An enzyme fraction from the green alga Chlorella pyrenoidosa that catalyzes the reversible epimerization of guanosine 5'-diphosphate D-mannose to guanosine 5'-diphosphate L-galactose brings about the incorporation of tritium from tritium-labeled water into the hexosyl moieties of those sugar nucleotides. The hexoses were degraded by periodate oxidation whereby the tritium was found to be equally distributed between carbon atoms 3 and 5. That observation was taken to imply that the epimerizations proceed via ene-diol intermediates.