Apparent viscosity was determined in vertical glass tubes (ID 30.2-132.3 microns) with suspensions of human red cells in A) serum, B) saline containing 0.5 g/100 ml albumin, C) plasma, and D) plasma containing Dextran 250 at a feed hematocrit of 0.45. Pressure-flow relationships were obtained in a range of pseudo-shear rates (mu) between 0.15 and 250 s-1. Relative viscosities in the nonaggregating suspensions (A and B) were found to increase monotonically with decreasing mu. The Fahraeus-Lindqvist effect was present in the entire range of mu. In the two aggregating suspensions (C and D), viscosities increased initially in larger but not small tubes with declining mu and fell in all tubes at some characteristic mu (usually below 10 s-1). Viscosity reduction was greater in the larger tubes and in suspensions with greater aggregation tendency. With suspension D, the Fahraeus-Lindqvist effect was eliminated in the lowermost shear-rate range. The cell-free marginal zone increased in width (to a maximum of approximately 40% of tube radius) as viscosity declined. Measurements of viscosity and cell-free marginal zone were also performed with suspension C in tubes mounted in horizontal position. In contrast to vertical tubes, a monotonic increase in viscosity was found with decreasing mu, associated with cell sedimentation and development of a cell-free layer only in the upper portion of the tubes.