Although microglia-mediated cytotoxicity has been extensively investigated, little is known about the potential microglial role in neuronal and glial support. Characterization of trophin elaboration by microglia and identification of responsive populations may define novel functions. We now report that microglia/brain macrophages express neurotrophins of the nerve growth factor (NGF) gene family in vitro and in vivo, suggesting that these cells promote development and normal function of neurons and glia. Moreover, neurotrophins promote microglial proliferation and phagocytic activity in vitro. We found that microglia express neurotrophins in a region-specific manner and that within any region only subpopulations elaborate trophins. Using an antiserum specific for neurotrophin-3 (NT-3) with the microglial/macrophage marker OX-42 on postnatal day 10 in vivo, double- labeled cells were identified in the cerebral cortex, globus pallidus, and medulla; NT-3 was undetectable in OX-42-positive cells in the ependyma, the external capsule, choroid plexus, and meninges. In contrast, ramified microglia in the adult brain did not exhibit NT-3 immunoreactivity, suggesting developmental regulation of microglial NT- 3 expression. In situ hybridization studies on purified microglial cultures confirmed that only subpopulations express the NGF and NT-3 genes, substantiating the existence of microglial heterogeneity. We tentatively conclude that microglial subtypes serve trophic roles in the normal brain, in addition to exerting well documented deleterious actions in illness and injury. Microglia were also responsive to neurotrophins: brain-derived neurotrophic factor (BDNF) and NT-3 increased [3H]thymidine incorporation in vitro, and NT-3 promoted proliferation. Moreover, NT-3 induced phagocytic activity, suggesting that the factor plays a role in processes associated with cellular activation.