A murine IgG2a monoclonal antibody with potential antineoplastic activity. Monoclonal antibody L6 binds to the L6 antigen, a cell surface glycoprotein overexpressed in many carcinomas, and induces antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity against L6-expressing tumor cells. This agent may be conjugated with various toxins in order to target their cytotoxic activity to tumor cells expressing the L6 antigen.