Chymotrypsin inhibitor 2 (CI-2), a serine proteinase inhibitor from barley seeds, has been crystallized and its three-dimensional structure determined at 2.0-.ANG. resolution by the molecular replacement method. The structure has been refined by restrained-parameter least-squares methods to a crystallographic R factor (= .sum..dblvert.Fo - Fo.dblvert./.sum. Fo ) of 0.198. CI-2 is a member of the potato inhibitor 1 family. It lacks the characteristic stabilizing disulfide bonds of most other members of serine proteinase inhibitor families. The body of CI-2 shows few conformational changes between the free inhibitor and the previously reported structure of CI-2 in complex with subtilisin Novo [McPhalen, C. A., Svendsen, I., Jonassen, I, and James, M. N. G. (1985) Proc. Natl. Acad. Sci U.S.A. 82, 7242-7246]. However, the reactive site loop has some significant conformational differences between the free inhibitor and its complexed form. The residues in this segment of polypeptide exhibit relatively large thermal motion parameters and some disorder in the uncomplexed form of the inhibitor. The reactive site bond is between Met-591 and Glu-60I in the consecutive sequential numbering of CI-2 (Met-60-Glu-61 according to the alignment of Svendsen et al. [Svendsen, I., Hejgaard, J., and Chavan, J.K. (1984) Carlsberg Res. Commun. 49, 493-502]). The network of hydrogen bonds and electrostatic interactions stabilizing the conformation of the reactive site loop is much less extensive in the free than in the complexed inhibitor.