Covalent immobilization of the pancreatic trypsin inhibitor onto a polymeric carrier was accomplished by introducing a double C = C bond in the inhibitor with a subsequent copolymerization of the activized inhibitor with acrylamide and N,N'-methylenebisacrylamide. To prevent a loss of the antitryptic activity under acylation of lysine residues of the reactive centre, the inhibitor was preliminary bound to a complex with trypsin, which was destructed by acidifying the solution before copolymerization. The antitryptic activity of the immobilized inhibitor was shown to be equal to the activity of the inhibitor in the native state.