1987a, b). More than 40 members of the family have been identified in viruses and plants as well as higher organisms. The serpins have developed by divergent evolution over a period of some 500 million years (Hunt & Dayhoff, 1980), most of the members retaining the presumed function of the original ancestral protein as serine proteinase inhibitors. Some, however, have lostthis function anddeveloped specialized roles as carriers of lipophilic molecules (thyroxine-and cortisolbinding globulins) or as peptide hormone precursors (angiotensinogen) or have no recognized function (ovalbumin). The best studied members are those in human plasma where there is a diversity of inhibitory specialization that illustrates the way in which the serpins have evolved in parallelwith their cognate proteases: antithrombin with thrombin, C]-inhibitor with C)-esterase, antiplasmin with plasmin, and so on. A key plasma serpin is a,-antitrypsin; this is an efficient inhibitor of trypsin, but its prime physiological role is as an inhibitor of the elastase released by leukocytes. Interest focused on arantitrypsin because its common genetic deficiency is as-sociated with the development of premature lung degeneration