The degree of delayed hypersensitivity to tuberculin purified protein derivative (PPD) was determined in 34 subjects by sequential skin testing with first, intermediate, and second strength PPD, and simultaneously the subjects’ lymphocytes were maximally challenged in vitro with PPD. A linear relationship existed between the minimal strength of tuberculin required to detect skin test sensitivity and the logarithm of the lymphocyte transformation index. in 25 of the 34 subjects, the size of the skin test response to any strength of tuberculin could be predicted within one standard deviation from the degree of lymphocyte transformation. the occurrence of some degree of lymphocyte transformation in all subjects who manifested skin test responses of 1 to 4 mm and 5 to 9 mm to some strength PPD compared to no lymphocyte transformation in subjects completely nonreactive to the 3 strengths of tuberculin suggested that most skin test reactions, regardless of size, have an in vitro, immunologic correlate. These data suggest that small skin test reactions, i.e., 1 to 4 mm, although not reactive by the established epidemiologic criteria, are significant immunologic events. This concept might help explain the booster effect that occurs occasionally with tuberculin tests and was observed in 2 of the 34 subjects in this study. Lymphocyte transformation might offer an additional method of assessing patients for tuberculin hypersensitivity that avoids the potential booster effect seen with skin testing.