DNA synthesis of erythroblasts in congenital dyserythropoietic anemia (CDA) type I and II was studied. In CDA type I, hypertetraploid and octoploid cells are present in all stages of maturation. In polychromatic normoblasts, only a few labelled cells are observed and the majority of cells show DNA values in the pretetraploid range. The results suggest a disturbance of mitotic division which becomes apparent already in the basophilic stage, and an arrest of DNA synthesis in the more mature nucleated red cells, dependent from cytoplasmic hemoglobin concentration. In CDA type II, a shift of the G1/S ratio was observed in mononucleated and binucleated polychromatic cells consistent with repression of DNA synthesis-induction earlier than normal. The binucleated cells are probably the result of normal karyokinesis not followed by cytoplasmic division