Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal bone marrow disorder, resulting from an acquired, somatic X-linked mutation of the PIG-A gene in an hematopoietic stem cell. Absence of PIG-A function in a cell prevents synthesis of the glycosylphosphatidylinositol (GPI) moiety, which anchors many different types of proteins to the cell membrane. Intravascular red cell destruction, the hallmark of the disorder, is caused by susceptibility of the abnormal erythrocyte to complement-mediated lysis; this sensitivity is due to lack of CD59, a potent inhibitor of the late components of complement and reactive lysis. In vitro studies from this laboratory have demonstrated transfer of GPI-linked proteins, CD55 and CD59, from normal to deficient cells and transfer is associated with resistance to hemolysis. Patients with PNH frequently require transfusion as their standard care. In addition, patients with all blood groups requiring transfusion will often receive compatible group O blood. Group O blood is prevalent in blood bank inventories; and red cell survival after transfusion is equal to that after transfusion of "in group" blood. The purpose of this study is to examine protein transfer of GPI-linked proteins from transfused cells to deficient cells obtained from patients with PNH. Patients with group A(1) blood will receive compatible group O blood so that donor and recipient blood cells can be discriminated. Flow cytometric studies will be performed subsequently to determine if transfer of GPI-linked protein to patients' cells has occurred.