Aminopeptidase P (APPro) is a metalloprotease whose active site includes a dinuclear manganese(II) cluster. The enzyme cleaves the N-terminal residue from a polypeptide when the second residue is proline. A complex of Escherichia coli APPro (EcAPPro) with an inhibitor, apstatin [N-(2S,3R)-3-amino-2-hydroxy-4-phenyl-butanoyl-L-prolyl-L-prolyl-L-alaninamide], has been crystallized. Apstatin binds to the active site of EcAPPro with its N-terminal amino group coordinated to one of the two Mn(II) atoms at the metal centre. The apstatin hydroxyl group replaces a hydroxide ion which bridges the two metal atoms in the native enzyme. The first proline residue of apstatin lies in a small hydrophobic cleft. The structure of the apstatin-EcAPPro complex has been refined at 2.3 A resolution with residuals R = 0.179 and R(free) = 0.204. The structure of the complex illustrates how apstatin inhibits APPro and suggests how substrates may bind to the enzyme, but the basis of the proline-specificity remains elusive.