D-Mannaric acid (II) was isolated and identified from human urine after oral administration of D-mannuronolactone (I). It was also isolated and identified from the incubation mixture containing I, nicotinamide adenine dinucleotide (NAD) and D-glucuronolactone dehydrogenase preparation purified from guinea pig liver, thus demonstrating the conversion of I into II in mammalian systems likewise the conversion of D-glucuronolactone (III) into D-glucaric acid (IV).