We fused spleen lymphocytes obtained from mice immunized against a human melanoma cell line and melanoma-mouse hybrid cells with the P3 X 63 Ag8 mouse myeloma in order to produce hybrids secreting antibodies against a human melanoma. Antibodies secreted by individual hybrids were tested for their reaction with a panel of human melanoma, colorectal carcinoma, and normal cells in an indirect radioimmunoassay, and they displayed different specificities and crossreactivities. Some reacted only with melanomas, whereas others crossreacted with normal human or human colorectal carcinoma cells. By analysis of competitive binding of mixtures of monoclonal antibodies, it was possible to delineate different epitopes on melanomas. Hybrids growing in nude mice and producing antimelanoma antibody suppressed growth of melanoma tumors.