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Recent evidence indicates that fluid shear stress on the membrane of leukocytes has a powerful control over several aspects of their cell function. The present study is aimed to examine numerically the magnitude and distribution of the fluid shear stress on the membrane of leukocytes in small postcapillary venules, where circulating leukocytes make their first contact with the endothelium. Using the Stokes equation for the motion of plasma, we computed the fluid shear stress on a leukocyte membrane in a microvessel for the cases when the leukocyte is freely suspended, as well as rolling along or attached to the vessel wall. The results indicate that the shear stress distribution on the leukocyte membrane is non-uniform with a sharp increase when the leukocyte makes membrane attachment to the vessel wall. In a microvessel of approximately 10 μm diameter, the shear stress on the membrane of a freely suspended leukocyte is estimated to be several times larger than the wall shear stress exerted by the undisturbed Poiseuille flow, and increases on an adherent leukocyte up to ten times. High temporal stress gradients are present in freely suspended leukocytes and on the vascular endothelium.