The inhibition of serine proteinases by both synthetic and natural inhibitors has been widely studied. Eglin c is a small thermostable protein isolated from the leech, Hirudo medicinalis. Eglin c is a potent serine proteinase inhibitor. The three-dimensional structure of native eglin and of its complexes with a number of proteinases are known. We here describe the crystal structure of hydrolysed eglin not bound to a proteinase. The body of the eglin has a conformation remarkably similar to that in the known complexes with proteinases. However, the peptide chain has been cut at the 'scissile' bond between residues 45 and 46, presumed to result from the presence of subtilisin DY in the crystallisation sample. The residues usually making up the inhibiting loop of eglin take up a quite different conformation in the nicked inhibitor leading to stabilising contacts between neighbouring molecules in the crystal. The structure was solved by molecular replacement techniques and refined to a final R-factor of 14.5%.