Suspensions of hardened human red blood cells (RBC) in normal plasma and in isotonic saline were investigated. A suspension of hardened RBC in isotonic saline is Newtonian at low shear rates and concentrations but slightly dilatant at high shear rates and concentrations. The average concentration inside a 99 μ capillary tube(D/d=12.3)is only about 85% of the concentration in the feed reservoir. The flow properties of a suspension of hardened RBC in saline flowing through a 99 μ tube can be predicted from a knowledge of the actual tube concentration and the flow properties obtained in very large tubes. The results found were compared to data for rigid sphere suspensions flowing through tubes about one order of magnitude larger than the sphere diameter.