In order to investigate whether the morphological abnormalities observed in congenital dyserythropoietic anemia type III (CDA III) have a cellular or an environmental origin; BFUE from the blood of a patient exhibiting a CDA type III were grown in vitro. The progeny derived from these BFUE were subsequently studied at light and electron microscopic level. Giant multinuclear erythroblasts which represent the most prominent finding of CDA III in bone marrow were also found in culture. Nuclear clefts found in vivo were also observed by electron microscopic studies performed on the erythroblasts growing in vitro. In each erythroid colony, morphologically normal and giant multinuclear erythroblasts were intermingled. This finding indicates that the two populations of erythroblasts derive from the same defective stem cell. The studies by indirect immunofluorescence of i antigen was preferentially expressed in the immature erythroblasts as in culture from normal subjects but not in the giant mature erythroblasts. This finding suggests that the excess of i antigen expression of CDA III in vivo is rather the indirect consequence of a stimulation of erythropoiesis than result of the disease.