This communication reports the association of changes in ultrastructure of Mycohacterium leprae with alterations in its permeability. To study morphologic changes of the organisms under different conditions (of temperature and exposure to NaOH and trypsin), ultrathin sections of the bacteria were cut and examined in an electron microscope. In the untreated bacilli and those washed with trypsin, the cytoplasmic membrane and the cell wall (peptidoglycan layer) remained intact; dapsone showed little effect on diphenoloxidase of the bacteria. M . leprne is unique among mycobacteria in possessing an unusual form of the enzyme diphenoloxidase. The antileprosy drug dapsone is a potent inhibitor of the enzyme, but it does not readily penetrate the bacteria where the cell envelope remains intact. The. cell wall of M . leprae exposed to -80 "C or washed with NaOH was partially detached from the cell membrane; dapsone readily penetrated these organisms and inhibited the bacterial enzyme. In the above preparations, the cytoplasmic membrane appeared undamaged and the bacteria remained viable, as evidenced by multiplication in mouse foot pads. At 50 "C, the peptidoglycan layer became completely separated from the membrane and the cytoplasm was partially denatured. These organisms were permeable to dapsone, but were no longer viable. At 100 "C, the structural organization of the bacilli was completely destroyed, and of course, they lost their enzyme activity as well as viability. Evidently, the intact cell wall layer mediates the exclusion of dapsone from M . leprae, and there is no correlation between its viability and permeability. The ultrathin sections also reveal the internal organization and cytoplasmic inclusions of M . leprne, as never before seen.