A melanoma-associated proteoglycan antigen is expressed by primary cutaneous and ocular melanomas, metastatic melanomas, nevus cells, some astrocytomas, and fetal fibroblasts, and it is shed into culture supernatant by both melanoma and nevus cells. The antigen is also expressed by tumor cells in vivo. Melanoma and nevus cells, but not normal melanocytes, were specifically stained by the immunoperoxidase procedure. The proteoglycan antigen, purified by immunoaffinity chromatography using a monoclonal antibody that specifically detects this antigen, was used to immunize rabbits. The resulting serum was tested by sequential immunoprecipitation and found to react with the same population of molecules detected by the anti-proteoglycan monoclonal antibodies. Furthermore, the reactivity patterns of the rabbit serum and of the monoclonal antibodies with a variety of tumor and normal cells were the same. Based on the these data, we conclude that the entire proteoglycan molecule is a melanoma-associated antigen. The monoclonal antibodies and immunoglobulin from the rabbit serum were tested in a double determinant immunoassay for the detection of antigen in a total of 339 sera from patients with various diseases. Elevated levels of circulating proteoglycan antigen were found in 76% of patients with a high metastatic melanoma tumor burden compared to 2% of healthy donors. A fraction (22%) of patients with light tumor burden or nonmelanoma neoplastic disease also had elevated levels of circulating proteoglycan antigen. The source of the antigen for the latter patients may be collagenous connective tissue which, as judged by immunoperoxidase staining, expresses the antigen in both normal and transformed tissues.