AbstractThree types of unusual, elongated erythrocytes have been studied in our effort to understand factors that regulate the shape and deformability of red blood cells. Measurements of deformability, morphologic response to membrane‐shape‐altering drugs, spectrin phosphorylation, spectrin extractability, and casein kinase activity were measured in llama cells, irreversibly sickled cells, and nonhemolytic human elliptocytes. The results of these studies support the concept that the physical properties of spectrin are important in maintaining the shape and deformability of the erythrocyte. Furthermore, changes in spectrin extractability and phosphorylation appear to be associated with changes in the viscoelastic properties of the membrane. However, it seems that spectrin's regulatory role is permissive rather than direct and act