The Serpins are a major family of proteins, most of which are involved in the regulation of proteinase activity. Current data indicate that inhibitor function is dependent on formation of tight, but reversible binary complexes, with carbohydrate being unimportant for this function. The reaction takes place in a reactive site loop common to all Serpins, with the key residue for complex formation being in the P1 reactive site position. However, other contact residues are also involved as shown by the variation in specificity in a number of animal proteins with the same P1 residue. The role of other amino acid residues in Serpins which aid in conferring specificity has not yet been established. However, the availability of methods for obtaining site specific mutations should soon make it possible to determine other contact points required for Serpin function, thus allowing for the design of inhibitors which are singularly targeted with a high reaction rate towards a given proteinase.