In 7 cases of untreated and 2 cases of treated acute leukaemia (AL) the erythropoietic cell proliferation was studied by the combined method using AFeulgen-cytophotometry and autoradiography after labelling with 3H-TdR in vivo (1 h after intravenous injection) or in vitro. A proliferation disturbance was observed, which was almost limited to the early polychromatic erythroblasts, consisting of an accumulation of diploid and unlabelled cells and a decreased proportion of cells in S. This proliferation defect was not present during complete remission. The results indicate the existence of out-of-cycle cells (G0-cells) in the erythroblasts of AL, which may be responsible for ineffective erythropoiesis in this disease.