J2E erythroid cells proliferate and differentiate in response to erythropoietin (epo), the red blood cell specific hormone. Using methylcellulose colony assays and suspension cultures we have demonstrated that nearly all the cells stimulated by epo synthesized haemoglobin. To achieve maximum production of haemoglobin J2E cells had to be treated with epo for only 6 h; hormone added subsequently did not enhance haemoglobin synthesis. Although virtually all viable J2E cells produced haemoglobin, the cells matured morphologically at different rates. Thus, upon exposure to epo J2E cells become committed to erythroid terminal differentiation but proceed in an asynchronous manner.