Cell surface melanoma-associated antigens can mediate cell-cell or cell-substrate adhesion, signal transduction, proteolysis, or immune recognition and play a key role in determining invasive and metastatic competence of the tumor cells. The melanoma-associated antigen, A32, was defined by a murine monoclonal antibody and was immunoprecipitated as a single 113 kDa integral membrane glycoprotein containing sialic acid and HNK-1 carbohydrate moieties. Immunohistochemistry revealed the presence of A32 antigen on most melanomas and nevi but not on normal epidermal melanocytes. Of the normal tissues tested, only endothelium, smooth muscle, cerebellum, and hair follicles expressed the A32 antigen. Tryptic peptides of the A32 antigen obtained after immunoaffinity chromatography showed sequence identity to MUC18 antigen, a member of the immunoglobulin supergene family. Melanoma cells adhered to affinity-purified A32 antigen immobilized to a solid phase, and the adhesion was blocked by either soluble A32 antigen or monoclonal antibody against the HNK-1 carbohydrate moiety. These findings, together with the observation that A32 antigen is concentrated in cell-cell contact borders, suggest that this antigen is an adhesion molecule with a possible role in tumor invasion and metastasis.