Galacturonic acid (GalA) is a major component of plant cell-wall-derived pectins. It can be also found in the cell-surface polysaccharides of different microorganisms, including several symbiotic and pathogenic bacteria. Uridine diphosphogalacturonic acid (UDP-GalA) is a likely donor for GalA during the biosynthesis of these polysaccharides. A highly efficient, yet simple, method is presented for generating and purifying UDP-[ 14 C]GalA. Commercially available UDP-[ 14 C]galactose was quantitatively oxidized (>95% conversion) to UDP-[ 14 C]GalA in the presence of high levels of galactose oxidase and catalase, at prolonged incubation times. Following this one-step enzymatic oxidation, UDP-[ 14 C]GalA was purified using a polyethyleneimine cellulose column with a single-step 1 M NaCl elution. The authenticity of the purified UDP-[ 14 C]GalA was verified by its relative mobility on thinlayer chromatograms, analysis of its chemical hydrolysis products, and 1 H NMR spectroscopy. Our yield of >90% is much higher than by previously described methods. The method may serve as a prototype for the preparation of other radiolabeled uronic acids and their nucleotide derivatives.