An antibody prepared against adult canary brain, 40E-C, stains ventricular zone cells that send long, unbranched processes into the forebrain parenchyma. We identify these cells as radial glia. The same antibody also stains a subset of brain astroglia and reacts with nonbrain material such as mesenchyme, Sertoli cells, and the Z-line of muscle. A weaker reaction is given by erythrocytes and some endothelial cells. 40E-C also reacts with the radial glia of the developing rat brain but fails to show any such glia in adult rodent brain. Western blot analysis shows that this antibody recognizes vimentin, a molecule shared by all 40E-C-positive cell types. We believe that the presence of radial glia in the adult avian forebrain and their apparent absence in mammals is related to neurogenesis in adulthood, which occurs in birds and much less or not at all in mammals. In addition, the presence of radial glia in adult birds may also relate to other, still-hypothetical, differences in the physiology of adult avian and mammalian brains.