The phenomenon of naturally occurring neuronal death and the protein nerve growth factor (NGF) are often referred to when new, putative neurotrophic factors make their débuts in the literature. Although these factors are shown to support the survival of embryonic neuron in vitro, it is doubtful if many of them are true neurotrophic factors in the sense that, like NGF, they participate directly in the regulation of naturally occurring neuronal death in vivo.