Summary. Using the micropipette technique, we examined the viscoelastic properties of the red blood cell (RBC) membrane which had been strongly coated with various components of complement (primary C3b, C3d, C4b, C4d) in vitro. The membrane elastic modulus (E), the viscosity index of the initial rapid phase of deformation (η_D1), the viscosity of the later slow phase of deformation (η_D2) and the viscosity of the recovery phase (η_R) were determined. Compared to control non‐complement coated RBCs, RBCs coated with C3d, either alone or with other complement components, showed significant increase in the values for elasticity and viscosities. Thus C3d fixation resulted in decreased membrane deformability. Changes in membrane viscoelasticity due to bound C3d were not enhanced by bound C4b, C4d, C5, factor Bb or ηp; presence on RBC membrane of the latter two complement components may partially reverse the effect of C3d fixation. Lipid fluidity of RBC membrane, examined by fluorescence depolarization, increased with fixation of all complement components except C5. These complement‐induced changes in membrane viscoelastic properties have potential pathophysiological and clinical implications. The data suggest that extravascular sequestration of human RBCs may be explained in part by increased membrane rigidity resulting from C3d fixation.