INVESTIGATIONS ON TUBERCULIN MEDIC BIIOUSHAL 997 standard old tuberculin by means of the Mantoux test, the two tuberculins having been titrated previously on sensitive guinea-pigs. On intracutaneous test in a large number of guinea-pigs the preparations were almost equal in potency, only the most sensitive animals indicating that the "dried dilutions " were a trifle stronger. 4. The reactions obtained with P.P.D. were very similar in character to the response with old tuberculin, both in

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... pigs and in human subjects. 5. Clinical comparison, both by measurement and by the relative number of positive and negative reactions obtained, indicated that the P.P.D. was slightly more potent than the old tuberculin, a fact that was only evident on the most sensitive guinea-pigs. 6. The results of the four clinical workers suggest that if the Mantoux test were to be used as a method of comparing the potency of two tuberculins it would be necessary to have observations on a large number of patients before any conclusion could be drawn. There would appear to be no advantage in using human subjects instead of guinea-pigs for the titration of old tuberculin. 7. In practice the deterioration of dilutions of old tuberculin in phenol saline is not sufficient to prevent their use for Mantoux tests if they have been kept up to six weeks at room temperature or nine weeks in the ice-chest. " DrOd dilutions " of P.P.D are much more stable, and have shown no detectable loss in potency after three years at room temperature. Our grateful acknowledgments are due to Drs.