We investigated the developmental expression of the synaptic antigen B16 in the outer and inner plexiform layers in the retina. We compared the expression of this protein with published accounts of retinal ribbon synaptogenesis. The time course of B16 expression was found to be similar, if not identical, to the time course of synaptogenesis in the photoreceptor terminal, supporting our idea of the synaptic nature of the B16 protein. This study provides further evidence which suggests that the B16 antigen is a ribbon associated protein. The morphology, location, and developmental expression of this antigen is coincident with the structure and development of the ribbon synaptic elements located in both the photoreceptor and bipolar terminals. The emergence of the B16 protein was also examined in retina from mice homozygous for the retinal degeneration (rd) mutation which affects photoreceptor synaptogenesis. The B16 expression in the rd retina parallels that in the wild-type retina at early stages of development. However, significant changes in the morphology and staining pattern of the B16 antigen in the rd retina occurring at later postnatal stages are consistent with the degeneration observed in the photoreceptor terminals.