Specimens of serum from ١٦٨ healthy subjects (٨٠ men and ٨٨ women, ages ١٨ to ٧٨ years)were divided into three groups: fresh, storage at ٤°C, and freezing at-٢٠°C . Periodic essays for the activity of Butrylcholinesterase (BChE,EC. ٣٫١٫١٫٨) were made on each frozen and unfrozen specimen. Serum frozen at-٢٠°C and left at ٤°C gave results similar to those from fresh specimens. Comparison of samples from men and women revealed significantly higher activity in men than in women (P<٠٫٠٠١). Activity was greatest in men aged ١٨-٥٠ years and then declined to slightly lower values. In women the activity was less in those under ٥٠ years old than in those above ٥٠.