A trypsin inhibitor was isolated from beans of Phaseolus vulgaris, cultivar. Kintoki, and the specific activity increased 200 times as high as that of the crude extract. It was homogeneous on several electrophoreses and the molecular weight was about 13, 000. The amino acid composition was characterized by high ratios of cystine, aspartic acid, and serine. It inhibited trypsin in a molar ratio of 1:1 and achymotrypsin in a molar ratio of 2:1. It, however, inhibited neither pepsin nor pronase. It was relatively stable to heat treatment in the acidic medium, but not in the alkaline medium. Neither pepsin nor pronase destroyed the inhibitory function.