In a 13-year-old patient with acute leukaemia the leukaemic blast cells, the nucleated red cells and the megakaryocytes were studied by cytophotometrie determination of the DNA content and autoradiographic labelling with 3H-TdR invitro. In the preleukaemic phase a striking proliferation defect in the early polychromatic erythroblasts was observed, consisting of an accumulation of cells in G1 and a decreased proportion of cells in S. In the megakaryocytes low DNA values from 2c-8c as compared to 4c-32c in normal megakaryocytopoiesis, and a decreased number of labelled cells between the ploidy stages was observed, indicating a severely restricted polyploidization capacity. These results suggest that in the preleukaemic stage a basic proliferation defect of acute leukaemic is becoming apparent in the so-called non-leukaemic cell systems, leading to peripheral mono- or pancytopenia.