The 14F7 monoclonal antibody (MAb) is an IgG(1) antibody that reacts specifically with GM3 (NeuGc) and with tissue sections of human tumors. We demonstrated here that this MAb is agglutinin that specifically agglutinated horse erythrocytes. Additionally, the capacity of 14F7 MAb to mediate cytotoxicity against GM3 (NeuGc)-positive murine myeloma cells, in vitro and in vivo, was evaluated. High concentrations of 14F7 MAb were needed to induce complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC) against the murine myeloma cells. The most relevant finding was the ability of this MAb to directly kill the target cells without participation of complement. This cytotoxicity was dependent on the temperature and MAb concentration and the number of the target cells. In vivo, the passive treatment with 14F7 MAb produced a strong anti-tumor activity, similar to the anti-tumoral response obtained with standard chemotherapy treatment.