A monoclonal antibody against chicken Thy-1 glycoprotein was utilised in an indirect binding assay and an immunohistochemical technique to investigate the developmental appearance of Thy-1 in the chicken nervous system. The largest increase of Thy-1 levels in chicken forebrain during embryonic development coincided with the major period of neuronal development and synapse formation but preceded the major period of myelination. Immunohistochemical localisation studies on sections of chicken cerebellum, retina and spinal cord during this period showed that Thy-1 first appeared in regions of tissue which contained differentiated synapses of axons; however, not all such areas were Thy-1 positive. Areas of tissue rich in actively dividing neuroblasts or postmitotic undifferentiated neurons, such as the external granular layer of the cerebellum, showed very little staining. The disappearance of Thy-1-specific staining with age observed in the white matter of sections of cerebellum and spinal cord was due to the masking effect of myelination and not to a loss of Thy-1 from neuronal membranes.