Butyrylcholinesterase purified from human serum as 6600-fold was heated at 37degrees, 40degrees, 45degrees, and 50degreesC for 24 hr. It was observed that the enzyme heated at 45degreesC for 24 hr converted to a stabilized form and followed Michaelis-Menten kinetics, whereas the enzyme samples, heated at the other temperatures for 24 hr, shown negative cooperativity with respect to its substrate, butyrylthiocholine. Even the sample heated at 45degreesC for 12 hr shown negative cooperativity. On the contrary to the heated enzyme at 40degreesC for 24 hr, the heated enzyme at 45degreesC for 24 hr could not be reactivated when it was kept at 4degreesC for 24 hr. In the kinetic studies, it was found that substrate analogs choline and benzoylcholine inhibited both the native enzyme and the enzyme heated at 45degreesC for 24 hr competitively, whereas succinylcholine was the partial competitive inhibitor of native enzyme but the pure competitive inhibitor of the heated enzyme.