A sulfatide-binding site on the globular end region of the long arm of laminin has been identified. Following proteolytic digestion with thermolysin, an intact fragment of the laminin A chain carboxyl-terminal domain exhibiting sulfatide-binding activity was isolated using gel filtration and heparin affinity chromatography. This fragment is composed of two peptides that are covalently linked by at least one disulfide bond and encompass the carboxyl-terminal 394 amino acids of the A chain. The clusters of charged residues in the primary structure of these fragments are sufficient for heparin-binding activity but not sulfatide binding since reduction and alkylation of the fragments abolished sulfatide binding under conditions in which heparin binding was retained. Thus, sulfatide binding requires an intact three-dimensional structure. The iodinated fragment bound to A2058 melanoma and T47D breast carcinoma cells and could be displaced by the unlabeled fragment. Based on incorporation of [35S] sulfate, both cell lines synthesize sulfated glycolipids that bind to laminin. In agreement with previous data that indicate a synergistic interaction of the sulfatide-binding domain with other laminin-binding sites on melanoma cells during attachment, the isolated sulfatide-binding fragment significantly inhibited interaction of labeled intact laminin with melanoma and breast carcinoma cells in direct binding assays.