Abstract High concentrations of red blood cell glycogen were visualized by electron microscopy and demonstrated biochemically in amylo-1,6-glucosidase- and phosphorylase-deficient red blood cells. Glycogen concentration decreased as a function of cell age. Similar incorporation rates of 14C-U-glucose into glycogen were observed in amylo-1,6-glucosidase-deficient and normal erythrocytes, characterized by an initial rise, followed by a plateau formation reflecting a steady state between glycogen synthesis and breakdown. A different pattern of kinetics was observed in phosphorylase-deficient cells, which in view of the lack of the degradative enzyme showed a continuous linear increase in radioactive glycogen formation leveling off only after exhaustion of substrate. Evidence that in amylo-1,6-glucosidase-deficient red blood cells the main metabolic activity affects the outer branches of the glycogen molecule was obtained directly by β-amylolytic degradation of the radioactive glycogen molecule and indirectly by a chase experiment substituting radioactive with nonlabeled glucose. Normal glycogen synthetase activity was found in all cases of amylo-1,6-glucosidase examined except in one family in which an unexpected low affinity of the enzyme to glycogen was found. The observation that both amylo-1,6-glucosidase- and phosphorylase-deficient red blood cells retain the capacity to incorporate glucose into glycogen indicates that glycogen synthesis in erythrocytes proceeds along the UDPG glycogen synthetase pathway and is not a result of a reverse activity of any of the degradative enzymes.