The effect of ligands on the tryptophan fluorescence of the purified monosaccharide transporter from human erythrocytes has been investigated. Cytochalasin B, D-glucose, and ethylideneglucose quench the fluorescence of the protein at longer wavelengths by 17%, 13%, and 8%, respectively. Propyl glucoside, another ligand, has no effect on the protein fluorescence. Values of the dissociation constants for cytochalasin B, D-glucose, and ethylideneglucose were determined from the concentration dependence of fluorescence change; these agree with the values obtained from the effects of these compounds upon the binding of [3H]cytochalasin B measured by equilibrium dialysis. There is no correlation between the effect of each ligand on the fluorescence of the transporter and the conformational state expected for its complex on the basis of other evidence. The fact that the quenching is greatest at longer wavelengths suggests that an exposed tryptophan residue(s), possible located at the ligand binding sites, is the perturbed one.