A 120 kDa protein, which is expressed mainly on the surface of chick sensory neurons during outgrowth, was identified by monoclonal antibody 1A12. Crossreactivity studies showed that this protein was identical to axonin 1, a member of the immunoglobulin superfamily which promotes neurite outgrowth. Using the 1A12 antibody, we show that in the peripheral nervous system of the chick, axonin 1 is present on the cell bodies and processes of cutaneous and visceral neurons, but not on muscle afferents. In the central nervous system, axonin 1 is present in sensory pathways, such as fibers of the dorsal funiculi in the spinal cord and the optic pathway. However, axonin 1 is only expressed on growing nerve fibers. Late in embryonic development, it is present only on a small population of dorsal root ganglion cells, and is entirely absent on optic fibers. The disappearance of axonin 1 in the visual pathway coincides with the arrival of optic axons at the tectum, suggesting its expression is down regulated by axonal contact with its target. The localization of this protein on the surface of neuronal membranes was confirmed by EM immunohistochemistry and by labeling live nerve cells and their processes in tissue culture. The restricted spatio-temporal expression of axonin 1, together with its expression on the surface of neuronal membranes suggests that it is important for the development of sensory projections.