In 6 cases initially showing a symptomatology of erythroleukaemia, the proliferation of erythroblasts was studied by combined Feulgen cytophotometry and autoradiography after labelling with 3H-TdR in vivo. A proliferation disturbance was observed consisting of an accumulation of diploid and unlabelled cells and a decreased proportion of cells in S. This defect occurred within the basophilic cell compartment in one case and within the early polychromatic cell compartment in all 6 cases. The results indicate the existence of out-of-cycle cells, which may be responsible for the inefficacy for erythropoiesis in this disease. The defect was present in all 4 cases with acute erythroleukaemia. In one of the two cases showing no deterioration of the physical and haematological condition over a period of years, an additional defect was observed, consisting of a striking accumulation of tetraploid unlabelled cells. Therefore, the technique used may be suitable as a diagnostic tool for evaluation of new types of ineffective erythropoiesis.