RBC microvesicles increase following transfusion in patients receiving washed RBCs. Our previous data demonstrated that transfer from RBC microvesicles was much more effective than cell-to-cell transfer. We examined the number of circulating RBC microvesicles in healthy controls and following transfusion of washed and unwashed RBCs. Gating was performed to include a microvesicle population that was smaller than erythrocytes and staining with antiglycophorin (top panel). Although the number of RBC microvesicles increased following unwashed RBCs (patient 2), there was a marked increase following transfusion of washed RBCs (patient 1) where the number of microvesicles increased from 10 microvesicles/μL to 1000 microvesicles/μL.