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 1s(-1) Hydrodynamic shear stresses of up to 1.1 dyn cm(-2), corresponding to a force on the bond of up to 110 pN, increased the off rate only modestly, to 3.5 s(-1). The data was adequately matched by a proposed equation(7) relating off rate to the exponential of tensile force on the bond and the bond interaction distance, and gave a bond interaction distance of 0.5 Angstrom. 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.