Hybridomas were generated by fusing SP2/0 mouse myeloma cells with spleen cells from mice that had been immunized with cultured human melanoma cells. One of the hybridomas secreted a monoclonal IgG1 antibody, 48.7, which binds to a cell surface antigen of cells from human melanomas and compound nevi. The presence of the target antigen in vivo was demonstrated immunohistologically by staining frozen sections of primary and metastatic melanoma by the peroxidase anti-peroxidase technique. Weak staining of some blood vessel cells was also seen, but other normal cells, including skin melanocytes, were unstained, as were cells from other tumor types. Antibody 48.7 immunoprecipitated polypeptides with apparent m.w. on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 250,000 and greater than 400,000.