Phosphatidylserine (PS), which is normally localized in the cytoplasmic leaflet of the membrane, undergoes externalization during aging or trauma of red blood cells (RBCS: ). A fraction of this PS is shed into the extracellular milieu. Both PS externalization and shedding are modulated by the oxidative state of the cells. In the present study we investigated the effect of calcium (Ca) flux on oxidative stress-induced membrane distribution of PS and its shedding and on the membrane composition and functions. Normal human RBCs were treated with the oxidant t-butyl hydroperoxide, and thalassemic RBCs, which are under oxidative stress, were treated with the antioxidant vitamin C or N-acetylcystein. The intracellular Ca content was modulated by the Ca ionophore A23187 and by varying the Ca concentration in the medium. Ca flux was measured by Fluo-3, PS externalization and shedding were measured by quantitative flow cytometry and membrane composition was measured by (1)H-NMR analysis of the cholesterol and phospholipids. The results indicated that increasing the inward Ca flux induced PS externalization and shedding, which in turn increased the membrane cholesterol/phospholipid ratio and thereby increased the RBC osmotic resistance. In addition, these processes modulated the susceptibility of RBCs to undergo phagocytosis by macrophages; while PS externalization increased phagocytosis, the shed PS prevented it. These results indicate that PS redistribution and shedding from RBCs, which are mediated by increased calcium, have profound effects on the membrane composition and properties and, thus, may control the fate of RBCs under physiological and pathological conditions.