Monoclonal antibody MB3.6 (IgG3) recognizes disialoganglioside GD3, which represents a major surface marker on most human melanoma cells. We demonstrate that this antibody effectively lyses four human melanoma cell lines expressing significant levels of GD3 on their surface by either of two mechanisms: antibody-dependent cellular cytotoxicity (ADCC) or complement-mediated cytotoxicity. However, a melanoma cell line that expresses minimal levels of GD3 on 13% of the cells shows insignificant lysis by MB3.6 by either of these two mechanisms, suggesting that a threshold level of antigen expression may be required for effective in vitro cytolysis. In addition, monoclonal antibody (MAb) MB3.6 effectively inhibits establishment and growth of human melanoma tumors in the nude mouse when injected 24 hr after subcutaneous inoculation of tumor cells. Furthermore, MB3.6 produces specific regression of established melanoma tumors when injected 7 days after the subcutaneous inoculation of tumor cells. In contrast, tumor growth in animals injected with the melanoma cell line expressing minimal levels of GD3 was not affected by MAb MB3.6. These data indicate that once appropriate levels of the GD3 ganglioside are expressed on human melanoma cells, MAb MB3.6 can mediate tumor cell killing in vitro and in vivo and, thus, may prove useful for effective immunotherapy of human malignant melanoma.