BSP-30K is a major acidic glycoprotein of bovine seminal plasma. It displays heparin-, gelatin-, and phospholipid-binding activities. BSP-30K binds to spermatozoa upon ejaculation and is thought to play a role in sperm capacitation. We have determined its amino acid sequence, disulfide bonds, and 0-glycosylation sites. BSP-30K consists of 158 amino acids arranged in a mosaic structure. BSP-30K has a unique 48-residue N-terminal extension which includes three 7-8- amino acid repeats and the six O-glycosylated threonine residues. The polypeptide stretch 49-71 is homologous to type 'A' domains found in heparin-binding proteins from other mammalian species. The C-terminal portion of BSP-30K is organized in a tandem of 40-44-residue domains each sharing the consensus pattern of the gelatin-binding fibronectin type II module. The mosaic structure of BSP-30K suggests that this glycoprotein might be a factor contributing to the different sperm-capacitating effects exerted by heparin in different mammalian species.