ABSTRACT The role played by neurotrophin-3 during the development of quail sensory ganglia was investigated using a mono-clonal antibody that specifically blocks the biological activity of this neurotrophin. Neutralisation of neu-rotrophin-3 was initiated during completion of gangliogen-esis. Neuronal cell counts indicate that about 30% of the neurons normally present in either the placode-derived ganglion nodosum or in a leg-innervating, neural crest-derived dorsal root ganglion are eliminated by the antibody treatment. In both ganglia, this reduction is seen early in development, and the results obtained with the ganglion nodosum indicate that neurotrophin-3 plays an essential role already during gangliogenesis. Neuronal numbers are also compared with those obtained after treatment with a monoclonal antibody to nerve growth factor, used either alone or in combination with the neurotrophin-3 antibody.