In this study, blood is collected from healthy subjects, processed into packed red blood cell units, and either immediately afterwards or 40-42 days later the packed red blood cells are labeled with biotin. The biotin-labeled red blood cells are then re-infused back to the donor (autologous transfusion). Blood samples are then taken from the subject every week for up to 70 days (10 weeks) to track survival of the labeled red blood cells.