A mouse monoclonal antibody (2137 mAb), which specifically bound to the alpha2 domain of HLA class I, rapidly induces cell aggregation accompanied by weak cytotoxicity against ARH-77 cells, suggesting that 2137 mAb had a potential for agonist antibody. In order to enhance this cytotoxicity, 2137 mAb was engineered to be a small bivalent antibody fragment, 2D7 diabody. The resultant 2137 diabody showed a strong cytotoxicity against ARH-77 cells. As a notable characteristic feature, the lethal effect of 2137 diabody was quite rapid, mediated by a caspase-independent death pathway. Furthermore, 2137 diabody also showed cytotoxicity against several leukemia and lymphoma cell lines, and mitogen-activated peripheral blood mononuclear cells (PBMC), but not for normal resting PBMC and adherent cell lines such as HUVEC. These results suggest that 2D7 diabody could be expected as a novel therapeutic antibody for hematological malignancies as well as inflammatory diseases. (C) 2004 Elsevier Inc. All rights reserved.