The steady‐flow rheological properties of suspensions of mixtures of normal and glutaraldehyde‐hardened human red blood cells in albumin‐containing saline were determined in the shear rate range of 0.1–1500 sec−1, for total cell volume concentrations (hematocrits) between 30 and 43.5% and over the entire range of normal‐to‐hardened cell ratios. Suspensions of only hardened cells were Newtonian at lower shear rates, becoming dilatant at high shear rates. Suspensions of just normal cells were pseudoplastic, becoming Newtonian at high shear rates. Suspensions of mixtures of the two types of cells showed mixed behavior, dependent on the mixture composition. For suspensions with a constant total cell volume concentration, the apparent viscosity at a given shear rate is a nonlinear function of the fraction of the cells (fH) which are hardened. Below a critical value offH, viscosity is a linear function offH, but the slope is low. Above the criticalfH, viscosity increases rapidly withfH. The criticalfHis associated with a critical hardened cell volume concentration where interactions between hardened cells become significant. It is expected that steady‐flow rheological measurements usually will not provide a sensitive method for determining changes in erythrocyte deformability distribution in blood or related suspensions, unless the abnormal cells are rigid and greatly distorted from the normal cell shape.