1. The potency of PPD tuberculin made from AN 5, a bovine strain, was high when WR medium was used and low when BAI medium was used. PPD tuberculin made from strains of the human type grown on BAI medium was intermediate in potency between the two bovine PPDs.
2. The standard deviation of the difference between responses to duplicate injections of human type PPD at 2·0 mg./ml. concentration was 2·59 in the Islington and 1·84 in the Weybridge experiment on tuberculous animals, and 1·11 at Islington and 1·13 at Weybridge on tubercle-free animals. Differences greater than 1·5 mm. were relatively infrequent on tubercle-free animals.
3. The Elliott ‘401’ electronic computer at Rothamsted was used to calculate discriminant functions and to estimate the discriminating power of different ‘tests’, each test involving an injection of a mammalian and an injection of avian tuberculin.
4. There were some differences between the mammalian preparations in their power to discriminate between tuberculous and tubercle-free animals, but these differences were closely allied to differences in potency. It seems probable that if concentrations of equivalent potency were used, there would be little if any difference in their discriminating power.
5. The contribution of avian tuberculin to the discrimination test was demonstrated; the greatest contribution came from a concentration of 0·25 mg./ml. used in the Islington experiment. The Weybridge results were quite consistent with the assumption that a maximum contribution is made by a concentration less than 0·5 mg./ml.
6. Methods for deducing rules for the interpretation of comparative tuberculin tests, using a mammalian and an avian tuberculin are illustrated and results given for the type of population sampled in these experiments.
7. The principle on which this rule might be modified where the population is believed to be free of infection, is outlined and discussed.
We should like to acknowledge the advice and constructive criticisms during the planning of the experiments of Dr A. W. Stableforth and Dr H. H. Green of Weybridge, and also the care with which Miss C. N. Hebert carried out the preliminary arrangement of the large quantity of data. Thanks are also due to the staff of the Islington abattoir for their willing co-operation.