The endogenous human tumor-associated galectin-3 (hL-31) is a functional molecule which acts as a receptor for ligands containing poly-N-acetyllactosamine sequences. However, little is known about its native ligand(s). In order to identify the ligand(s), the human melanoma cell line A375 was metabolically labeled with [3H]glucosamine, and total cell extracts and serum-free conditioned medium of the labeled cells were affinity-purified on immobilized recombinant hL-31 followed by elution with lactose, the specific sugar inhibitor of the lectin. Cellular ligands for hL-31 were found to be composed of the two lysosome-associated membrane proteins, LAMP-1 and LAMP-2, while secreted ligands consisted of two glycoproteins of 98 and 70 kDa. N-terminal protein microsequencing revealed that the 98 kDa and 70 kDa species share the same N-terminal sequence. The functional relevance of these secreted ligands was demonstrated by their ability to inhibit lectin-mediated hemagglutination in a manner similar to the specific sugar inhibitor lactose. Computer-assisted sequence library searches have identified the 98 kDa human melanoma secreted ligand to be the Mac-2-binding protein (Mac-2-BP), also known as the human lung tumor L3 antigen.