AbstractA covalently bound complex of acetylcholinesterase with poly[N‐(2‐hydroxypropyl) methacrylamide] has been obtained, showing a 70‐fold prolongation of the acetylcholinesterase activity survival in mouse blood after intravenous injection of the modified enzyme if compared with the non‐modified acetylcholinesterase. The kinetics of the modified and native acetylcholinesterase thermoinactivation at 50°C and proteolytic inactivation at 37°C have been studied. At pH 7.5 the thermoinactivation rate constant of the modified enzyme was 74 times smaller than that of the native acetylcholinesterase. The protective effect of the polymer against the proteolytic inactivation was much less pronounced, being 3 times for chymotrypsin and 13 times for trypsin. The polymer‐modified enzyme initiated antibodies in mice which interacted with the native acetylcholinesterase but the polymer‐modified enzyme itself did not givein vitroimmunoprecipitation neither with the antiserum obtained against it nor against the native acetylc