Paroxysmal nocturnal hemoglobinuria (PNH) is a rare disease that is caused by a monoclonal stem cell defect. The affected cells lack the carbohydrate linkage between phosphatidylinositol and a group of membrane proteins of which three protect the cell against complement lysis. The absence of these three proteins, DAF(CD55), C8BP and HRF20(CD59), makes cells from the erythropoiesis, thrombopoiesis and myelopoiesis extensively sensitive to complement attack and affected patients suffer from intravascular hemolysis, thrombosis and increased susceptibility to infections. In this study we describe a swift and specific assay for the detection of CD55- and CD59- erythrocytes, which is suitable for screening of possible PNH patients.