In order to restore confidence in the tuberculin test as such it is necessary to reinstitute the standardization of commercially available test materials. Satisfactory conditions of production, careful biological comparison, and electrophoretic analysis are necessary to insure the uniformity of the purified protein derivative (PPD) or of any tuberculin. From 25,538 records of patients examined during a 15-year period, data were obtained for a study of the relation of pulmonary tuberculosis to the results of tuberculin (PPD) tests. The data imply that the degree of sensitivity to tuberculin constitutes important information to the clinician. The authors conclude that the use of the standard two-dose test has been highly satisfactory and should not be modified in clinics that serve areas where tuberculosis continues to be a major public health problem.