A comprehensive approach to target screening, hit validation, and binding site determination by nuclear magnetic resonance (NMR) spectroscopy is presented. NMR (19)F signal perturbation was used to screen a small compound library and identify candidate ligands to the target of interest. Ligand dissociation constants were measured using a pegylated form of the protein, which resulted in a 2-fold increase in the strength of the saturation transfer difference signal. The initial small-molecule hits were further optimized by combining a residue-specific labeling strategy, to identify the specific sites of interaction with the protein, with a second site screening approach based on relaxation enhancement using a paramagnetic probe. The advantages of this combination strategy in the identification and optimization of weak binding chemical entities early in a program are illustrated with the discovery of a low micromolar ligand (K(d) = 20 microM) for Nurr1 and identification of the binding site location through residue-specific (15)N isotope labeling and derivatization of Cys residues with 2-mercaptoethanol-1-(13)C.