Tubelculosis and Johne's' disease in cattle nmay be diagnosed by the injection into the skin of appropriate allergenis (tubeiculin and johnin) prepaied from cultures of the organisms which cause these diseases. Animals which are or have been diseased show a reaction to the allergens in the form of a thickening of the skin immediately surrounding the site of injection. Unfortunately, when using current preparations of the allergens, cows with tuberculosis may exhibit some reaction to johnin, and vice versa. Even though the intensity of reaction is greater to the allergen of the disease from which the animal is suffering, some confuson in diagnosis results. The Pathological Division of the Bureau of Animal Industry has been carrying out studies which have as their purpose the development of allergen preparations more specific for each disease, and at the same time potent. The general results of these studies will be published elsewhere, but certain points relative to the experimental designs may be summarized here.' In the experimental work on this problem, animals artificially sensitized to either tuberculin or johnin have been employed. The intensity of reaction to the allergen preparations may be measured by the amount of skin thickening produced using the technique of Dr. H. W. Johnson of the Bureau (1944). In preliminary tests it was found that a satisfactory degree of reaction to the allergen and a satisfactory measurement of the skin thickening could be obtained on the neck-flank, back and upper and lower side regions of the sensitized animals (Figure 1). Further, a total of 50 to 100 separate injections could be distributed over these regions, with reactions to the individual injections occurring independently. Before starting the experimental program proper, the importance of various factors which might affect the response to allergens was studied in a series of uniformity trials. To illustrate the major findings, data from one of these trials is cited. Here a preparation of johnin was injected into johnin-sensitized cows. The material was administered at two concentrations in each of four regions (Figure 1) on each side of five cows. Summarized results and analysis are given in Tables 1, 2, and 3.