Metal affinity precipitation has been successfully developed as a simple purification process for the proteins that have affinity for the metal ions. The copolymers of vinylimidazole with N -isopropylacrylamide are easily synthesized by radical polymerization. When loaded with Cu(II) and Ni(II) ions, these copolymers are capable of selectively precipitating proteins with natural metal-binding groups or histidine-tagged recombinant proteins.