Proteinase inhibitor II' from adzuki beans was subjected to peptic digestion. One of the resulting fragments, which inhibited chymotrypsin but not trypsin, was composed of 27 amino acid residues. The fragment was confirmed to be derived from the chymotrypsin-inhibitory domain of the original inhibitor. Another fragment, which inhibited trypsin only, contained 38 amino acid residues and consisted of two peptide chains. One of them, consisting of 25 amino acid residues, corresponded to the original reactive site region for trypsin. These fragments were also obtained from inhibitor II by peptic digestion. These findings, confirm that these inhibitors, which do not inhibit chymotrypsin and trypsin simultaneously, have separate and independent domains for the inhibition of each enzyme. The active fragments are homologous in chemical structures with the two fragments from soybean Bowman-Birk proteinase inhibitor. However, unlike the fragments from Bowman-Birk inhibitor, our chymotrypsin-inhibitory fragment was a potent inhibitor of the enzyme and was as resistant as the intact inhibitor to the attack of excess chymotrypsin. The trypsin-inhibitory fragment had a lower anti-tryptic action than the original inhibitor and was gradually inactivated by trypsin. These differences between our fragments and those of the Bowman-Birk inhibitor are probably a result of the replacement of a few amino acid residues in the reactive site regions.