In plasma clot cultures fetal, newborn and adult sheep bone marrow cells responded to Ep [erythropoietin] giving rise to large numbers of colonies of erythroid cells. In fetal and newborn, but not adult, sheep the marrow produced EEC [endogenous erythroid colonies] in the absence of added Ep. The EEC-forming cells represent a transitory population of erythroid progenitors, persisting for approximately 2 mo. post-partum. The activity of these cells in the normal newborn sheep displayed a cyclic pattern, which was abolished in the presence of high circulating Ep levels. In fetal and newborn sheep, EEC formation was inhibited by prior neutralization of Ep present in culture media with anti-Ep, suggesting that as in patients with PV [polycythemia vera] the proliferation and/or differentiation of EEC-forming cells of sheep was subject to Ep control.