Several proteins which strongly inhibit trypsin have been found in adzuki bean seeds. Two of them, designated as adzuki proteinase inhibitors (API) I-A and I-A', were analyzed for their amino acid sequences by conventional methods. Inhibitors I-A and I-A' exhibited strong homology with other Bowman-Birk type proteinase inhibitors from leguminous seeds in spite of belonging to different genera. Inhibitors I-A and I-A' consisted of 78 and 72 amino acid residues and their molecular weights were 9,100 and 8,300, respectively. Inhibitor I-A' lacked the six amino acid residues of the amino terminus of inhibitor I-A and had an asparagine residue in place of the aspartic acid residue at position 40 of inhibitor I-A. The results showed the occurrence of some genetic variants of proteinase inhibitors in adzuki bean seeds. Inhibitor I-A was a double-headed one, and the reactive sites for trypsin were Lys-Ser and Arg-Ser bonds. Therefore, inhibitor I-A' was also assumed to be a double-headed one having Lys-Ser and Arg-Ser bonds as the reactive sites for the enzyme.