Chymotrypsin and neurotoxic esterase (NTE) have some similarities. After inhibition of concentrated (80-800 micro M) chymotrypsin by aryl saligenin cyclic phosphates it is known that aging occurs and some phenolic material becomes attached to protein. This binding has now been shown to be a manifestation of non-specific reaction with any available electrophile such as Tris, reduced glutathione (GSH), or protein. The reaction is therefore not a model for the 100% efficient transfer of alkyl groups to protein which occurs during aging of NTE inhibited by dialkyl phosphates.