C1-inhibitor (serpin G1) is a 105 kDa inhibitor which functions as a major antiinflammatory protein in the body. It has its effects via inhibition of the proteases of the complement system and contact system of coagulation, as well as several direct effects mediated by its unique highly glycosylated N-terminal domain. The serpin controls a number of different proteases very efficiently and for some of these the function is augmented by the cofactor, heparin. Here, we describe the preparation of human plasma and recombinant C1-inhibitor and the basic methods required for their characterization, using the complement enzyme C1s as an example of a target enzyme.