The automated ektacytometer enables rapid measurement of red cell deformability by using small aliquots of blood (25 microliter). By subjecting red cells to varying shear stress in suspending media of different osmolalities, one can identify separate contributions of membrane viscoelastic properties, internal viscosity, and surface area-to-volume ratio to overall cellular deformability. The influence of drugs on red cell deformability can also be investigated. Analysis of the diffraction pattern of red cells that are aligned in the ektacytometer at minimal shear stress enables improved measurement of red cell diameter. Modern computerized image analysis can further improve the reliability of this measurement. Volume measurement of red cells of unusual shape is possible. since, under certain conditions, they transform into uniform ellipsoids in the ektacytometer. Aided again by improved image analysis, simple accurate computation of red cell volume becomes possible Ektacytometric measurement may provide a sensitive assessment of the overall functional integrity of living red cells or subpopulations of red cells. As such, these measurements are of research interest and may have major clinical utility.