SELECTINS tether to the blood vessel wall leukocytes that are flowing in the bloodstream and support subsequent labile rolling interactions as the leukocytes are subjected to hydrodynamic drag forces^1,2. To support this rolling, selectins have been proposed to have rapid bond association and dissociation rate constants, and special mechanical properties linking tensile forces and bond dissociation^3-6. We have visualized transient tethering and release of neutrophils in hydrodynamic flow on lipid bilayers containing densities of P-selectin below those required to support rolling. We report here that transient tethers had first-order kinetics and other characteristics suggesting a unimolecular interaction between P-selectin and its glycoprotein ligand (PSGL-1). The unstressed dissociation constant (off rate) was 1 s^-1. Hydrodynamic shear stresses of up to 1.1 dyn cm^-2, corresponding to a force on the bond of up to HOpN, increased the off rate only modestly, to 3.5 s^-1. The data was adequately matched by a proposed equation7 relating off rate to the exponential of tensile force on the bond and the bond interaction distance, and gave a bond interaction distance of 0.5Å. This distance is compatible with hydrogen and metal coordination bonds between P-selectin and PSGL-1. Fast on and off rates, together with the high tensile strength of the selectin bond, appear necessary to support rolling at physiological shear stresses.