The human platelet glycoprotein thrombospondin (TSP) binds specifically and with high affinity to sulfatides (galactosylceramide-I3-sulfate). Binding of 125I-TSP to lipids from sheep and human erythrocytes and human platelets resolved on thin layer chromatograms indicates that sulfatides are the only lipids in the membrane which bind TSP. Binding to less than 2 ng of sulfatide could be detected. TSP failed to bind to other purified lipids including cholesterol 3-sulfate, phospholipids, neutral glycolipids, and gangliosides. Binding of 125I-TSP was inhibited by unlabeled TSP, by low pH, and by reduction of intersubunit disulfide bonds with dithiothreitol. A monoclonal antibody against TSP (A2.5), which inhibits hemagglutination and agglutination of fixed activated platelets by TSP, strongly inhibited TSP binding to sulfatides. A second monoclonal antibody (C6.7), which inhibits hemagglutination and aggregation of thrombin-activated live platelets, weakly inhibited sulfatide binding. Binding was inhibited by high ionic strength and by some monosaccharide sulfates including methyl-alpha-D-GlcNAc-3-sulfate. Neutral sugars did not inhibit. Fucoidan, a sulfated fucan, strongly inhibited binding with 50% inhibition at 0.3 micrograms/ml fucoidan. Other sulfated polysaccharides including heparin and dextran sulfates were good inhibitors, whereas hyaluronic acid and keratan sulfate were very weak.