An acquired membrane defect is believed to be responsible for the maintenance of the sickled shape in oxygenated irreversibly sickled cells (ISC), because the hemoglobin S in these cells is not in the aggregated, "sickled" state. In the present study, it is demonstrated that the acquisition of the membrane defect in vitro depends on cellular metabolism. Only if cellular ATP is almost completely depleted while the cells are sickled, do they become unable to resume the biconcave disk shape upon reoxygenation. If calcium is omitted from the incubation buffer, ISCs are not generated despite metabolic depletion. This suggests an action of ATP mediated through calcium metabolism similar to that which prevents membrane stiffening in normal red cells. No ISCs were produced by repeated sickling and unsickling. Thus, a membrane alteration occurring as a consequence of metabolic depletion seems to be a more important factor in the generation of ISC than sickling-unsickling induced fragmentation.