Affected erythrocytes in patients with paroxysmal nocturnal hemoglobinuria (PNH) have been detected as complement-sensitive cells by the complement sensitivity assay. Decay-accelerating factor (DAF), a molecular mass of 70 kDa complement-regulatory membrane glycoprotein, has been reported to be deficient on affected PNH blood cells. In the present study, DAF on erythrocytes from 12 patients with PNH were stained by an immunocytochemical method and the ratio of DAF+ erythrocytes was compared with their laboratory data concerning hemolysis, almost all normal human erythrocytes stained positively for DAF. In contrast, various percentages of DAF+ erythrocytes were found in the patients with PNH. The percentages of DAF+ erythrocytes correlated positively with the total amounts of DAF on erythrocytes measured by an enzyme-linked immunosorbent assay (ELISA), negatively with % hemolysis in Ham's test and in sucrose hemolysis assay and with percentages of PNH type III (PNH-III) erythrocytes. In some patients with PNH, subpopulations of erythrocytes weakly-positive for DAF were demonstrated in addition to DAF- erythrocytes. The immunocytochemical method for staining DAF on erythrocytes developed in the present study is useful for the detection of affected PNH erythrocytes and applicable to further studies on membrane defects in PNH.