The present invention provides a therapeutic agent, for diseases in which neoplastic proliferation of plasma cells occurs, which is characterized by containing an anti-human CD48 monoclonal antibody and having cellular cytotoxicity to cells expressing human CD48 and the monoclonal antibody. In addition, the present invention provides a method for screening an active ingredient of a therapeutic agent for diseases in which neoplastic proliferation of plasma cells occurs, the method including the following processes:(1) a process of sorting a substance that binds specifically to human CD48 and(2) a process of sorting a substance having cellular cytotoxicity.
