Tissue from an eosinophilic granuloma of bone was examined with the electron microscope. In several of the histiocytes were rod-shaped structures, one end of which was frequently distended into a sac. The structures are identical to granules which have been described in Langerhan cells of skin. We believe that the granules originate from an infolding of plasma membrane and that their presence in both Langerhans cells and histiocytes indicates that the two cell types have some functions in common.