In order to characterize changes in membrane‐bound immunoglobulins and complement components, red cells (RBCs) were separated into young and old populations by simple centrifugation. Old RBCs had reduced mean corpuscular hemoglobin volume, increased mean corpuscular hemoglobin concentration, and reduced sialic acid. Using radioactive anti‐ antiglobulin techniques, old RBCs were shown to have more IgG, IgM, IgA, and C3d on their surfaces than did young RBCs; there was no increase on old RBCs of C3b, factor B, C4b, or C5. Similar results were observed with RBCs strongly coated with C3d in vivo from a patient with cold agglutinin disease. RBCs taken into ethylenediamine tetraacetate, washed thoroughly in saline, and then stored for prolonged periods in Alsever's solution or kept in autologous ethylenediamine tetraacetate plasma, at 4 degrees C, showed no increase in RBC‐bound C3d with increased storage time. If, however, blood was taken into citrate‐ phosphate‐dextrose and maintained at 4 degrees C in autologous plasma, a significant increase in RBC‐bound C3d was observed in the mixed‐cell population with prolonged storage time. Order donor blood units, taken into citrate‐phosphate‐dextrose and stored at 4 degrees C as packed red cells, showed higher levels of RBC‐bound C3d in the mixed‐cell population than did units stored for a shorter time. In no case did donor unit RBCs give a positive direct antiglobulin test on serologic testing with anti‐C3d. The findings complement data already collected on membrane and cytoplasmic changes in aging RBCs and may contribute to an understanding of RBC senescence.