The ultrastructure of globule leucocytes isolated from rat tracheal epithelium is described. The isolated globule leucocytes appear as rounded mononuclear cells with a diameter of 6 to 8 microns. The often indented nucleus is usually eccentrically located in the cell. The characteristic intracytoplasmic membrane-bound granules vary greatly in electron-density, size, shape, and internal structure. On the basis of their internal structure, four types of granules could be distinguished. They may reflect different stages in the process of accretion and utilization of the granule content. The prominent juxta-nuclear Golgi complex is presumably involved in the formation of granules. Striking in the isolated tracheal globule leucocytes are bridge-like connections between several granules, the significance of which is still unknown. Besides the granules and the extended Golgi complex, the cytoplasm contains inconspicuous organelles. Except for the bridge-like connections between intracytoplasmic granules, the ultrastructure of the isolated globule leucocytes resembles very well the ultrastructure of similar cells previously described in tissue sections of trachea and other organs of different mammalia. It is concluded that the used isolation method produces globule leucocytes which are morphologically well preserved and apparently undamaged. This may be a significant step towards understanding the function and origin of these peculiar cells.