We have raised a hybridoma, 4.2, which forms an IgM antibody to an antigen expressed on cultivated cells from most human melanomas, more infrequently on cells from other human tumors, and very weakly on cultivated fibroblasts, T cells and B cells. The antigen can be detected in biopsy material of human melanoma and, in smaller amounts, in fresh samples of normal human lung and kidney. Preliminary biochemical characterization indicates that it is associated with the ganglioside fraction. We feel that the antigen (referred to as 4.2) has a sufficient degree of tumor specificity to be rated as a human melanoma-associated antigen.