Summary Treatment of erythrocytes by low concentrations of digitonin results in the formation of elongated, bulged membrane areas free from particles and intramembraneous tubular structures emerged from these domains. At higher concentrations the tubular structures are present also outside of the membrane. A second, not bulged type of particle-free areas is more or less rounded, but mostly of rectangular shape. The rate of this type of domains increases with the concentration of digitonin. Breaks in these areas lead to a formation of sheets which results in a breakdown of the membrane structure finally into a brittle mass of many small sheets lying irregularly one upon the other. Already during the first steps of this membrane breakdown hemolysis takes place. Cross-linking of the proteins with glutaraldehyde does not entirely block up the membrane alterations. In fixed erythrocytes the formation of elongated domains is restricted, no tubular structures are present and no dislocations of particles can be observed. Nevertheless smooth sheets localized also outside of the membrane are formed. However the investigated ghosts are not stabilized by glutaraldehyde against the effects of digitonin. Particle dislocations as well as all the other membrane alterations are present. The implications of the obtained results are: 1. The elongated domains and the tubular structures are probably not digitonin-cholesterol-complexes. 2. The formation of crystalline regions by digitonin-cholesterol-complexes destroys the membrane structure.