We describe a new method for the quantitative measurement of weak forces between adhesion domains formed by receptor-ligand pairs. The method relies on the microinterferometric analysis of adhering vesicles subjected to an external force produced by a magnetic bead adhered to the membrane. The forces exerted on pinning centers can be deduced using classical elasticity theory. The method is applied to measure the binding strengths between solubilized solid supported integrin receptors alpha (IIb)beta (3) Of blood platelets and lipid-coupled cyclic peptides containing arginine-glycine-aspartate (RGD) tripetide ligands, embedded in the vesicle, that are recognized selectively by the integrins. We find that unbinding takes place at force levels far below those found in single-molecule studies. We interpret our findings in terms of the fracturing of the receptor-ligand pairs by the torque generated by the force which is much more effective than the traction.