Analysis of the humoral immune response of patients with melanoma has identified a small group of individuals with antibody to cell surface antigens that are restricted to autologous melanoma cells. These antigens, referred to as Class I or unique tumor antigens, are demonstrated by reactions between serum and cultured melanoma cells from the same patient and absorption tests with autologous and allogeneic normal and malignant cells to determine antibody specificity. Five Class 1 melanoma antigens have been defined to date, but insight into the nature of these antigens has been limited because antibodies identifying these antigens lacked detectable immunoprecipitating activity. We have now defined a Class 1 melanoma antigen (designated FD) that is immunoprecipitated by autologous antibody. FD antigen is identified by an IgG antibody present in the sera of patient FD, and peak titers of this antibody in tests with cultured autologous melanoma cells are in the range of 1:2048. By absorption tests, FD antigen could not be detected on any other cell type, including 33 allogeneic melanomas. Prolonged culture of FD melanoma cells resulted in decreased expression of FD antigen, but sublines could be obtained with stable antigen expression. FD antigen is trypsin and heat sensitive, neuraminidase resistant, and is shed in the culture medium. Immunoprecipitation of 125I-labeled cell membrane preparations revealed a 90,000 dalton component of pI 5.5. Serum immunoprecipitating activity could be absorbed by autologous melanoma cells but not by autologous B cells or allogeneic cell lines. A component of the same molecular mass could be precipitated from lysates of cells metabolically labeled with [3H]mannose. The membrane form of the FD antigen binds strongly to Con A-Sepharose and can be eluted with methyl-alpha-D-mannoside. The identification of a precipitating Class I antigenic system of melanoma facilitates efforts to generate monoclonal antibodies to this tumor antigen and to clone its coding sequence.