A trial with a candidate anti-leprosy vaccine based on killed Mycobacterium leprae was started in Norway in 1983 to evaluate its toxicity and efficacy to induce cell-mediated immunity (CMI) in BCG-vaccinated healthy volunteers. The vaccinated subjects were found to be free of unacceptable side-effects and their T cells showed elevated proliferative response to M. leprae up to 1 year postvaccination. When tested in 1991, 8 years after vaccination, peripheral blood mononuclear cells from the same volunteers showed a persistent high proliferative response to M. leprae. From a total of 147 T-cell clones established from these subjects, 26 clones were specific to M. leprae and the remaining T-cell clones responded to M. leprae as well as to BCG and other cultivable mycobacteria. The epitopes recognized by the M. leprae-specific T-cell clones were present on several protein antigens including the 18 kDa and the 65 kDa heat shock proteins. A dominant epitope, peptides 38-50 on the M. leprae 18 kDa heat shock protein, which was recognized by M. leprae-specific T cells 1 year after vaccination, was also recognized 8 years after vaccination by the same donor. This is the first report demonstrating the unique property of killed M. leprae with respect to the induction of long-lasting T-cell reactivity towards M. leprae antigens in humans.