Microvesicles from stored RBC units transfused to patients express primarily CD59 on their surface. To assess the GPI-anchored protein content of erythrocyte microvesicles, we examined aliquots from 3 U blood received by a patient. Gating was performed to include a microvesicle population that was smaller than erythrocytes and staining with antiglycophorin. (A) Scattergrams of microvesicles binding to antiglycophorin and anti-CD55 and anti-CD59 showing primarily expression of CD59. (B) Stored RBC units were washed 3 times per blood bank procedure in unbuffered saline and the number and composition of microvesicles assessed in the washed and unwashed preparations. More CD59-rich microvesicles were found in the washed preparation. Microvesicle number per 10 mL washed and nonwashed blood is seen above. (C) RBC microvesicles were sorted by flow cytometry on the basis of staining with anti–CD59mAb-PE and antiglycophorin-FITC. Electron microscopy was performed on the positive preparation at × 18 000 magnification using a Zeiss model Axioskop microscope, a Zeiss camera (model Mc80 DX), with a × 60 objective.