alpha 2-Macroglobulin (alpha 2M) was identified in chemically defined spent medium of cultured human melanoma cells. This wide-spectrum protease inhibitor was subsequently shown to be synthesized and was secreted by 10 of 13 cultured melanoma cells but not by neoplastic cells of other histologic origins (0/5 breast, 0/2 colon, 0/2 lung, 0/2 glioblastomas, 0/1 neuroblastoma, 0/4 lymphoid). alpha 2M was also produced by one strain of fetal lung fibroblasts, as was previously shown, but not by two strains of normal fetal uveal melanocytes or two strains of fetal skin fibroblasts. Biochemical characterization of melanoma alpha 2M indicated that it has a similar tetrameric structure but is smaller in size than authentic serum alpha 2M. Preliminary studies indicated that melanoma alpha 2M can bind to protease and has possible functional capability.