A hemopoietic cell line FDC-P1 that requires either IL-3 or GM-CSF to survive and proliferate was infected with retroviruses that expressed either c-fms, which encodes the receptor for M-CSF, or v-fms, which is an oncogenic derivative of c-fms. The expression of c-fmsallowed FDC-P1 to grow in the absence of IL-3 or GM-CSF provided that M-CSF was present. The M-CSF did not, however, induce macrophage differentiation. The expression of v-fmsallowed FDC-P1 to grow in the absence of any added hemopoietic growth factors, including M-CSF, although the addition of M-CSF enhanced v-fmsactivity. V-fmscell lines grew to a higher cell density in suspension and were tumorigenic.