Growth kinetics of certain hemopoietic cells and target cells of Friend spleen focus-forming virus (SFFV) were studied in mice treated with supralethal irradiation (900 rads) and infusion of bone marrow cells, or with myleran. In supra-lethally irradiated mice, the presence of SFFV and SFFV-induced tumor cells corresponded with that of normal colony-forming cells but with somewhat different growth rates. In myleran-treated mice, however, SFFV and SFFV-induced tumor cells were present when normal colony-forming cells were not observed. Thus, the results indicate that normal colony-forming cells were probably not the target cells for SFFV. The results are consistent with the view that at least two or possibly three types of target cells were present; the target cells for SFFV replication might be different from those for SFFV-induced tumor cells.