The significance of the deficiency of the major complement-regulatory membrane proteins, decay-accelerating factor (DAF) and CD59, to the lysis of paroxysmal nocturnal hemoglobinuria (PNH) red blood cells was investigated. DAF and CD59 were demonstrated to be deficient simultaneously on affected PNH red blood cells (PNH-III) by two-color FACS analysis. At least in some patients with PNH, PNH-I was also revealed to be deficient partially in DAF. Purified DAF and CD59 ameliorated the complement sensitivity of PNH red blood cells, partially and completely, respectively. Functional blocking of these molecules on nomrla human red cells by monoclonal antibodies to DAF and CD59 rendered A or AB type blood cells complement-sensitive but not O or B blood type blood cells. The differences of complement-sensitivity among blood types were revealed to reside on the step of binding of C9 to C5b-8, i. e. C9 can bind to C5b-8 more on A type blood cells than on O type blood cells. We conclude that the deficiency of DAF and CD59 play a major role for the complement sensitivity of PNH red blood cells and that other factors reported to be deficient in PNH do less than these two proteins.