During the course ofour study on parathion (organophosphorus insecticide) poisoning we happened on the discovery that the human blood serum promoted the decomposition of this insecticide. A procedure for the estimation of the promoting activity of blood serum in splitting parathion was developed to examine this phenomenon quantitatively. In this procedure, the mixture of serum, emulsion of parathion and bicarbonate-carbonate buffer of pH 10 in a fixed proportion was incubated at 37℃ for an hour, and the amount of paranitrophenol liberated from parathion was measured by colorimetry to express the splitting activity in terms of γ of paranitrophenol per ml of serum. A new enzyme that had not hitherto been described was supposed to be responsible for the parathion-splitting activity of serum after careful examination of its property with special reference to the ambient conditions (pH of the medium, temperature, amount of substrate used, inhibitor, etc.). The enzyme was quite different from alkaline phosphatase, cholinesterase and A-esterase (E-600 esterase) which had been believed to be concerned with the detoxication of parathion. Normal range of parathion splitting activity of human serum by the proposed method of the author was 134 ± 45 γ/ml for males and 180 ± 62 γ/ml for females.