Untreated leprosy patients were examined with respect to lymphocyte transformation in vitro after stimulation with mycobacterial and other microbial antigens, allogeneic lymphocytes, or nonspecific mitogens. Methods were used to circumvent technical variability. The results were compared with those obtained in controls matched for age, sex, race, and environment. No evidence was found for a generalized impairment of lymphocyte transformation in vitro, whereas a specific defect towards Mycobacterium leprae was demonstrable in lepromatous leprosy patients. The response to M. leprae, investigated in untreated and treated leprosy patients, decreased along the leprosy spectrum. Moreover, the results of the one-way mixed lymphocyte cultures showed that lymphocytes from leprosy patients had a normal stimulator and responder capacity, when they were tested against a panel of allogeneic lymphocytes. The influence of serum factors was investigated in untreated leprosy patients in the mixed lymphocyte culture. On average, tuberculoid as well as lepromatous sera showed a low-level depressive effect, but some sera showed a stimulatory effect. Therefore, a depressive effect of serum factors cannot be considered to be a general feature of leprosy. The correlation between the Mitsuda type of lepromin skin test and the lymphocyte reactivity in vitro to M. leprae was studied, and a positive correlation was found.