A method is detailed for the assay of isoniaaids. It depends upon the reduction of ferricyanide by the hydrazide in acid solution with subsequent estimation of the color produced with the aid of a photoelectric colorimeter. The method is simple, relatively raid, and requires apparatus commonly available in chemical, pharmaceutical, ancf clinical laboratories. The method is very sensitive for 2 μg. in 1 ml. of original test solution (2 parts per million) and in 5 ml. of reaction solution can readily be determined.