The Hu paraneoplastic antigen is an RNA binding protein important in the development and maintenance of neurons. Hu has homology with the product of Elav, a gene that is essential for Drosophila visual system development. Using immunohistochemical staining, the Hu antigen was identified in most retinal neurons but was not detected in the photoreceptors. Hu is, therefore, not essential for the maintenance of all differentiated neurons. Hu immunolabeling was consistently found in glia in the optic nerve. The presence of the Hu antigen in a subset of glial cells emphasizes the heterogeneity of glial phenotype and raises the possibility that these cells may retain properties of undifferentiated neural stem cells.