AbstractA neuron‐survival assay with dissociated ciliary ganglion neurons was used to examine the level of neurotrophic activity in organs of the rat and chicken. All tissue homogenates enhanced neuron survival at increasing protein concentrations, although 50% survival activity (1 trophic unit, TU, per ml) occurred at distinctly different levels in the different organs. Liver, spleen, T‐cells, and submandibular gland from the rat were very low in survival‐promoting activity (0.1–1.1 TU per mg of protein). Serum and lung were slightly richer (2.4–2.8 mg). Heart, brain, and skeletal muscle stimulated survival well (6.2–13 TU/mg). Unexpectedly, the highest activities were found in the rat kidney (44 TU/mg). The activity in adult chicken organs was less varied (1.6–9.1 TU/mg). Higher trophic activities were found in the chicken embryo, particularly at day 18 of development (20–30 TU/mg). The up to 100‐fold difference in trophic level between organs should make it feasible to use subtractive hybridization, differential screening, and transient expression in eukaryotic cells for molecular cloning of the ciliary neuro