E. coli L-asparaginase was modified with poly(N-vinylpyrrolidone-co-maleic anhydride). The modified enzyme exhibited the decrease of immunoreactivity towards anti-asparaginase serum with increase of modification degree. The immunoreactivity could be completely restrained although activity of the modified asparaginase was retained by 8.3% of the native one at the highest modification degree. The modified asparaginase also showed much greater stability against protease. After one hour incubation in presence of trypsin, the modified enzyme retained about 80% of its original activity while the native enzyme retained only 14%. As a result of modification the pH optimum of enzyme activity shifted by about 0.8 pH unit in the alkaline direction. The apparent optimum temperature of the modified enzyme was about 10 degrees C higher than that of native enzyme.