A monoclonal antibody directed against the cell surface protein claudin 6 (CLDN6), with potential immunostimulating and antineoplastic activities. Upon administration, the anti-CLDN6 monoclonal antibody ASP1650 binds to CLDN-6 and may stimulate the immune system to exert both an antibody-dependent cellular cytotoxicity (ADCC) and a complement-dependent cytotoxicity (CDC) mediated immune response against CLDN-6-expressing tumor cells. This may inhibit tumor cell growth. CLDN-6, a tight-junction protein and embryonic antigen, is expressed on a variety of tumor cells but is not expressed on normal, healthy adult cells.