Two telomers containing amino end-group are synthesized and attached covalently to peroxidase by using coupling agents. When the enzyme is modified with a telomer having polystyrene as a main chain, it dissolves in organic solvents and exhibits catalytic activity in water-immiscible solvent. The catalytic activity increases as the polarity of the solvents decreases. The enzyme modified with a telomer having poly- N-isopropylacrylamide chain varies its solubility drastically from water-soluble to water insoluble at a certain temperature, being accompanied by change in the catalytic activity. The solubility variation is reversible against temperature change and allows the catalytic activity to be controlled reversibly.