A new NMR method for the study of ligand protein interactions exploits the unusual lifetimes of long-lived states (LLSs). The new method provides better contrast between bound and free ligands and requires a protein-ligand ratio ca. 25 times lower than for established T-1 rho methods, thus saving on costly proteins. The new LLS method was applied to the screening of inhibitors of urokinase-type plasminogen activator (uPA), which is a prototypical target of cancer research. With only 10 mu M protein, a dissociation constant (K-D) of 180 +/- 20 nM was determined for the strong ligand (inhibitor) UK-18, which can be compared with K-D = 157 +/- 39 nM determined by the established surface plasmon resonance method.