Using a polyclonal antibody against postnatal cerebellar cells, we have isolated a new, brain-specific member of the lipid-binding protein family (BLBP). Members of this family, such as cellular retinoic acid-binding protein, have been shown to carry small hydrophobic signaling molecules between cellular compartments. The expression of BLBP is spatially and temporally correlated with neuronal differentiation in many parts of the mouse CNS, including postnatal cerebellum, embryonic spinal cord, and cerebral cortex. In situ hybridization and immunocytochemistry show that BLBP is transiently expressed in radial glia in both the embryonic ventricular zone and the postnatal cerebellum. Subcellular localization studies by immunoelectron microscopy demonstrate that BLBP is present in the nucleus as well as the cytoplasm. Affinity-purified anti-BLBP antibodies block glial and neuronal differentiation in primary cell cultures, but have no effect on cell proliferation or adhesion. Based on these results, we propose that BLBP is required for the establishment of the radial glial fiber system in developing brain, a system that is necessary for the migration of immature neurons to establish cortical layers.