Because of the important role played by interleukin-2(IL-2) in T cell growth and differentiation, we investigated the effect of exogenous IL-2 on the proliferative response of peripheral blood mononuclear cells(PBMCs) from 77 leprosy patients. The proliferative responses of PBMCs from lepromatous leprosy(LL) or borderline lepromatous leprosy(BL) patients to M. leprae were significantly lower(cpm 6,051 ± 803 for LL type; 4,951 ± 2,529 for BL type) than those from tuberculoid leprosy(TT) or borderline tuberculoid leprosy(BT) patients (28,853 ± 28,916 for TT type; 15,884 ± 334 for BT type). To investigate the effect of exogenous IL-2, purified IL-2 was added at the start of culture at 100 unit/ml. There was an apparent increase in ³H-thymidine incorporation of M. leprae-stimulated PBMCs(18,723 ± 6,503) in the presence of IL-2 compared to the results without IL-2(6,051 ± 803) in LL patients. Twenty nine out of 33 LL patients belonged to the responders to IL-2 and four patients were nonresponders. Therefore we conclude that the defective cell mediated immune response in LL patients may result from diminished production of IL-2, but we can not exclude the possibility of diminished expression of the IL-2 receptor. And we suggest that the immunologic heterogeneous response of an individual to M. leprae is important to the pathogenesis of clinical disease in the same LL patients.