Abstract Cooke and Stull (1, 2) employed phosphotungstic acid in hydrochloric acid for the standardization of pollen-extracts, and established a protein-nitrogen unit which was taken as equal to 0.00001 mg of nitrogen per ml of extract. This method has become one of those most widely used for the standardization of extracts used in allergic practice and research. If the actual active substance in pollen-extract were known, then methods for its exact measurement might be evolved. Since, however, its nature is not specifically known, allergists are forced to accept various procedures which, at best, give only an approximation of the relative strength of successive extracts. The experiments described below point to the fact that the method of Cooke and Stull, considered the least inexact of the various procedures, is not scientifically acceptable. The total of so-called protein-nitrogen units said to be measured by this method in any one extract varies with a number of physical and chemical factors not usually controlled.