The occurrence of gangliosides on nerve cells of the developing retina was studied by fluorescence-activated cell analysis and sorting, using fluorescent cholera toxin as marker. This toxin binds to GM1; neuraminidase converts several other gangliosides into GM1. Without pretreatment by this enzyme weak binding of toxin is detected at later stages of development, whereas pretreatment leads to considerable toxin binding at earlier stages. The number of cells binding toxin as well as the amount bound per cell increase with developmental age of the retina. Cells binding a given amount of toxin vary strongly in size. Cell sorting was used to separate postmitotic cells from proliferating cells. Proliferating cells have little binding capacity, while postmitotic cells bind relatively large amounts of toxin. Localization of gangliosides which bind toxin in the developing retina was studied in cryostat sections. At an early stage (day 6) toxin binding is localized in the inner layer of the developing retina which contains the ganglion and other postmitotic cells, but is not found in the outer layer of matrix cells. At later stages complex staining patterns evolve with binding predominantly in the nerve fiber layers.