Protein active SH radical of sera and orgaus was examined in rabbits and patients with parathion poisoning. Advanced informations in this experiment were as follows. 1. In parathion poisoning, serum protein active SH radical was markedly inhibited. 2. Inhibition and recovery of serum protein SH radical correlated with that of serum cholineterase activity. 3. The decrease of serum protein active SH radical was relative to the symptoms of poisoning in their degrees, and it took about two weeks to restore in serious cases. 4. In rabbits subcutaneously injected with 5 mg/kg of parathion, protein active SH radical of organs was slightly inhibited only in the initial stage, thereafter reactive increase was observed. In the case of kidney, however, it decreased considerably and its recovery took much longer time in comparison with the others. In the fatal cases with the subcutaneous injection of 100 mg/kg parathion, the protein active SH radical of rabbit organs tended to increase generally. From the above-mentioned, parathion inhibited not only cholinesterase but also protein active SH radical of SH ferment as the cases of arsenic or heavy metal poisoning.