We describe here the use of 3 established nerve growth factor (NGF) targets (cultured neurons from dissociated chick embryo sympathetic ganglia, and chick embryo or neonatal mouse dorsal root ganglia) to investigate neuronotrophic activities in conditioned media (CMs) from: (i) chick embryo heart; (ii) purified mouse Schwann cells; and (iii) clonal rat Schwannoma RN22 cells. In chick sympathetic and mouse dorsal root ganglionic cultures, all 3 CMs supported survival of the same number of neurons as did mouse submaxillary NGF, and in most cases no increased survival resulted from concurrent administration of NGF and any one CM. NGF and CM activities were quantitated in each of the responsive cell systems. No differences were seen when either test population was used for the same agent, or when different CMs were examined on the same test cells. The CM activity, unlike that of NGF, was not blocked by even excess amounts of antiserum against mouse submaxillary NGF. The neuronotrophic activity of CMs appears to reside with macromolecular constituents. None of the CMs displayed trophic activity on chick embryo dorsal root ganglionic neurons. However, at least one of them (RN22 medium) had drastic effects on these ganglionic cells even in the presence of NGF, leaving open the possibility of a 'toxic' factor overriding putative trophic agents.