Microdensitometry showed that the membrane profiles of normal cultivable mycobacteria were very asymmetric (outer layer denser and thicker than the inner layer), while the profiles of normal-looking M. leprae in lepromatous patients, in experimentally infected armadillos and in nude mice were approximately symmetric; moreover, the membrane of M. leprae was thicker than that of cultivable species. Using two cytochemical methods for the ultrastructural detection of periodic acid-Schiff (PAS)-positive molecules (the Thiéry procedure, and staining with phosphotungstic acid at low pH) we found that the membrane of cultivable mycobacteria, growing in vitro or in vivo, had PAS-positive components exclusively in the outer layer, while the normal-looking M. leprae in patients and in armadillos had membranes with PAS-positive components in both layers. The membranes of damaged cultivable mycobacteria, in vivo or in vitro, and of damaged M. leprae, in patients or armadillos, were PAS-negative.