Erythropoietin (epo) added to liquid cultures of mouse bone marrow cells effected both the numbers of spleen colony-forming cells (CFU) in the cultures and the types of spleen colonies formed from these cells in irradiated hosts. Epo caused an increase in the number of CFU detected in cultures on the second day; this effect persisted through day 10, with the maximal increase occurring on the seventh day. The magnitude of the rise on day 7 was proportional to the amount of epo added. The increase in spleen colonies found with cells cultured in the presence of epo was due solely to erythroid colonies. After seven days in culture without epo, there was a peak of cells that formed non-erythroid colonies. This peak did not appear when the cells were cultured in the presence of epo.