A product of fluorescence reaction of 3-hydroxykynurenine with p-toluenesulfonylchloride was separated and its structure was investigated. A reaction product of 2-amino-3-hydroxyacetophenone, which has the same configuration of carbonyl, amino and hydroxyl groups, with p-toluenesulfonylchloride was also studied. It was shown that the product from 3-hydroxykynurenine was p-toluenesulfonylester of the 3-hydroxyl group and p-toluenesulfonylamide of the aliphatic amino group, and that the product from 2-amino-3-hydroxyacetophenone was p-toluenesulfonylester of the 3-hydroxyl group.