The mechanical properties of sickle erythrocyte membranes were evaluated in the ektacytometer. When ghosts from the total red blood cell population were examined, the rigidity of the resealed ghosts and their rate of fragmentation by shear stress (t( 1/2 )) were normal. However, fractionation on Stractan density gradients revealed that sickle cells were heterogenous in their membrane mechanical properties. The ghosts from dense cell fractions exhibited both increased rigidity and decreased stability. Presumably, these altered mechanical properties are a reflection of the well-documented biochemical damage found in irreversibly sickle cell membranes. Nevertheless, neither of the alterations in mechanical properties are likely to be significant elements in the hemolysis of sickle cell anemia. Earlier studies of abnormal erythrocytes suggest that increases in membrane rigidity per se do not increase hemolysis, and they are, therefore, unlikely to do so in this case. The stability of membranes from the dense cell fractions was reduced to about two thirds of the control value. Comparison with the results of studies of red blood cell membranes with genetically defective or deficient spectrin suggests that a reduction in t( 1/2 ) of 50% is not associated with significant increases in the rate of hemolysis. Although altered ghost stability and flexibility can be demonstrated in dense sickle cells, these changes in membrane mechanical properties are not likely to be significant factors in the hemolytic process.