Chymotrypsin was immobilized with interpolymer complexes formed by the cationic polymer poly(allyltrimethyl ammonium chloride-co-acrylamide) [P(TM-co-AAm)] and poly(acrylic acid) (PAA). The introduction of a small amount of cationic groups led to a much stronger polymer-polymer interaction between P(TM-co-AAm) and PAA. The characteristic pH sensitivity of this kind of complex provided the possibilities of controlling the activity of the immobilized enzyme and separating the immobilized enzyme from the batch by changing the pH of the medium. Compared with the free enzyme, the immobilized chymotrypsin had higher thermal stability, acid-base stability, and stability in use.