Deuterium and carbon-13 labelled glycerols have been fed to Streptomyces calvus fermentations and isotope incorporation into the fluorine containing antibiotic nucleocidin have been evaluated by 19F-NMR. A single deuterium atom was incorporated from [2H5]- and (R)-[2H2]-glycerol into C-5' of the antibiotic, suggesting that an oxidation occurs at this carbon after ribose ring assembly from glycerol (pentose phosphate pathway), during nucleocidin biosynthesis.