The influence of the shape of the red blood cell during stomatocyte-echinocyte transformation on its deformability was studied by microsieving through pores with diameters of 2.6, 4.5, and 6.9 μm. A stomatocytic transformation was produced by chlorpromazine (0.02, 0.1, and 0.5 mmol/L) and an echinocytic transformation by sodium salicylate (7.5, 30, and 120 mmol/L). For spherostomato-cytes, an increase in filtration resistance through 2.6 and 4.5 μm pores was observed, whereas for spheroechino-cytes, a decrease in filtration resistance through 2.6  μm pores was found. Larger pores (6.9  μm) were not sensitive to those shape changes. The changes in deformability can be explained by the fact that the surface area of (sphero)-stomatocytes decreased, whereas that of (sphe-ro)-echinocytes increased; the cell volume remained essentially constant. Echinocytes produced by 24-hour adenosine triphosphate depletion differed from drug-induced echinocytes: they had an increased cell volume at constant surface area and consequently an increased filtration resistance through 2.6- and 4.5-μm filter pores. Shape changes with spicule formation are therefore not a homogeneous entity, and cell geometric factors (eg, surface area and volume) must be assessed with care. The viscosity of red cell suspensions at a hematocrit level of 45% was higher for drug-induced echinocytes than discocytes or stomatocytes at all shear rates tested. We conclude that the normal discocyte represents an optimum shape for the flow in vivo since a stomatocytic transformation could impair the passage through the microcirculation (decrease in cell filterability) and an echinocytic transformation could impair the flow in larger vessels (increase in blood viscosity).