Decay-accelerating factor (DAF), a complement-regulating glycoprotein, has been shown to be expressed on hematopoietic progenitors and their progeny cells in normal individuals but not on abnormal cells in patients with paroxysmal nocturnal hemoglobinuria (PNH). Fluorescence histograms of bone marrow cells showed the heterogeneity of DAF expression on their cell membranes, suggesting mixed hematopoietic cell populations in PNH. We have previously shown that DAF is a maturational protein expressed on normal hematopoietic cells. In order to elucidate the relationship between DAF expression and cell maturity in PNH, we fractionated bone marrow cells according to amount of DAF and cultured these cells in methylcellulose for clonal assay of erythroid burst-forming units (BFU-E) and granulocyte-macrophage colony-forming units (CFU-GM), followed by reanalysis of DAF expression on their progeny. The matured cells from the bursts/colonies in cultures with DAF-negative PNH marrow cells had no or little DAF, but those grown from DAF-positive PNH progenitors showed nearly as much of this factor as those grown from normal progenitors. These results clearly indicate that there are at least two distinct populations of hematopoietic progenitors with respect to the membrane expression of DAF and that abnormalities occur at the level of the stem cell in PNH.