Selectins are cell adhesion molecules that bind carbohydrate ligands and promote interaction between leukocytes and the vessel wall in vascular shear flow. Selectin-ligand bonds have high mechanical strength, allowing initial tethering to the vessel wall through one or few bonds, and have fast on and off rates, permitting rolling in response to hydrodynamic drag. The L-selectin molecule on leukocytes binds to peripheral node addressin on high endothelial venules of lymph nodes to mediate leukocyte rolling and binds to a ligand on neutrophils to mediate rolling of leukocytes over one another. Here we describe a surprising mechanism for regulation of these interactions, both in vitro and in vivo. Shear above a critical threshold is required to promote and maintain rolling interactions through L-selectin, but not through E-selectin, P-selectin or VCAM-1. The shear threshold requirement for L-selectin may be physiologically important in low shear to prevent inappropriate aggregation of leukocytes and interaction with the vessel wall.