We studied how rheological properties of the blood influenced capture and rolling adhesion of leukocytes, as well as their margination in the blood stream. When citrated, fluorescently-labelled, blood was perfused through glass capillaries coated with P-selectin, leukocytes formed numerous rolling attachments. The number of attached leukocytes increased as haematocrit was increased between 10-30%, and was essentially constant from 30-50%. In EDTA-blood, adhesion was absent, and the flux of marginated cells varied little with increasing haematocrit. However, the velocity of marginated leukocytes increased monotonically while the volumetric flow rate was constant, implying that the flow velocity profile became blunted and wall shear rate increased. Thus, increasing haematocrit promoted attachment for a given total flow rate, without increasing margination, even though wall shear rate and blood viscosity increased. Blood was diluted to 20% haematocrit with plasma, 40kD dextran (to reduce red cell aggregation) or 500kD dextran (to enhance aggregation). Increasing aggregation correlated with increasing leukocyte adhesion, and with more slow-flowing leukocytes near the wall. Thus, flowing erythrocytes promote leukocyte adhesion, either by causing margination of leukocytes or by initiating and stabilising attachments that follow.