Mechanically speaking the red cell membrane consists of a lipid bilayer and a membrane skeleton. The bilayer confers bending, the skeleton shear elasticity to the membrane. Changes of red cell shape are usually invoked by changing the spontaneous curvature of the bilayer. The resulting shape is then a balance of shear tensions in the skeleton and bending moments in the bilayer. As to the bending elasticity, two contributions are distinguished. The first (called bilayer-couple bending) originates from the 2D isotropic elasticity of the two monolayers and their fixed distance. The second (called single-layer bending) comprises the intrinsic bending resistance of each monolayer. Accordingly, the total spontaneous curvature (ξ) is subdivided into ξ c (in bilayer-couple bending) and ξ s (in single-layer bending). Both quantities depend on the average cross-sectional area of the molecules in each monolayer. In addition, ξ c depends on the number of molecules and ξ s on their average cone angle. While ξ c is to some extent amenable to measurement, there is no quantitative information on ξ s . Therefore, model calculations were performed for ranges of these quantities. The following types of experiments were considered: (a) asymmetric incorporation of exogenous molecules; (b) interleaflet redistribution of endogenous molecules; (c) exchange of endogenous molecules against exogenous ones. The calculations show that the bilayer-couple effect dominates the behavior in cases (a) and (b). This may be the reason why single-layer bending has been neglected so far in most quantitative assessments of red cells shape change.