The role of erythrocyte (red blood cell; RBC) aggregation in affecting leukocyte (white blood cell; WBC) margination in postcapillary venules of the mesentery (rat) was explored by direct intravital microscopy. Optical techniques were refined and applied to relate the light-scattering properties of RBCs to obtain a quantitative index of aggregate size (G), which, under idealized conditions, represents the number of RBCs per aggregate. WBC margination, defined as the radial migration of WBCs to the venular wall and their subsequent rolling along the endothelium, was measured as the percentage of the potentially maximal WBC volumetric flux within the microvessel lumen (F _WBC ^∗). In normal blood,F _WBC ^∗ increased exponentially fourfold, and G increased from 1 to 1.15 as wall shear rates (γ˙) were reduced from a steady-state value of ∼600 to <100 s⁻¹ by proximal occlusion with a blunt microprobe. Enhancement of aggregation by infusion (iv) of dextran 500 (428 kDa), to attain a systemic concentration of 3 g/100 ml, resulted in a four- and sevenfold increase in G andF _WBC ^∗, respectively, as γ˙was reduced below 100 s⁻¹. Inhibition of RBC aggregation by infusion of dextran 40 (37.5 kDa) causedF _WBC ^∗ to fall to one-half of its steady-state level for γ˙ < 100 s⁻¹. Thus it appears that the well-known increase of WBC margination with reductions in γ˙ is strongly dependent on the occurrence of RBC aggregation. Increasing the extent of RBC aggregation during reductions in γ˙ also increased the firm adhesion of WBCs to the endothelium because of an enhanced probability of contact between leukocytes and the postcapillary venular wall.