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T cell proliferation and IFN-γ production of peripheral blood mononuclear cells from 25 healthy controls and 39 leprosy patients were tested against BCG-bacilli and culture filtrate, Mycobacterium leprae and purified antigen 85 (the major secreted 30-32 kD protein antigen) from M. bovis strain BCG, In lepromin negative healthy controls, blastogenesis was low to M. leprae and completely negative to antigen 85, IFN-γ levels were very low, close to detection limits. In all lepromin positive controls, significant proliferation and IFN-γ secretion was found in response to M. leprae and antigen 85, In the group of lepromatous leprosy (LL) patients, 25/29 of patients (with either positive (13) or negative (12) lymphoproliferative response to BCG) were unreactive to M. leprae or to antigen 85, Four LL patients with positive T cell response to BCG responded with detectable lymphoproliferative response and IFN-γ secretion to antigen 85, All tuberculoid (TT) leprosy patients responded to BCG, M. leprae and antigen 85, Hence, T cells from leprosy patients and controls demonstrate a marked parallelism of responsiveness towards whole M. leprae and purified antigen 85 from M. hovis BCG, suggesting strong cross-reactivity between the two species and underlining the biological importance of such secreted antigens. [ABSTRACT FROM AUTHOR]