The serpins (SERine Proteinase INhibitors) are a family of proteins with important physiological roles, including but not limited to the inhibition of chymotrypsin-like serine proteinases. The inhibitory mechan- ism involves a large conformational change known as the S-->R (stressed-->relaxed) transition. The largest structural differences occur in a region around the scissile bond called the reactive centre loop: In the native (S) state, the reactive centre is exposed, and is free to interact with proteinases. In inhibitory serpins, in the cleaved (R) state the reactive centre loop forms an additional strand within the beta-sheet. The latent state is an uncleaved state in which the intact reactive centre loop is integrated into the A sheet as in the cleaved form, to give an alternative R state. The serpin structures illustrate detailed control of conformation within a single protein. Serpins are also an unusual family of proteins in which homologues have native states with different folding topologies. Determination of the structures of inhibitory serpins in multiple conformational states permits a detailed analysis of the mechanism of the S-->R transition, and of the way in which a single sequence can form two stabilised states of different topology. Here we compare the conformations of alpha(1)-antitrypsin in native and cleaved states. Many protein conformational changes involve relative motions of large rigid subunits. We determine the rigid subunits of alpha(1)-antitrypsin and analyse the changes in their relative position and orientation. Knowing that the conformational change is initiated by cleavage at the reactive centre, we describe a mechanism of the S-->R transition as a logical sequence of mechanical effects, even though the transition likely proceeds in a concerted manner.