This article reported the multiplication of mycobacterium leprae in immune-deficient animals-nude mice (NIH/nu/nu). Mycobacterium leprae taken from multibacillary leprosy patient were inoculated with 1.0 x 10(4)/foot pad into right hind foot pad(FP) of nude mice and CFW mice. The results of bacterial enumeration showed that acid-fast bacillus (AFB) in the nude mice continuously multiplied. At 21 months after inoculation, the numbers of AFB increased by over 108,000-folds and reached greater than 1.08 x 10(9)/FP (greater than 5.4 x 10(10)/g tissues), while the CFW mice developed only 3.68 x 10(6)/FP and exhibited their characteristic limited growth pattern. It is indicated that the phase of logarithmic growth was prolonged in nude mice. The AFB multiplied were identified as mycobacterium leprae by following identification test: (1) failure to growth on artificial media; (2) loss of acid-fastness by pyridine extraction; (3) typical growth curve of mycobacterium leprae in foot-pad and sensitivity to antileprosy drugs. The histopathological examination of nude mice foot-pad inoculated were found lepromatous lesions, in which enormous numbers of AFB were seen on 476 days postinoculation, AFB disseminated into the fear, nose, sciatic nerve, testicle, lymphnodes, liver, spleen, lung, and noninoculated foot-pad. As mentioned above, it is considered that experimental leprosy was established in nude mice.