A method is presented to determine the Labusch parameter &agr; from the measurement of magnetic stiffness &eegr;. Quantitative results on various YBa2Cu3O7−&dgr;samples are reported. For all samples &eegr; is a function of the external magnetic field and does not depend on magnetic history. For small magnetic fields &eegr; obeys aB1.5behavior. At fields above 120 mT the melt grown material and the thin film turn over to aB2dependence. The results can be understood in terms of a crossover from nearly single vortex characteristics to the elastic interaction of vortices in the flux line lattice. ©1995 American Institute of Physics.