Abstract: C1-inhibitor (C1-Inh), the major regulatory protein of the classical pathway of complement activation, can be purified in a new, simplied three step procedure which includes PEG fractionation, jacalin-agarose chromatography and hydrophobic interaction chromatography on phenyl-Sepharose which takes advantage of the marked hydrophilicity of the inhibitor. This procedure has major advantages over those which have been the most frequently used. This method may be highly adaptable to bulk purification for clinical use or for performing analytical or functional studies on genetically or pathologically altered C1-Inh from clinical specimens.