The present study regards the correlation between the percent of viable M. leprae (as determined by the mouse foot pad technique) and the quantitative ultrastructural analysis of M. leprae cells in 6 armadillo's samples and 1 nude mouse foot pad. The quantitative ultrastructural study of 3 LL patients and 1 M. leprae-infected nude mouse was correlated to the Morphological Index. The results show that most M. leprae cells with continuous undeformed cell walls, continuous symmetric membranes, ribosomes and fibrilar nucleoids are viable bacilli. Some cells with the above ultrastructural pattern may be dead bacilli that did not yet enter the macrophage-induced degradative process that results in the disposal of the bacteria. Our results also show that degenerating M. leprae cells largely predominate in most samples studied. This means that, even in the absence of anti-leprosy treatment, dead M. leprae cells accumulate in the host's tissues. This point has to be taken into account in the calculation of the generation time of M. leprae in vivo, the dynamics of the leprosy bacillus in susceptible hosts being influenced by the simultaneous occurrence of growth, death and degradation. Since known facts in regards to the physiology of bacterial membranes make it difficult to accept the PAS-symmetric membrane of viable M. leprae as the membrane of growing bacilli, a search of M. leprae cells with asymmetric membranes was undertaken in appropriate samples from nude mice. Several M. leprae cells with normal ultrastructure and Thiéry-asymmetric membranes were found in the foot pads of one mouse. Although this observation must be confirmed in another sample, it suggests that M. leprae would not be an exception to the general concept that the membranes of all growing Gram-positive bacteria have PAS-positive components located only in the outer layer. The M. leprae cells that have normal ultrastructure and symmetric membranes and that are viable would represent some sort of resting cells, that is, living but not growing bacteria. All the, 11, 263 individual bacillary profiles scored in the ultrastructural study included in the present study exhibited the micromorphological characteristics of acid-fast bacteria.