The presence of parathion-splitting enzyme in the blood and various organs was described in the preceding papers of this series of study. A study was attempted to look for the activators of the enzyme in the hope that the activators might be used for the therapy of parathion poisoning. 118 kinds of drugs were examined, and sodium thiosulphate, methionine, azan, fructose, glucose and chloeletin were found to activate the parathion-splitting activity of human blood serum. Of these drugs the glucose has been known to benefit the cure of parathion poisoning. Its beneficial effect will be accounted for by the accelerated decomposition of parathion to less toxic paranitrophenol in the presence of glucose. Atropine, scopolamine, blocking agents of parasympathetic ganglion and anticholinesterase drugs, which exhibit remarkable curative effect for parathion poisoning, failed to activate the parathion-splitting enzyme. On the contrary, some stimulants of sympathetic nervous system (ephedrine) and antihistamine drugs were the activators of the enzyme. In view of the parathion-splitting enzyme and its activators the detoxicants which are used for the treatment of parathion poisoning (sodium thiosulphate and glucose) are recognized to owe its beneficial effect to the promotion of the decomposition of the poison. The similar mechanism holds in the case of sympathetic stimulants and antihistamine drugs, but the effect of parasympathetic anesthetics, the most effective drugs for the treatment of parathion poisoning, is independent of the parathion-splitting enzyme.