This chapter describes the in vitro expression of serpins. All serpins do not inhibit proteinases but those that do show a distinctive unfolding tendency at relatively low denaturant concentration. Upon cleavage in the reactive site loop (RSL), the conformation is dramatically stabilized, which is unusual because proteolysis usually has the opposite effect on most proteins. This transition from an unstable to a stable conformation is almost certainly caused by the formation of a unit of secondary structure known as the A-sheet. Inhibitory serpins are conformationally unstable in the virgin form but stabilized by proteolysis in the RSL. Checks of conformational stability before and after RSL cleavages are, thus, a useful tool in assessing serpin integrity. Many serpins form complexes with proteinases that resist dissociation under conditions that are expected to unravel proteinases. This is a characteristic not shared by other natural active-site-directed proteinase inhibitors. © 1994, ACADEMIC PRESS, INC.