A practical laboratory experiment is described that illustrates the application of fluorescence resonance energy transfer to the study of protein-ligand binding. The affinities of wild-type and mutant human carbonic anhydrase II for dansylamide were determined by monitoring the increase in ligand fluorescence that occurs due to energy transfer from tryptophan residues near the enzyme active site. In a subsequent experiment, the binding constant of azetazolamide, a weaker fluorophore but a stronger ligand, is measured by competition with dansylamide. This simple experiment introduces students to the widely used technique of fluorescence spectroscopy and to the determination of protein-ligand binding constants.