Olfactory neuronal progenitor cells were partially purified from the olfactory epithelia of 14.5-day-old mouse embryos by an immuno-killing method with an antibody against neural adhesion molecule (N-CAM). Immunostaining study showed that about 60% of the population after immunokilling, was class-III beta-tubulin-/keratin-neuronal progenitor cells and that they differentiated into N-CAM+/class-III beta-tubulin+ neurons in a chemically-defined medium within 1 day in culture. A part of them proliferated prior to their neuronal differentiation as determined by the incorporation of bromodeoxyuridine (BrdU) into neuronal nuclei. Leukemia inhibitory factor (LIF), but not ciliary neurotrophic factor or interleukin-6, increased the proportion of the BrdU-labeled neurons in vitro. These results suggest LIF promotes the neurogenesis in the olfactory neuronal progenitor cells.