The KM231 mAb recognizing sialyl Lewis(a) (sLe(a)) epitope of glycoprotein or glycolipid expressed on various human cancers was used to prepare bispecific antibody (BSAb) containing anti-CD3 x anti-sLe(a) mAb. The effect of anti-CD3 x anti-sLe(a) BSAb on the induction of cytotoxicity by activated T cells was investigated. The activated CD3+ T cells expressing CD8 or CD4 were induced from human peripheral blood mononuclear cells by culture with recombinant IL-2 plus immobilized anti-CD3 mAb. The activated CD8+ and CD4+ T cells showed marginal cytotoxicity against tumor cells by themselves. However, addition of anti-CD3 x anti-sLe(a) BSAb resulted in a great augmentation of their cytotoxicity against gastrointestinal tumor cells. The BSAb also triggered IL-2 production of CD4+ helper/killer T cells during lysis of tumor cells. Moreover, the BSAb was demonstrated to have a potent in vivo antitumor activity against human colon cancer implanted in nude mice by combination with CD4+ helper/killer cells. These results demonstrated that sLe(a) antigen might be a good target molecule for BSAb-directed adoptive tumor immunotherapy.