Human C4 binding protein (C4bp), which is a macromolecular weight (Mr 450,000-590,000) cofactor of C3b/C4b inactivator (I), is composed of 6 or 8 disulfide-linked polypeptide chains of Mr 75,000. Chymotrypsin cleaved C4bp into two major fragments; a large fragment of Mr 160,000, which contained carbohydrate chains and was composed of disulfide-linked polypeptide chains of Mr 25,000, and a small fragment of Mr 48,000, which was a single polypeptide chain and had the cofactor activity of C4bp. These results suggest that chymotrypsin liberates a functional domain-containing Mr 48,000 fragment from each subunit chain of C4bp and yields a core fragment derived from a disulfide-knot domain connecting each subunit chain of C4bp.