The neurotrophin family of trophic factors influences survival and function of neurons in both the peripheral and central nervous systems. Critical information regarding physiological function of these factors may be gained by examining their localization in the brain. Here we report the immunocytochemical characterization of antisera directed against brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin 4/5. These antisera provide important tools to localize the bioactive neurotrophin proteins. Correspondence between protein distribution and previously determined messenger RNA expression was observed in some brain regions, such as hippocampus and cortex. However, neurotrophin proteins were also detected in neurons which have no apparent corresponding messenger RNA, indicating that the proteins may be transported from the sites of synthesis in certain populations. Immunocytochemical double-labelling analysis also indicated that a sub-population of neurotrophin-positive cells were labelled with an astrocyte marker (glial fibrillary acidic protein) as well, demonstrating that trophic molecules are localized to glial cells as well as neurons in vivo. Thus, the use of antisera specific for individual neurotrophic factors has indicated potential cellular sites of action.