An immunotoxin of a humanized monoclonal antibody C242 (MoAb HuC242) conjugated with a derivative of cytotoxic agent maytansine, DM4, with potential antitumor activity. huC242-DM4 is generated based on MoAb C242, which is raised against a cell surface superantigen, CA242, found in a variety of human tumor cells. Upon binding and entry, the immunoconjugate releases the maytansinoid agent DM4, which binds to tubulin, thereby affecting microtubule assembly/disassembly dynamics. As a result, this agent prevents cell division and reduces cell growth of cancer cells that express CA242.