We report the amino acid sequence of a 299-residue segment from the alpha chain of the human platelet membrane glycoprotein Ib. This includes the complete sequence of the amino-terminal tryptic fragment of 290 residues comprising the von Willebrand factor-binding domain. Two primary sets of overlapping fragments were obtained by cleavage of the S-carboxymethylated protein at methionyl and lysyl bonds following treatment with cyanogen bromide and Achromobacter protease I, respectively. Additional fragments were obtained by treatment of native glycocalicin with trypsin, Staphylococcus aureus V8 protease, and Serratia marcescens protease. Analysis of all these fragments provided data that allowed determination of the continuous sequence corresponding to approximately half of the alpha-chain polypeptide. This region of glycoprotein Ib is largely hydrophobic and contains only two N-linked and one O-linked carbohydrate chains. A hydrophilic region exists between residues 215 and 299, which contains a cluster of 10 negatively charged residues at 269-287. This area is likely to attract positively charged molecules. The hydrophilic, highly glycosylated (at serine and threonine residues) region corresponding to the previously described "macroglycopeptide" and representing the carboxyl-terminal half of the alpha chain is likely to begin at residue 292. The determined sequence of the alpha chain of glycoprotein Ib contains a region (residues 29-193) with seven repeats, which is indicative of gene duplication and is highly homologous to human leucine-rich alpha 2-glycoprotein. This protein sequence agrees completely with that deduced from the cDNA sequence reported by Lopez et al. [Lopez, J.A., Chung, D.W., Fujikawa, K., Hagen, F.S., Papayannopoulou, T. & Roth, G.J. (1987) Proc. Natl. Acad. Sci. USA 84, 5615-5619].