The growth of cord blood and bone marrow myeloid mononuclear cells in response to single and multiple growth factors has been examined. Single factors such as IL-3, IL-6, GM-CSF, and stem cell factor did not optimally stimulate growth of cord blood CFU-GM. However a synergistic stimulation was observed when the factors were combined. This effect was seen in both the presence and absence of added fetal calf serum.