Leukemia inhibitory factor (LIF)/differentiation-inhibiting activity (DIA)/human interleukin for DA cells (HILDA) is a cytokine with biologic activities involving a variety of different types of target cells. Here we have tested LIF/DIA for possible effects on the growth and differentiation of normal human hematopoietic cells in culture. As a single agent, LIF/DIA had no effect on colony formation by CD34- positive human bone marrow cells. However, LIF/DIA was as effective as either interleukin-6 (IL-6) or granulocyte colony-stimulating factor (G- CSF) in the enhancement of IL-3-dependent colony formation of very primitive blast colony-forming cells. Studies using neutralizing antibodies against IL-6 or G-CSF demonstrated that this was not due to induction in culture of either of the other known synergistic factors for blast cell colony formation. A 1-day delay in the time course of appearance of blast cell colonies grown in the presence of LIF/DIA relative to those grown in the presence of IL-6 suggests that the different synergistic factors may operate through different mechanisms, although we cannot rule out that high doses of LIF/DIA might yield accelerated blast cell colony formation. Our findings provide evidence that LIF/DIA may play an important role, along with IL-6 and G-CSF, in the regulation of early hematopoietic stem cells.