Astrocytes derived from adult bovine corpus callosum contain large amounts of mitogenic activity for capillary endothelial cells and astrocytes. On the basis of radioimmunoassay, bioassay and immunoneutralization studies, 99.5% of this activity consists of basic fibroblast growth factor (bFGF). Immunoblot and gene expression analysis are also consistent with the synthesis of bFGF. The remainder of bioactivity [0.5%] is accounted for by acidic fibroblast growth factor (aFGF). These data suggest that astrocytes are a source of brain-derived bFGF and to a much lesser extent also of aFGF. Considering that bFGF has been proposed to play a role in the development and differentiation of the nervous system through its angiogenic and neurotropic properties, its presence in astrocytes provides a rationale for the complex regulatory and neurotropic functions attributed to these cells. Also, the ability of bFGF to stimulate the growth of astrocytes themselves suggest a role for the mitogen in normal and aberrant proliferation of astrocytes.