Accepted by Guest Editor Y. Isogai) The flow properties of aggregating red cell suspensions flowing at low rates through horizontal tubes are analyzed using a theoretical model based on microscopic observations. It is assumed that aggregated red cells form a cylindrical core which is eccentrically located in a horizontal tube due to sedimentation in a steady-state. The cylindrical core surrounded by a unifonnly dispersed suspension of individual red cells moves with a uniform velocity along the tube axes. The velocity distribution within the tube is calculated on the assumption of unidirectional, incompressible flow. From this result the relative viscosity is obtained as functions of the size, eccentricity and hematocrit of the core and a discharge hematocrit. It is shown that the relative viscosity increases monotonically with increasing the eccentricity. The simplified model also shows the presence of a certain condition under which the relative viscositiy of aggregating red cell suspensions becomes lower than that of nonaggregating suspensions. INTRODUCTiON When blood flows slowly through a tube, normal human red blood cells form aggregates termed rouleaux. Under very weak external forces, rouleaux become large and link to form secondary three-dimensional structures. Aggregation is a reversible process, that is, rouleaux are easily broken by an external force and formed again when the external force is removed. Therefore, red cell aggregation is one of the most significant determinants of flow properties of blood at low shear rates. Intravascular red cell aggregation has been observed in microcirculation under normal conditions(1,2,3) as well as under pathological conditions. On the analogy of a well-known behavior of bulk viscosities( 4) at low shear rates measured in rotational viscometers, it has been considered that intravascular aggregation may greatly increase flow resistance and cause flow cessation(5,6,7,8). However, the effects of red cell aggregation on the flow properties of blood in narrow tubes are more complicated than the effects on bulk properties, because aggregation in narrow tubes is associated with phase separation.