The plasma protease C1 inhibitor (C1Inh, SERPING1) forms proteolytically inactive stoichiometric covalent complexes with the C1r and C1s proteases (Sim et al. 1979a). This effectively disassembles the C1 complex, releasing inactive C1r:C1Inh and C1s:C1Inh complexes (Arlaud et al. 1979, Sim et al. 1979b, Ziccardi & Cooper 1979). C1Inh also inhibits and controls certain non-antibody-induced as well as spontaneous C1 activation. Thus C1Inh plays an important role in regulating nonspecific complement activation (Ziccardi et al. 1983). C1Inh is also a major physiological inhibitor of kallikrein (Ratnoff et al. 1969), coagulation factors XIa and XIIa (Forbes et al. 1970), and the enzymatically active fragments derived from factor XIIa (factor XIIf) (Schreiber et al. 1973).