When endo-uronidases act on glycosaminoglycans, the reaction products have hexuronic acid residues at the reducing terminals. An analytical method for hexuronic acids at the reducing terminals was devised for hyaluronate oligosaccharides having hexuronic acid residues at the reducing terminals. The procedure is as follows: Hexuronic acid residues at the reducing terminals of hyaluronate oligosaccharides were tritiated with reduction using NaB[3H]4 and the products were hydrolyzed with trifluoroacetic acid and nitrous acid. As a result, the tritiated and reduced hexuronic acid residues, that is aldonic acids, were liberated from the reducing terminals. After passing them through anion and cation ion-exchange resins, the aldonic acids were lactonized. The lactones were developed on paper chromatography, and their radioactivities determined on the paper. The method is also useful for discrimination between glucuronic acid and iduronic acid at the reducing terminals of glycosaminoglycans.