Mucoprotein of sera and organs was examined in rabbits and patients with parathion poisoning to clarify the mechanism of poisoning and the advanced information was acquired as follows. 1. Serum mucoprotein was considerably inhibited in parathion poisoning and its inhibition correlated with the degrees of parathion poisoning as the cases of serum cholinesterase and protein active SH radical. Moreover, its recovery was quicker than that of serum cholinesterase and protein active SH radical. 2. Mucoprotein was, in the rabbits subcutaneously injected with 5 mg/kg of parathion, inhibited considerably in all organs, especially in the liver and then followed by the kidney, brain, spinal cord, spleen, intestine, stomach, muscle, lung, and heart, in the order mentioned. But in the case of kidney, both the inhibition and recovery of mucoprotein were delayed markedly. In the fatal cases with the subcutaneous injection of 100 mg/kg parathion, mucoprotein decreased considerably in all organs, especially in the case of the liver where it decreased to 13.8 per cent. From the above-mentioned, parathion absorbed into a living body, combine with SH radical, especially of lower molecule such as mucoprotein mainly fixed in liver, brain, spinal cord etc. These SH radical consumption was supposed to be a cause of the inhibition of cholinesterase activity.