The effects of erythrocyte rigidity on the steady flow viscosity and the oscillatory flow viscoelasticity are examined over a wide range of shear rates. It is found that by hardening the erythrocytes, both the viscosity and the viscoelasticity are generally increased. In addition, the shear rate dependence of the viscoelasticity undergoes an abrupt change in character at higher shear rates. This effect in the viscoelasticity is identified as dilatancy. The elastic component of the complex modulus of viscoelasticity is a particularly sensitive indicator of the rigidity of the cells. These rigidity effects are identified for normal cells, chemically hardened cells, aged cells, and osmotically modified cells.