A monoclonal antibody (MoAb) against extracellular melanin with tumor targeting activity. Anti-melanin monoclonal antibody PTI-6D2 binds to extracellular melanin, a melanocyte pigment which is released during tumor cell turnover from dead melanoma tumor cells, while avoiding the binding of melanin in normal, healthy tissue because of melanin's normal intracellular location. Upon labeling with the beta-emitting radioisotope rhenium Re 188 (PTI-188), this MoAb may target multiple melanoma (MM) cells, thereby delivering a cytotoxic dose of radiation specifically to the targeted tumor cells.