Both hemostasis and thrombosis occur as a result of platelet adhesion to the subendothelial matrix, platelet activation, and platelet aggregation. The first stage in hemostasis and thrombosis is the binding of the platelet membrane receptor, glycoprotein (GP) Ib-IX complex, to its ligand, von Willebrand factor (VWF), in the subendothelium. In particular, the A1 domain of VWF is responsible for binding GP Ib-IX. After immobilizing A1 on a 2.0 μm diameter polystyrene bead, we optically trapped the bead using a titanium-sapphire laser tuned to 830 nm. The A1-coated bead was then moved towards a transfected Chinese hamster ovary cell which expressed the GP Ib-IX complex, and allowed to adhere to the cell. We subsequently detached the cell from the bead at different constant loading rates, ranging over three orders of magnitude, by using a piezoelectrically-driven translational stage. Displacement of the bead was simultaneously monitored from the trapping center using a quadrant photodetector to determine the force required to detach A1 from GP Ib-IX. These dynamic measurements of unbinding force emphasize the important role that shear rate plays in the initial stage of thrombus formation.