The regulation of transferrin-receptor synthesis was studied in J2E erythroid cells induced to differentiate with erythropoietin. Nuclear run-on assays demonstrated that transcription of the transferrin-receptor gene rose markedly after erythropoietin treatment. In addition, transferrin-receptor mRNA was stabilised and this was associated with an increase in the activity of the RNA-binding protein IRP (iron regulatory protein). As a result of increased transcription and mRNA stabilisation, steady-state RNA levels increased 10-20-fold. However, despite these large increases in mRNA, translation only doubled; consequently, modest increases in total protein and surface transferrin receptors were observed. Moreover, this rise in transferrin receptors was transient, and correlated with a burst of proliferation shortly after erythropoietin treatment. The expected inverse relationship between transferrin receptors and ferritin did not occur during J2E maturation as translation of both ferritin subunits increased when transferrin-receptor mRNA levels rose. Analysis of mutant J2E clones incapable of synthesising haemoglobin revealed that surface transferrin-receptor levels were only 15-25% that of the parental erythroid line. We propose that the surface expression of transferrin receptors in J2E cells is governed by three factors: basal levels essential for normal growth in culture; elevated levels needed for haemoglobin synthesis; and a transient erythropoietin-induced increase that is required for the final burst of proliferation. It was concluded that the regulation of transferrin-receptor production in erythropoietin-stimulated J2E cells is complex and that there are several sites of control.