The influence of accessory cells on the generation of neurons and neuronal survival has been studied in vitro using an immortalised, cloned cell line, Ast-1, which has many of the functional and phenotypic characteristics of cells of the astrocytic lineage. It was found that monolayers of Ast-1 cells were equivalent to monolayers of primary astrocytes in their ability to promote the generation of neurofilament positive neurons from neuroepithelial cells obtained from embryonic day 10 (E10) mice; and both were superior to NIH 3T3 cells. Ast-1 cell monolayers were also found to provide a suitable substrate for the prolonged survival (at least 3 days in vitro) of neurofilament positive neurons obtained from E17 mice, whereas neurons plated onto NIH 3T3 cells were all dead after 2 days. Medium conditioned by Ast-1 cells displayed similar biological activities to that of the monolayers: it increased the number of neurons generated from the E10 neuroepithelial cells, whether they were plated directly onto glass coverslips or onto monolayers of NIH 3T3 cells; and it increased the survival of E17 neurons plated directly onto glass coverslips. In addition, the Ast-1 conditioned medium was shown to promote the survival of the neuroepithelial cells. These results confirm that one of the mechanisms by which astrocytes or their precursors may regulate neuronal development is by secreting soluble growth factors, as has been previously documented in the case of fibroblast growth factor (FGF) (Hatten et al., 1988; Drago et al., 1991a). However, it appears in this system that FGF is not responsible for the demonstrated biological activities, and the Ast-1 action appears to be mediated by putatively novel factor(s).