1. Spleen explants of transfustion-induced polycythemic mice were incubated in vitro with and without erythropoietin.
2. After 24 hours incubation with the erythropoietin, immature large erythroblasts appeared, whereas the mature small erythroblasts were first observed after 48 hours incubation.
3. Marked radioiron incorporation into the heme was observed after 48 hours incubation with erythropoietin.
4. The control incubations without erythropoietin did not show these findings.
5. These data strongly suggest that erythropoietin induced the differentiation of erythroblasts from stem cells in vitro.