Anti-idiotype monoclonal antibody carrying the internal image of ganglioside GM3. Murine anti-idiotype monoclonal antibodies were generated against a human IgM monoclonal antibody (L612) that recognizes ganglioside GM3 on human melanoma. Hybridomas secreting antibodies that bound specifically to L612 were selected by enzyme-linked immunosorbent assay using L612 and three negative control human IgMs, including monoclonal anti-GM2 and anti-GD2 antibodies, as well as purified serum IgM, as antigen sources. GM3-binding inhibition and cell-binding inhibition assays were used to identify seven anti-idiotype monoclonal antibodies that recognized determinants located within the antigen-combining sites of L612. To determine whether these anti-idiotype monoclonal antibodies possessed the internal image of the original antigen, we immunized syngeneic BALB/c mice with one of the anti-idiotype monoclonal antibodies, 4C10, coupled with keyhole limpet hemocyanin. Sera from the immunized mice reacted strongly with an antigen-positive M12 melanoma cell line and with purified GM3. Because L612 detects and kills melanoma tumor cells in vitro and in vivo in the presence of complement without affecting normal tissues, anti-idiotype monoclonal antibodies carrying the internal image of GM3 may be an effective tool for active specific immunotherapy in patients with melanoma. 