Brain topography may have its earliest expression as spatial gradients of molecules controlling the deposition of neurones and neuronal processes. In the vertebrate visual system there is evidence that the stereotyped alignment of central retinal projections relies on an initial spatially organized distribution of molecules in both the retina and its central target nuclei. We used an immunological approach to look for molecules that are so organized and produced a monoclonal antibody (JONES) which shows a pronounced dorsal to ventral gradient of binding in the rat retina throughout the period when retinal ganglion cell axons are forming topographically organized projections within the central nervous system (CNS). Binding is present throughout the radial thickness of the retinal epithelium in regions where postmitotic neurones are generated but is not associated with any consistent histological characteristic of the tissue. The antibody was shown to bind on the cell surface of freshly dissociated retinal cells, and dorsal retinal quadrants were found in vitro to have nearly twice as much antigen as ventral retinal quadrants. Initial biochemical characterization of the target epitope reveals that it is a lipid present in chloroform/methanol extracts from perinatal retina and is sensitive to neuraminidase digestion.