The substrate specificity of a novel serine protease isolated from soybean seeds, cultivar Keburi, was investigated using various peptide-MCAs and several neuropeptides involving single and paired basic amino acid sequences. The protease was quite specific for arginine residue at the P1 site of the active center, and it recognized paired Arg-Arg and cleaved at the linkage between Arg-Arg or after Arg-Arg in peptide and protein molecules. This is the first protease in plant tissues which resembles in substrate specificity the arginine-specific serine proteinases from porcine gastric and intestinal mucosa, recognizing paired basic amino acid sequences.