The serine proteinase inhibitor from barley seeds, chymotrypsin inhibitor 2(CI-2), has been crystallized in a molecular complex with subtilisin Novo (EC 3.4.21.14). The crystal structure of this complex has been determined at 2.1-A resolution by the molecular replacement method and partially refined by restrained-parameter least-squares methods. The present crystallographic R factor (SigmaFo[unk] - [unk]Fc/Sigma[unk]Fo[unk]) is 0.193. CI-2 is a member of the potato inhibitor 1 family; it is a serine proteinase inhibitor lacking disulfide bonds. Comparison of the subtilisin molecule in this complex with the native subtilisin shows that the two molecules are very similar in structure. The inhibitor binds in a mode presumably resembling that of a true substrate, but it is not cleaved. This is in accord with the reported structures of other serine proteinase-inhibitor complexes. CI-2 consists of a four-stranded mixed parallel and antiparallel beta-sheet against which an alpha-helix packs to form a hydrophobic core. A wide loop crossover connection between parallel strands 2 and 3 of the beta-sheet contains the reactive-site bond. The conformation of the four residues to either side of the reactive-site bond is similar to that of the analogous residues in the third domain of the turkey ovomucoid inhibitor (Kazal family); the overall polypeptide chain fold of these inhibitors and the location of the reactive site in the respective chains are different.