A proteolytic enzyme, hydrolyzing N-benzoyl-D,L-arginine-p-nitro-anilide (BAPAase), has been isolated from the buckwheat seeds (Fagopyrum esculentum). The enzyme was purified 400-fold and was homogeneous according to isoelectrofocusing and disc electrophoresis in polyacrylamide gel. The molecular weight of the BAPAase was determined to be 65000 by gel-chromatography and 70000 by polyacrylamide gel electrophoresis. The sedimentation coefficient of the BAPAase was found to be 4.3 S, the isoelectric point--pH 4.5. The enzyme split peptide, esteric and amide bonds formed by carboxyl groups of lysine and arginine in synthetic substrates. The enzyme did not hydrolyse fibrinogen, did not activate chimotrypsinogen, weakly hydrolyzed histones and casein and strongly--protamine. The BAPAase did not hydrolyse albumins and globulins from the buckwheat seeds, and weakly hydrolyzed glutelins. The study of the products of the hydrolysis of salmine and sturine by BAPAase showed that the enzyme split internal peptide bonds in these substrates, and, thus, it is an endopeptidase.