The molar concentrations of the catalytic sites of serum cholinesterase (ChE--EC 3.1.1.8.) and cholinesterases (ChEs-acetylcholinesterase (AChE)--EC 3.1.1.7. and ChE) from brain and perfused liver of male birds, rats, swine and sheep were determined. A positive correlation between the molar concentrations of the catalytic sites of ChEs and the resistance of the animals to some organophosphorus compounds (OPhCs) was found. In addition, the present study also showed that the difference of the molar concentrations of catalytic sites of ChEs in the brain, blood serum and liver can cause varied resistance to some OPhCs.