Normal human sera were analyzed for the presence and molecular form of two human melanoma-associated antigens (MAAs), the 250 "melanoma-specific" glycoprotein and the 100 K "common tumor antigen". The 250 K MAA was not synthesized by any cultures other than human melanoma and was not detectable in normal human serum. In contrast, the 100 K MAA, which is present in spent medium of cultured human melanoma, carcinoma and fetal melanocytes but not of adult normal cells, was found in normal human serum in nanogram quantities. This serum form of the 100 K MAA was also found in pooled sera of higher apes but not of lower species. The cell-derived form of the 100 K MAA, present in spent culture medium, had a similar phylogenetic distribution. The molecule was produced by cultured brain glia from gorilla, but not by melanoma cells from miniature swine or dog. The 100 K MAA from gorilla glia had a mol. wt identical to the molecule produced by human melanoma cells. Molecular characterization of this MAA in normal human serum showed that it was heterogeneous in size and was present in fractions greater than 100 kd after analytical HPLC gel sieving under non-denaturing conditions. In contrast, MAA from spent culture medium of melanoma cells was 100 kd or less in chemically defined medium (CDM) with no protein supplement, but had a higher mol. wt in CDM with BSA or fetal calf serum supplement, similar to the serum form of the molecule. An association of the 100 K MAA with albumin was demonstrated by analytical HPLC gel sieving and SDS-PAGE analysis of monoclonal antibody immunoprecipitates. The 100 K MAA was dissociated from albumin in normal human serum by treatment with SDS and fractionation by gel sieving. Under these conditions 100 K MAA from serum co-migrated with similarly treated 100 K from melanoma cells. These results indicate that the 100 K MAA is a normal serum constituent which forms a strong, non-covalent association with albumin and is evolutionarily restricted to higher apes or humans.