The great variation in degree of sensitiveness of tuberculous guinea pigs to tuberculin is a factor of considerable importance in the standardization of tuberculin, as well as in many research problems concerning tuberculosis. The cause for such fluctuations is at the present time entirely unknown. During the course of an experimental study of tuberculin extending over a period of eight years it had been noted that the skin sensitiveness of tuberculous guinea pigs was at times of such a low degree as to prevent any quantitative study of the potency of tuberculins by that method. The minimal lethal dose test was therefore resorted to (1). At other times, as at the time of writing, the sensitiveness was so great that the most delicate titration of potency by means of the skin test has been possible.