Serologic and immunochemical assays showed that human melanoma-associated antigens (MAA) identified with operationally specific xenoantisera were neither spatially nor structurally associated with beta 2-microglobulin (beta 2-mu), the light chain of the HLA-A,B antigen molecular complex; i.e., cultured melanoma cells coated with a specific anti-beta 2-mu xenoantiserum maintained their reactivity with anti-MAA xenoantisera. Furthermore, soluble MAA were not bound by a beta 2-mu immunoadsorbent. Finally, MAA were shed into the culture medium of melanoma cells and then were immunoprecipitated with specific anti-MAA xenoantisera, analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, they appeared as two distinct structures with molecular weights of 240,000 and 94,000 but comprised no structure with the characteristic 12,000 molecular weight of beta 2-mu. Conversely immunoprecipitates obtained by the reaction of spent culture medium of [3H]valine-labeled melanoma cells with anti-beta 2-mu xenoantiserum had the 12,000-molecular-weight component but no structures with the molecular weights established for MAA. Thus the data refute the contention that serologically detectable MAA have a molecular structure similar to that of HLA antigens.