Quantities of a 100,000-mol wt human melanoma-associated antigen (MAA), defined by both monoclonal and polyclonal xenoantibodies, were measured in sera of 32 patients with malignant melanoma and 15 carefully documented healthy volunteers by use of an amplified sandwich enzyme-linked immunoabsorbent assay. Detectable levels of MAA were found in all normal adult sera. Five of 10 patients with stage III and 9 of 16 patients with stage IV disease had antigen levels that exceeded. the 95th percentile of serum levels in the normal donors (P less than .001). Elevated levels of the 100K MAA were associated with evidence of residual tumor (P less than .008): Only 3 of 13 patients with no evidence of tumor had abnormal values, whereas 9 of 16 patients with evident tumor had increased serum levels. Levels of the 100K MAA were unrelated to the levels of soluble immune complexes as detected by the fluid-phase C1q binding test. Molecular evaluation of the 100K MAA in serum indicated that the molecule associated with albumin in a strong, noncovalent manner. When the complex was dissociated by treatment with sodium dodecyl sulfate, the 100K MAA in serum comigrated with the molecule from spent culture medium of human melanoma cells. Antigen levels in sera were measured under these dissociating conditions. These studies suggest that measurement of the 100K MAA may be useful in monitoring tumor burden in patients with malignant melanoma.