The role of host hematopoietic stem cells in the formation of tumor colonies in the spleen of (C57BL/6 X DBA/2) F1 mice after grafts of spleen cells from Friend virus (FVP)-infected donors has been investigated. Hematopoietic stem cell compartments of recipient mice were destroyed by Myleran treatment or gamma-ray irradiation. A single injection of Myleran reduced the pluripotent hematopoietic stem cells (CFU) and the erythropoietin responsive cells (ERC) in polycythemic mice to around 1% of that of controls. Repeated injections of erythropoietin (EPO) restored the erythropoietic precursor cell (ERC) population. Pretreatment of polycythemic hosts with Myleran totally suppressed the tumor colony forming ability of grafted Friend virus-infected spleen cells, whereas it had no effect on tumor colonies produced by inoculation of true tumoral Friend cells. After EPO injections in such Myleran-treated recipients, with a consequent appreciable ERC repopulation, splenic colonies again occurred. Similar results were obtained in hosts whose ERC populations were damaged by irradiation. These data strongly suggest that splenic colonies result from the proliferation of the host cells transformed by virus released by Friend virus-infected cells and not from the proliferation of donor tumor cells.