Abstract.Human arylesterase is localized in liver microsomes where the presence of different electrophoretic bands corresponding to the serum bands can be recognized.Serum arylesterase is mainly a result of liver activity and its high level might be explained by a low rate of elimination in urine. The behaviour of arylesterase towards inhibitors shows certain similarities to that of some of the proteases, such as trypsin.The clinical value of serum arylesterase determination in assessing liver function is confirmed by its isoenzyme behaviour in cirrhossis and porto‐caval shun