Paroxysmal nocturnal haemoglobinuria (PNH) is an acquired clonal disorder with a deficiency of glycosylphosphatidylinositol (GPI) anchored proteins. Homologous restriction factor 20KD (HRF20, CD59) is a GPI-anchored and major complement-regulatory protein which plays a key role in the haemolytic mechanism of PNH. We examined the differentiation stage at which the PNH abnormality occurs, by means of flow cytometric analysis of HRF20 expression. Non-phagocytic mononuclear marrow cells were labelled with anti-HRF20 monoclonal antibody and sorted into either HRF20-negative or -positive fractions. The sorted cells were cultured in methylcellulose and their progeny in the colonies or bursts were analysed for HRF20 expression. All colonies and bursts from HRF20-negative fractions remained negative, whereas those from HRF20-positive fractions were either positive or negative. The possibility of a sorting error was excluded, because the secondary colonies from the HRF20 positive primary colonies consisted of both positive and negative progeny. These results suggest that there are several stages during differentiation from early progenitors to mature cells, at which the PNH abnormality becomes manifest.