Kunitz soybean trypsin inhibitor (STI) was specifically coprecipitated during precipitation of Cu(II)-loaded copolymers induced by increase in temperature and ionic strength. The copolymers used consisted of 1-vinylimidazole and N-vinylcaprolactam or N-isopropylacrylamide. The elution of STI was achieved by solubilization of the STI-Cu(II)-polymer complex in the presence of an excess of the competing ligand, imidazole, and a subsequent precipitation of the polymer with STI remaining free in solution in a purified form as judged by Sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE). To the best of our knowledge this is the first reported successful metal affinity precipitation of protein in a heterobifunctional format.