The administration of a Mg-deficient diet to adult rats for 4-5 wk causes anemia. The RBC are slightly smaller and flatter, and have a reduced hemoglobin component and a decreased osmotic fragility. 51Cr survival time is shortened when the affected cells are infused into normal adult rats. In vitro synthesis of heme and globin, as measured with 14C-glycine was the same as in control rats with "reticulocytosis-rich" anemia produced by bleeding or immunologically. The activity of a series of enzymes associated with energy production also failed to yield findings specific for Mg deficiency. It is suggested that the anemia of Mg deficiency is hemolytic and results from the combination of a reversible extrinsic defect and of an irreversible structural defect in the RBC.