The Bowman-Birk trypsin inhibitor (BBI) from wheat germ (I-2b) consists of 123 amino acid residues with two inhibitory loops. The crystal structure of a bovine trypsin-wheat germ trypsin inhibitor (I-2b) complex (2:1) has been determined at 2.3 Å resolution to a final R-factor of 0.177. A distance of 37.2 Å between the contiguous contact loops allows them to bind and inhibit two trypsin molecules simultaneously and independently. Each domain shares the same overall fold with 8 kDa BBIs. The five disulfide bridges in each domain are a subset of seven disulfide bridges in the 8 kDa BBIs. I-2b consists of ten β-strands and the loops connecting these strands but it lacks α-helices. The conformations of the contiguous contact loops of I-2b are in a heart-like structure. The reactive sites in both domains, Arg 17 and Lys 76, are located on the loop connecting anti-parallel β-strands, β1/β2 and β6/β7. Strands β1 and β6 are in direct contact with trypsin molecules and form stable triple stranded β-sheet structures via hydrogen bonds.