Lymphocyte transformation in response to PHA, M. leprae and BCG with the use of measurements of 3H-thymidine uptake, was studied in 10 normal controls, and 71 leprosy patients of different types histologically classified according to the Ridley-Jopling scale. Lepromin and PPD tests were done in each one of them. It was found that there was a generalized depression of immune responsiveness to PHA in all forms of leprosy. The immune responsiveness of lymphocytes to M. leprae continuously and gradually increases from the lepromatous (LL) towards the tuberculoid (TT) end of the spectrum. The loss of immune response of lymphocytes from lepromatous patients to M. leprae is specific as evidenced by their capacity to respond selectively to BCG when PPD-positive, irrespective of their place in the leprosy spectrum.