Human blood serum and various organs of dogs were determined for its content of parathion-splitting enzyme by the procedure presented in the preceding paper. Ten per cent saline suspension of the relevant homogenized tissues was used for the estimation of parathion-splitting enzyme of the organs. (1) The activity of the enzyme in blood serum of normal individuals was 134 ± 44.6 γ/ml for men and 180 ± 62 γ/ml for women. (2) Of the various organs of dogs the blood serum was the most abundant in the enzyme, and the liver, the lung, the bone marrow and the kidney followed in the order given. The distribution of the enzyme was not appreciable in other organs. (3) The blood, the liver, the lung and the bone marrow were thought to play a major part in the detoxication of parathion, because the afore-mentioned decomposition of this poison to the less toxic paranitrophenol was believed to imply detoxication. The blood will constitute the most important and the liver the next-important organ of detoxication, if volume of the organs is taken into account.