This research will determine if red blood cell (RBC) units from iron-deficient volunteer blood donors fail to meet U.S. Food and Drug Administration (FDA) standards for 24-hour post-transfusion recovery. Over 15 million RBC units are donated annually in the United States. Despite fulfilling all requirements for blood donation, almost two- thirds of the women, and half of the men, who are regular blood donors are iron deficient. RBCs from iron- deficient donors may be specifically damaged by refrigerated storage, thus decreasing their post-transfusion recovery and lifespan in circulation. Our overarching hypothesis is that a substantial proportion of RBC units from iron-deficient volunteer donors are suboptimal. To this end, we propose a prospective, double-blind, randomized, placebo-controlled study of iron-deficient, regular blood donors. In Aim #1, we will determine whether RBCs from iron-deficient blood donors meet the FDA standards for 24-hour post-transfusion recovery. In Aim #2, we will determine whether intravenous iron repletion improves the 24-hour recovery of a second, similarly stored, autologous RBC unit. This project will fill critical gaps in knowledge by (i) identifying an unrecognized source of impaired RBC recovery in standard products, (ii) identifying an innovative way to identify donors whose RBC units have impaired tolerance for refrigerated storage using RBC zinc protoporphyrin levels, and (iii) developing a clinical strategy to avoid these adverse effects through iron repletion. This new information will help identify ways to improve the safety and efficacy of RBC transfusions. For example, RBCs that do not circulate cannot deliver oxygen. Thus, measures to improve post-transfusion RBC recovery could lead to sustained improvements in patient outcomes, particularly in those requiring chronic transfusion, where long-term circulation of the maximal number of transfused RBCs is vital, such as patients with sickle-cell disease and β-thalassemia.