Gel filtration of human seminal plasma on Sephadex G-100 revealed four zones displaying the activity of trypsin inhibitors. The inhibitor from the zone corresponding to the molecular mass of 30-40 kDa was obtained in an electrophoretically homogeneous form. The purification procedure included gel filtration on Sephadex G-100, anion-exchange chromatography on DEAE-Sephadex A-50, ammonium sulfate precipitation and hydrophobic chromatography on phenyl-Sepharose 4B. The inhibitor thus obtained has a specific activity of 1.29 IU/mg protein in a caseinolytic test, molecular mass of about 36 kDA and pI of 7.2. The glycosidase and lectin analysis of the carbohydrate component of the protein revealed the presence of neuraminic (sialic) acid and galactose (galactosamine). The amino acid composition of the protein was determined. Antibodies to this protein were obtained; the high specificity of the protein for human sperm was demonstrated The inhibitor was found to be immunochemically identical to previously described prostatic beta-globulin.