Chicken myeloid cells transformed by the v-myb-or v-myc-containing leukemia viruses, E26 and OK 10, respectively, require chicken myelomonocytic growth factor (cMGF) for proliferation in vitro. Upon superinfection with retroviruses carrying oncogenes of the src gene family, these myeloid cells acquire the ability to grow in the absence of exogenous cMGF. Conditioned medium prepared from superinfected E26 cells contains a growth-stimulating activity similar in biological and immunological properties to cMGF. This activity is reduced by more than 80% following absorption of conditioned media with antiserum against cMGF. Incubation of superinfected E26 cells with an immunoglobulin fraction of antiserum against cMGF inhibits their proliferation, indicating that the cells are dependent on the secreted factor. We conclude that viral oncogenes of the src family can induce chicken myeloid cells to produce a cMGF-like factor(s) that stimulates proliferation of these cells in an autocrine fashion.