The antichymotrypsin, antitrypsin, and anticholinesterase potencies of four homologous series of organophosphorus inhibitors are compared: O-(n-alkyl) methylthiophosphonates, O-(n-alkyl)-S-(n-butyl) methylthiophosphonates, O-(n-alkyl)-S-(β-ethylmercaptoethylene) methylth-iophosphonates, and their methylsulfomethylates. As sources of α-chymotrypsin and trypsin, commercial preparations from Worthington Biochemical Corporation and Leningrad Myasokombinat were tested. Bimolecular constant of the reaction rate was used as the measure of antienzyme potency. In all cases, the antichymotrypsin efficiency was lower, while the antitrypsin-essentially higher than the anticholinesterase activity of the studied inhibitors. These differences were found to much depend both on the inhibitor structure and on the nature of the cholinesterase preparations.