gamma-Seminoprotein (gamma-SM), a glycoprotein from human seminal plasma, was isolated in highly purified form by ion-exchange chromatography on a Mono Q column. The main form, fraction M, was homogeneous by PAGE at pH 8.3 and by SDS-PAGE. The complete amino acid sequence of gamma-SM was determined with the aid of fragments generated by cleavages with cyanogen bromide, clostripain, chymotrypsin and Staphylococcus aureus V8 protease. The fragments were aligned with overlapping sequences. The single polypeptide chain of gamma-SM contains 237 amino acids with a calculated Mr of 26079. A single N-linked carbohydrate side chain is attached to Asn45. The complex structure of this oligosaccharide has been determined recently [van Halbeek H. et al. (1985) Biochem. Biophys. Res. Commun. 131, 507-514]. Sequence comparison with serine proteases shows a high degree of homology, especially with the kallikrein family. The residues in the vicinity of the active site of serine proteases are also highly conserved in gamma-SM, indicating the participation of His41, Asp96 and Ser189 in its active site. gamma-SM hydrolyzed M-casein with a pH optimum at 8.0, but failed to hydrolyze any of the synthetic substrates tested. This proteolytic activity could be inhibited with diisopropylfluorophosphate, phenylmethylsulfonyl fluoride, Zn2+ or Hg2+ ions.