Activation of the classical pathway of the complement system 1s brought about by the bmding of the first component of complement, Cl, to antibodyantigen complexes. Cl is a glycoprotein complex, contaming 3 types of subumts, Clq, Clr and Cls, of which the two latter are se&e protease zymogens [l]. The interaction of Clq with antibody in immune complexes leads to the sequential activation of Clr and Cls by hmlted proteolysls [2]. The proteases Cir and Cis are both of mol. wt 83 000, and have structures sirmlar to that of plasmm [3]. As with other plasma proteases, the actlvitles of Cir and Cis are regulated by protease inhibitors. In plasma there are 7 well-characterised protease inhibitors, each capable of mhlbitmg several serine proteases [7, 8]. AdditIonal plasma serine protease mhbitors have been described [7]. Although Cir and Cis are both known to form stable complexes urlth Cilnhlbitor (Ci In)[4, 5], httle information is available on their activity towards other protease mhibltors. A simple, and widely applicable techniqu_e, involvmg incubation of “‘I-labelled Cir and Cls~ th plasma, followed by gel filtration, affinity chromatography and SDS-polyacrylamide gel analyses has been used to mvestlgate