Deficiency of decay-accelerating factor (DAF) occurs in blood cells in paroxysmal nocturnal hemoglobinuria (PNH), characterized by an unusual susceptibility to hemolysis by complement activation. This study examined DAF expression on hematopoietic progenitors from normal individuals and PNH patients using a fluorescence-activated cell sorter (FACS) with monoclonal antibodies to DAF. Nonphagocytic mononuclear marrow cells expressing different density distributions of DAF were sorted into DAF-, DAF+/-, DAF+, and DAF++ fractions. The cells from each fraction were cultured in methylcellulose and assayed for CFU-E, BFU-E, CFU-GM, and CFU-Mix. The percentages of distribution of DAF- negative normal progenitors increased in the order of CFU-E, CFU-GM, BFU-E, and CFU-Mix, whereas those of DAF-positive cells inversely decreased in this order. These results indicate that DAF expression may accompany differentiation from CFU-Mix to CFU-E. On the other hand, most progenitors in PNH patients had little, if any, expression of DAF on their cell surfaces. These findings were supported by another approach using a complement-dependent cytotoxicity method with the anti- DAF monoclonal antibodies. Abnormal expression of DAF was found on the progenitors in the bone marrow as well as on mature cells circulating in the blood in PNH.