We report here on a new amphiphilic homopolymer that binds noncovalently to proteins. Thispolymer not only binds to the target protein chymotrypsin with submicromolar affinity but also stabilizes thenative structure of the protein. Since the polymer-protein binding process is based on electrostaticinteraction, the bound protein can be released from the polymer surface and reactivated either by increasingthe ionic strength or by adding complementary cationic surfactants. The electrostatic binding of polymer tothe protein results in a marked change in the substrate specificity of chymotrypsin.