A syngeneic, monoclonal, anti-idiotype antibody, G6D9, was raised against the mouse monoclonal IgM HNK-1. G6D9, characterized as an IgG1-kappa, inhibits the binding of HNK-1 to the myelin-associated glycoprotein (MAG). G6D9 does not interfere with the binding of various human monoclonal anti-MAG IgM to their specific antigen. G6D9 binds HNK-1 hybridoma cells on their surface and within their cytoplasm, as demonstrated using indirect immunofluorescence. In the presence of complement, G6D9 is cytotoxic for HNK-1-secreting cells. A cell lysis of 32% was observed and compared to lysis obtained with other antibodies directed against mouse lymphocyte antigens. The use of G6D9 as a tool to study specific cytotoxicity and immunotherapy in vivo is discussed.