It is shown that published results on the anomalous properties of blood when flowing through rigid tubes can be explained by considering the blood as a suspension of red cells in serum. The rise in viscosity at low rates of flow is attributed to rouleaux formation. The variation of viscosity with tube diameter is shown to be of the form expected if there were a layer of low viscosity fluid adjacent to the wall. Published results of the variation of viscosity with hematocrit reading, when corrected for this wall effect, are shown to give a linear relation between fluidity and red cell concentration. It is shown that published results of the variation of viscosity with tube diameter, the drop in red cell concentration and the difference in velocity of red cells and serum in narrow tubes, may all be explained by an inward mechanical displacement of cells near the wall as they enter a tube. Submitted on July 30, 1956