Complement Factor H (FH) is an abundant, non-enzymic plasma/serum glycoprotein, which has a major role in regulating activation of the complement system. It can be purified from human plasma/serum by affinity chromatography, using a monoclonal anti-FH antibody as ligand. Other affinity chromatography ligands, including cardiolipin and trinitrophenyl-bovine serum albumin (TNP-BSA), can be used to purify human FH and also FH from a wide range of vertebrates, including mammals, birds, bony fish. Human FH protein concentration can be quantified by sandwich ELISA. The activity of FH is generally measured by assays which detect the cleavage, by complement factor I, of the complement protein C3b to form iC3b. Cleavage occurs only in the presence of a cofactor, and FH is one of a small number of cofactors for this reaction.