A 12-yr-old white girl with congenital dyserythropoietic anemia (CDA) type II was studied. Excessive cytoplasmic membranes (appearing like "double membranes") were found in the majority of the normoblasts. There was marked decrease in the uptake of tritiated thymidine in the binucleated normoblasts as demonstrated by radioautography. The results suggest that the cells with more severe structural cytoplasmic abnormalities and/or decreased DNA synthesis are destroyed within the bone marrow, and the circulating red cells are derived from a less abnormal population of precursors.

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