Antibodies in the serum of melanoma patient AU precipitate an antigen from 125I-labelled extracts of cultured autologous melanoma cells. The antigen, which is probably not a cell surface component, is present in other pigmented melanomas but not in non-pigmented melanomas or other tumor cell types, and the amount of antigen is correlated with the degree of pigmentation. These conclusions are based on absorption experiments with 11 pigmented melanomas, 8 non-pigmented melanomas, 3 astrocytomas, 12 carcinomas of various histological types, I leukemia, 2 EB-virus-transformed B lymphocyte lines, and human erythrocytes. The antigen was also detected in cultured human melanocytes. It has a molecular weight of 70,000, an isoelectric point of pH 5.3, and it binds to concanavalin A-Sepharose. Ninety-six sera from other melanoma patients were examined and none of them precipitated this antigen. As described previously, the serum from patient AU also has antibodies to a unique (Class I) tumor antigen found only on AU melanoma cells. The pigmentation-associated, differentiation antigen and the unique antigen are clearly different in their distribution, but some relationship between these unusual antibody responses is possible.