Alpha-1-B glycoprotein (A1BG) which is a plasma protein with no known function belongs to the immunoglobulin superfamily. Although the human A1BG has been known for four decades, and the information about the amino acid sequence, chromosomal assignment, and even genetic polymorphism in different populations have been known, no biological function has been suggested. A1BG (Mr approximately equal to 63,000) consists of one polypeptide chain of 474 amino acids with four glucosamine oligosaccharides. The polypeptide has five intrachain disulfide bonds and consists of five repeating structural domains, each containing about 95 amino acids and one disulfide bond. Recently human cyctein-rich secretory protein 3 (CRISP-3) has been found to be a binding partner of A1B, suggesting the A1BG-CRISP-3 complex displays a protecting function from a potentially harmful effect of free CRISP-3 in circulation.