A panning method has been developed to enrich Langerhans cells (LC) from murine epidermis. In standard culture media, the enriched populations progressively lose viability over a 3-d interval. When the cultures are supplemented with keratinocyte-conditioned medium, LC viability is improved and the cells increase in size and number of dendritic processes. Accessory function, as monitored by stimulating activity in the mixed lymphocyte reaction (MLR), increases at least 10-20-fold. The conditioned media of stimulated macrophages and T cells also support the viability and maturation of cultured LC. A panel of purified cytokines has been tested, and only granulocyte/macrophage colony-stimulating factor (GM-CSF) substitutes for bulk-conditioned medium. The recombinant molecule exhibits half-maximal activity at 5 pM. Without activity are: IL-1-4; IFN-alpha/beta/gamma; cachectin/TNF; M- and G-CSF. A rabbit anti-GM-CSF specifically neutralizes the capacity of keratinocyte-conditioned medium to generate active LC. However, GM-CSF is not required for LC function during the MLR itself. We conclude that the development of immunologically active LC in culture is mediated by GM-CSF. The observation that these dendritic cells do not respond to lineage-specific G- and M-CSFs suggests that LC represent a distinct myeloid differentiation pathway. Because GM-CSF can be made by nonimmune cells and can mediate the production of active dendritic cells, this cytokine provides a T-independent mechanism for enhancing the sensitization phase of cell-mediated immunity.