Erythropoietin (epo) induced differentiation of the J2E erythroid cell line is characterized by haemoglobin synthesis, together with morphological changes and an immediate increase in proliferation In this manuscript we have shown that the size of J2E cells decreased during differentiation and the nucleus to cytoplasm ratio was reduced appreciably. Furthermore, major ultrastructural alterations occurred-mitochondria, rough endoplasmic reticulum and Golgi apparatus decreased in size and number with maturation, while nuclei condensed considerably before extrusion. The use of mitotic indices, H-3-thymidine uptakes and flow cytometry confirmed that the immature J2E cells undergo enhanced replication shortly after epo stimulation. In addition, we demonstrated that cell division ceased as the cells entered the final stages of erythroid differentiation. Thus the J2E line provides a useful model, not only for haemoglobin synthesis, but for all aspects of erythroid terminal differentiation.