Abstract: A method of isolating a protein from a mixture containing the protein, the method including providing an antibody immobilized on a solid support, which antibody is reactive with the protein complexed with a ligand and substantially unreactive with the protein not complexed with the ligand; contacting the mixture, in the presence of the ligand, with the immobilized antibody to bind the protein, complexed with the ligand, to the immobilized antibody to form an immune complex; and contacting the immune complex with a compound having a binding affinity for the ligand higher than the binding affinity of the protein for the ligand, to release the protein from the immobilized antibody.