When the membrane-assocd. product of in vitro mannan synthesis by an E. coli inner membrane-outer membrane mixt. with GDP-mannose-14C was extd., it migrated in Na dodecyl sulfate-polyacrylamide gel electrophoresis with a mobility characteristic of the carrier-mannan complex (Kopmann, H.J.; Jann, K., 1975). When the mannan-14C was liberated from the carrier, reduced with NaB3H4, acid hydrolyzed, and then analyzed by paper chromatog., 14C was detected in the same position as mannose, whereas 3H was obsd. in the same position as glucitol, and no radioactivity was in the mannitol position. For cells labeled with glucose-14C, the in vitro system yielded mannan-14C contg. a terminal sugar residue that, after its release, had the same mobility as authentic glucose. When this residue, obtained from the mannan labeled with glucose-14C in vivo, was treated with glucose oxidase, the product comigrated with gluconic-3H acid in paper electrophoresis at pH 5.4. Thus, the reducing end of 08-mannan is glucose.