Side effects of cyclosporine treatment include anemia. Most recent studies have found that anemia may be caused by triggering of suicidal erythrocyte death (eryptosis), i.e. activation of an erythrocyte scramblase and phosphatidylserine exposure at the erythrocyte surface. Phosphatidylserine exposing cells are rapidly cleared from circulating blood by phagocytosis. Stimulators of erythrocyte membrane scrambling include cytosolic Ca(2+) and ceramide, which are increased by entry through Ca2+-permeable cation channels and by activation of a sphingomyelinase, respectively. The present study has been performed to test for an effect of cyclosporine on eryptosis. Erythrocytes from healthy volunteers were exposed to cyclosporine, and phosphatidylserine exposure (annexin V binding), cell volume (forward scatter), cytosolic Ca2+ activity (Fluo3-dependent fluorescence), ceramide formation (anti-ceramide-FITC antibody), and 45Ca2+ uptake were determined by flow cytometry and tracer flux measurements, respectively. Exposure of erythrocytes to cyclosporine triggered annexin V binding and significantly enhanced the increased annexin V binding both following glucose depletion and after hyperosmotic or isotonic cell shrinkage. However, cyclosporine significantly decreased cytosolic Ca2+ activity and did not stimulate 45Ca2+ uptake. Instead, cyclosporine transiently stimulated ceramide formation, decreased the cytosolic ATP concentration and potentiated the decline of cytosolic ATP concentration following glucose depletion. Elevated ceramide levels and ATP depletion, in turn, sensitize the erythrocytes for the eryptotic effects of Ca2+. The present observations may provide a mechanistic explanation for the anemia following treatment with this important immunosuppressive drug.