Myeloid precursors can be grown in vitro in the presence of specific growth factors; however, their expansion is limited by a competing process of terminal differentiation^1,2. Proto-oncogenes seem to be involved in cellular proliferation and/or differentiation and may also play a role in the myelopoietic process^3,4. Murine myeloid precursors which are grown in vitro with growth factors respond with augmented self-renewal upon infection with recombinant retroviruses carrying the v-myc or v-src oncogenes^5–11, suggesting a synergism or complementation between some viral oncogenes (v-onc) and certain growth factors. We now show that the combination of two v-onc genes (raf and myc) induces the selective proliferation of monocytic cells from fresh murine bone marrow (BM) in the absence of a specific growth factor supplement. Depending on the culture conditions these cells can either differentiate and cease to proliferate or grow continuously, thus mimicking the alternative pathways that can be followed by committed BM stem cells in vivo.