Examinations were made on the method for measurement of serum pyrophosphatase activity, and a direct method was devised in which the whole procedure can be carried out with one test tube. In this method, deproteinization has been omitted and the protein is dissolved by the addition of sodium dodecyl sulfate to the reaction mixture. Examination of various conditions for enzymatic reaction revealed that the optimal concentration of pyrophosphoric acid is 5mmol/l and that of magnesium, 1mmol/l. As a buffer solution, 0.1mmol/l of Tris-HCl buffer was found to be satisfactory, and the optimal pH was at 9.0.