The in vivo cloning method for hemopoietic cells was applied in experiments aimed at analysis of the regulation of erythropoiesis in the embryonic as compared to adult mouse. Transfusion-induced polycythemia caused complete suppression of erythroid clones otherwise produced by bone marrow from adult donors. Fetal liver cells, on the other hand, did produce erythroid clones in the spleens of polycythemic recipients, indicating that fetal hemopoietic cells undergo erythropoietic differentiation under conditions of a severe suppression of erythropoietin. The earlier the embryonic stage of the donor liver cells, the less susceptible were the erythroid clones produced by these cells in polycythemic recipients. There is, thus, a correlation between the in situ duration of the embryonic pattern of control, and that manifested in the clones produced by embryonic cells. In all experiments reported in the present study with clones produced by fetal cells, the suppression was more intense in female polycythemic recipients than in males. This observation is discussed in relation to the possible differences in mechanism of regulation between adult and fetal erythropoiesis.