C4b-binding protein (C4BP) is a soluble, 570 kDa large glycoprotein, present in plasma at a concentration of approximately 200 mg/L. C4BP is the main inhibitor of the classical and lectin pathways of complement, where it controls C4b-mediated reactions. Here, we describe a method for purification of C4BP from human plasma, which is based on barium chloride precipitation, anion exchange chromatography, and gel filtration. We also describe a functional assay, in which C4BP's cofactor activity to factor I, in the degradation of C4b, can be assessed.