Sucrose has been found to decrease the fluorescence observed during the standard fluorometric assay of tryptophan. The quenching is enhanced with increasing concentrations of sucrose. This phenomenon is not due to a metal contaminant in the sucrose, nor to a direct interaction of the sugar with the fluorophore, norharman, which results from the oxidation of a tryptophanformaldehyde complex by FeC&. Sucrose and other keto-sugars reduce the Fea+ ions in the standard reaction mixture to Fez+ and thereby decrease the amount of FeC& available for oxidizing the tryptophan complex to norharman. The increase in the amount of ferrous ions produced in the reaction mixture is directly proportional to the degree of quenching of fluorescence that is observed. Aldoses, on the other hand, do not significantly reduce the Fea+ ions to Fez+, nor do they diminish the fluorescence yield of the reaction. Since the concentrations of sucrose that lead to this interference are in the range of those often used to prepare tissue samples, it is important that appropriate corrections be applied to ensure accurate measurements of tryptophan in biological fluids.