Erythropoiesis of two patients in the early stage of acute leukaemia and two patients with refractory anaemia and hypercellular bone marrow (preleukaemia?) was studied with the cytophotometric-autoradiographic method. Megaloblastoid erythroblasts show a decreased proliferative activity in comparison to the morphologically normal cells and are in the maturation stage of E4 (early polychromatic normoblasts) mainly in the Gt-phase; they are therefore largely comparable to the megaloblastoid erythroblasts in erythroleukaemia. Erythroblasts in preleukaemia with nuclear abnormalities are found in a high percentage in the G2-phase or are unlabelled with a DNA content of between diploid and tetraploid value. They show a similar proliferative behaviour to the megaloblasts in pernicious anaemia. Early polychromatic erythroblasts arrested in G2-phase can differentiate without mitosis into tetraploid mature erythroblasts (E5). They can divide elsewhere endomitotically, produce binucleated Es or take up the DNA synthesis and become polyploid. E4 with nuclear abnormalities do not proliferate and are mainly found in the premitotic phase.