A Bowman–Birk type trypsin inhibitor I-2, M_r = 14 000, 123 amino-acid residues, isolated from wheat germ, and its complex with trypsin have been crystallized. For I-2 two morphologically different crystal forms were obtained. Crystal form 1 is tetragonal, P4₁22 or P4₃22, with a = 55.45 (2), c = 129.1 (2) Å and V = 3.97 (2) × 10⁵ ų. The crystals diffract X-rays very anisotropically, to less than 6 Å resolution normal to the c* direction, but up to 3 Å resolution in the other directions. Crystal form 2 is monoclinic, space group C2. The cell parameters show significant variation even for crystals in the same batch. The median parameters are: a = 83.9, b = 41.5, c = 45.7 Å, β = 95.9° and V = 1.58 × 10⁵ ų. The diffraction pattern is isotropic and reflections up to 2.2 Å resolution were observed. The crystals of the complex between bovine trypsin and I-2 (2:1) belong to the orthorhombic space group P2₁2₁2₁ with a = 73.49 (2), b = 120.56 (3), c = 70.04 (2) Å and V = 6.206 (5) × 10⁵ ų. The crystals diffract up to 2.3 Å resolution, and contain one complex of 60 100 Da in an asymmetric unit.