Monoclonal antibody A2B5, clone 105, obtained from a mouse immunized with 8-day chicken embryo retinas, reacts with most neuronal cells in chicken retina as revealed by immunofluorescence studies [G.S. Eisenbarth, F.S. Walsh, and M. Nirenberg (1979), Proc. Natl. Acad. Sci. USA 75: 4913-4917]. The antibody binds to many antigens, presumably gangliosides, present in ganglioside fractions from chicken brain, chicken retina, and human brain, as detected by autoradiography of thin-layer chromatograms. Most of the antigens, which are found in the mono-, di-, tri-, and polysialoganglioside fractions, do not correspond in chromatographic mobility to any of the major gangliosides of these tissues, as revealed by orcinol reagent. Apart from the fact that only one neuraminidase-labile sialyl residue is required for binding, the carbohydrate sequence recognized by the antibody is not known. There is a qualitative and quantitative change in the ganglioside antigens in chicken brain and retina during development. The less sialylated antigens appear first, followed by the more sialylated ones. The adult tissues contain little ganglioside antigen.