Eleven infect rats with murine leprosy were inoculated several time with lithiocarmine and pyrrole blue. One can see that the same cells which phagocytize the dye granules, in vital staining, were the same which contained bacilli. One can observe in the cell both dye particles and staining bacilli together. It is possible to see the transformation of the histiocytes into lepra cells. In inner organs examined we have always ascertained that the lepra cell was derived from an element of the reticuloendothelial system and the dye particles and bacilli were seen in the cells of reticulo-endothelium of the spleen, bone marrow, lymph nodes, Kupffer cells and lung histiocytes.