Because of the widespread use of 2-deoxy-2-[18F]fluoro-D-glucose (FDG) prepared by the "Julich" method or its variants it was decided necessary to determine the major chemical impurities present in the final product. An analytical system for quantifying FDG was developed using pulsed amperometry after separation by high-performance anion exchange chromatography. With this system a heretofore unidentified impurity, 2-deoxy-2-chloro-D-glucose (C1DG, ca 20-2000 micrograms; typically < 100 micrograms), was found in our preparation and in those from other laboratories using the "Julich" method. C1DG arises from Cl- ion displacement during the labeling procedure where Cl- ion comes from several sources, and Cl- ion displacement from the HCl used in the hydrolysis step. FDG mass was present in the same preparations at a level of ca 1-40 micrograms. Other major chemical constituents were glucose (ca 1-6 mg) and mannose (ca 10-18 micrograms). Glycerol, arising from sterilizing filters, was also detected in most preparations. Although C1DG is a chemical impurity which has not been detected previously in nca FDG preparations, its biochemical and pharmacological properties are similar to FDG and 2-deoxy-D-glucose. Thus it is unlikely that the presence of small quantities of C1DG found in typical FDG preparations (ca 100 micrograms) would have adverse pharmacological or toxicological consequences that would limit continued application of this radiopharmaceutical in basic and clinical studies.