Ghosts are post-hemolytic residues of red blood cells. It is generally assumed that these residues are devoid of intracellular structure and consist primarily of the cell membrane. Hence ghosts are widely used in the study of composition, structure, and function of the red blood cell membrane. There are possibly as many different types of ghosts as there are ways of hemolyzing red blood cells a. The present review on the preparation and properties of red blood cell ghosts is confined to ghosts made by hypotonic hemolysis. These ghosts have a composition which, in many respects, is similar to or even identical with that of the membrane of the intact cell. Hence, "hypotonic ghosts" have been more intensely investigated than those prepared by other techniques. In spite of the fact that in all preparations of hypotonic ghosts hemolysis is induced by the mechanical stress brought about by an osmotic pressure difference, the properties of the final product still depend on the precise conditions existing at the instant of the disruption of the membrane and on the treatment of the resulting ghosts thereafter. The discussion of the effects of such modifying factors constitutes the bulk of this paper.