Malignant mesothelioma (MM) is an aggressive malignancy arising from the mesothelial cells. It is generally associated with a history of asbestos exposure and has a very poor prognosis. Due to lack of efficacy of conventional treatments, novel therapeutic strategies are urgently needed to improve outcomes. Recently we showed that CD26 is preferentially expressed on epithelial type of MM cells but not on normal mesothelial cells. We have developed a highly biological active humanized anti-CD26 mAb and this antibody inhibited growth and invasion of MM cells and induced long-term survival of tumor transplanted SCID mice. It is conceivable that CD26 is a new therapeutic target for MM. Phase I/II clinical trial for MM has been already starting in France and we plan to start the clinical trial for MM as soon as possible in Japan.