Glycocalicin, a major glycoprotein of the platelet glycocalyx, is obtained in soluble forms following platelet homogenization and has been purified to homogeneity. Glycalicin has a molecular weight of 148,000 (+/- 5,000) as determined by gel electrophoresis. It contains 60 grams % carbohydrate (46.6 mol %) comprising galactose, N-acetylgalactosamine, N-acetylglucosamine, and sialic acid as its principal sugars in a ratio of 2:1:1:2, but with a small amount of glucose (2.3 mol %), mannose (1.2 mol %), and fucose (1.9 mol %). The principal amino acids are serine and threonine (4.9 and 7.6 mol %), leucine (6.7 mol %), proline, (6.8 mol %), and aspartic and glutamic acids (4.7 and 5.8 mol %). Tryptic digestion of glycocalicin yielded a macroglycopeptide (Mr = 118,000 +/- 5,000) identical with that previously obtained from intact platelets (Pepper, D.S., and Jamieson, G.A. (1970) Biochemistry 9, 3706-3713) and a peptide of molecular weights 45,000 (+/- 2,000) which contained only 7 mol % carbohydrate. This peptide showed a significant enrichment of serine, glycine, and glutamic acids compared with glycocalicin and together these amino acids comprised over 50 mol % of the peptide. Purified glycocalicin gave a single precipitin line with antiserum prepared in chickens. It showed reactions of partial identity with both the macroglycopeptide and the (non-glyco)peptide obtained by tryptic digestion and these showed lines of partial identity with each other. These results suggest that at least three determinants are present in the intact molecule. Glycocalicin gave precipitin reactions with wheat germ agglutinin and with the lectin of Agaricus bisporus.