Mab 202, a mouse monoclonal IgM antibody which recognizes sialic acid alpha 2----3 galactosyl residue in monosialogangliosides and reacts with human melanoma cells but not with normal cells, was administered to a melanoma patient by either intralesional injection or intravenous infusion. Mab 202 induced regressions of the metastatic tumors without side effects. Histopathologic examination showed remarkable degenerative and necrotic changes in the tumor, around which lymphocytes, eosinophils, plasma cells and macrophages infiltrated. Immunoperoxidase staining revealed Mab 202 binding to melanoma cells. Clinical and pathologic evidence suggested that Mab 202 has cytotoxic effects against melanoma cells. Mab 202 may therefore be useful in the treatment of human malignant melanoma.