The barium salt of 6-phosphogluconate was prepared according to the method of Seegmiller and Horecker (18). Solutions were prepared for use by dissolving the material in HC1, removing the barium with potassium sulphate, and adjusting the pH to 7.4. Various samples of barium phosphogluconate contained 1.7 to 2.0 /iM of organic phosphorus per mg. and 0.01 to 0.03 ?? of inorganic phosphorus per mg. The fructose content, according to the method of Roe (16), was not more than 0.03 ?? per mg., and the glucose-6-phosphate content, according to assay with glucose-6-phosphate dehydrogenase (11) was 0.08 to 0.13 ?? per mg. of the barium salt. The phosphogluconate content was calculated from the organic phosphorus content, after correction for the impurities. Ribose-5-phosphate was prepared by acid hydrolysis of adenylic acid or adenosine triphosphate according to the procedure described by LePage and Umbreit (12) and Albaum and Umbreit (1). A preparation of TPN+ of 70% purity was generously supplied by Dr. M. A. Mitz of Armour Laboratories. Oxidized glutathione (GSSG) was purchased from Schwartz or was prepared by oxidizing GSH with H202 as previously described (5). Tris (trishydroxymethyl amino-