The signal transduction system of erythropoietin (Epo) and the accompanying molecular control mechanism of proliferation and differentiation of erythroid progenitors remains largely unknown. In this study, the effect of Epo on the expression of nuclear oncogenes was investigated in two murine cell lines which respond to the hormone in different ways: ELM-1-1 cells proliferate independently of Epo, but differentiate in response to the hormone, while the growth of DA-IER cells is absolutely dependent on Epo or interleukin (IL) 3. The cell lines were stimulated with Epo or IL-3, and total RNA was extracted. Then expression of nuclear proto-oncogenes (c-myc, c-fos and c-myb) was analyzed by northern blotting. The change in c-fos expression observed during the first two h following stimulation with either stimulant were common to both cell lines; a rapid adn temporary increment. Before stimulation, c-myc adn c-myb were strongly expressed in both lines. No apparent change in c-myc expression was observed during the first two h of stimulation, while c-myb expression in ELM-I-1 cells was slightly reduced 1 h after stimulation with Epo but not with IL-3. Three daays after stimulation with Epo. but not with IL-3, only ELM-I-1 produced hemoglobin and expressed a lower amount of c-myb mRNA. These data suggest the importance of c-fos sin the early signaling system of Epo. and the involvement of c-myb in erythroid differentiation but not in proliferation.