Preliminary crystallographic studies of a Bowman-Birk type protease inhibitor, AB-I, from adzuki beans (Phaseolus angularis) 'Takara,' and its complex with trypsin were carried out. AB-I, MW 9100 with 82 amino acid residues, crystallizes in a trigonal space group, P3121 (or P3221), with the following unit cell dimensions: a = 68.7, c = 99.7 A. The asymmetric unit contains two dimer molecules. Structure analysis at 5 A resolution revealed the rough appearance of the dimer. The complex between AB-I and trypsin also could be crystallized in a tetragonal space group, P41212 (or P43212), with the unit cell dimensions, a = 55.4 and c = 181.5 A, and Z = 8. The crystallinity seems to be much better than that of the crystals of the inhibitor alone. The other type of inhibitor from adzuki bean, AB-IIa, was also crystallized.