A. G. OGSTON1, in a recent note in Nature on the “Interpretation of Experiments on Metabolic Processes, using Isotopic Tracer Elements", introduced the important concept that an enzyme might be able to distinguish between the chemically ‘identical’ groups of a symmetrical compound. Ogston pointed to the dangers of the contrary assumption, and as one example he questioned the conclusion that citric acid was not an intermediate in the Krebs cycle of oxidations in so far as this conclusion was based upon the finding that the fixation of isotopically labelled carbon dioxide led to the formation of asymmetrically labelled α-keto-glutaric acid. It had been assumed2,3,4 that if citric acid were on the pathway leading to α-ketoglutaric acid, the latter would have had isotopic carbon equally distributed between the two carboxylic acid groups.