inhibitory humanized MoAb shows clinical responses in some breast cancer patients, both with MoAb alone and in combination with Cisplatinum or other anti-cancer drugs. A mouse-human chimeric anti-ErbB-2 MoAb CH401 was established and characterized in our laboratory. CH401 is able to kill cancer cells overexpressing ErbB-2 both in vitro and in vivo. The analysis of this tumor growth inhibition by CH401 made it clear that the cytotoxicity was induced by apoptosis. These results may suggest that CH401 has a therapeutic potential for ErbB-2 overexpressing cancers. This approach may be particularly valuable as a new type of cancer therapy.