In spite of the progress that has already been made, the diagnosis of tuberculosis in its minimal stage, when its recognition and treatment offers the greatest benefit to the patient, is still extremely difficult. In the vast majority of children, the presence or absence of tuberculous infection usually may be determined by the application especially of the Pirquet or Mantoux tests. These tests, although undoubtedly specific, when positive show only that at some time the person has been infected by the bacillus of tuberculosis, but give no information as to the activity of the tuberculous lesion. In addition, a negative result from the beforementioned tests does not exclude tuberculosis, particularly in advanced or terminal stages of the disease and also in children for a variable period following certain diseases such as measles, scarlet fever, diphtheria, influenza, erysipelas or typhoid fever. The Pirquet and Mantoux tests, nevertheless, continue to be of