The role of granulocyte-macrophage colony-stimulating factor (GM-CSF) and Flt3 ligand in the in vivo development of Langerhans cells (LC) was assessed, considering both the steady-state levels of LC in the epidermis and the rate of LC recovery after depletion following lipopolysaccharide (LPS) treatment. The density of LC was determined by counting following IA-specific immunofluorescent staining of epidermal sections from mouse ears. LC levels were compared in beta common chain receptor null (beta c(-/-)) mice that fail to respond to GM-CSF interleukin-5 (IL-5), in GM-CSF transgenic mice with elevated GM-CSF levels, and in mice given daily injections of Flt3 ligand. In the steady state, LC levels were increased in GM-CSF transgenic mice and present at reduced levels in beta c(-/-) mice but unchanged in Flt3 ligand-injected mice. Application of LPS to the ears of control BL/6 mice led to an approximately 70% reduction in LC 4 days later, with recovery beginning by day 8 and a return to normal levels by 2 weeks. This recovery was significantly delayed in beta c(-/-) mice and unchanged in Flt3 ligand-injected mice. These results suggest that GM-CSF (but not Flt3 ligand) enhances recruitment/maturation of LC even though GM-CSF is not essential for their formation.