Using affinity chromatography with the monoclonal antibody 271A6, which binds selectively to telencephalic regions of the rabbit brain, we have purified a telencephalon-specific antigen to apparent homogeneity and characterized it as a membrane glycoprotein. The telencephalon-specific membrane protein (named "telencephalin") has a molecular weight of about 500,000 and is composed of four subunits each of mol. wt 130,000. Its digestion with N-glycanase reduced the subunit mol. wt by 23,000, indicating that each subunit has several N-asparagine-linked oligosaccharide chains. Immunohistochemical analysis using polyclonal antibody against the purified telencephalin shows that expression of the entire protein is restricted to the telencephalon. In addition, segment-specific expression of telencephalin was observed in all mammalian species examined (mouse, rat, guinea-pig, rabbit, cat and monkey). The telencephalon is the most rostral segment of the brain, and comprises the cerebral neocortex, paleocortex, hippocampus, septum, striatum and olfactory bulb. The present results indicate that all regions of the mammalian telencephalon express the segment-specific membrane glycoprotein, telencephalin, and suggest that telecephalin is involved in functions specific to the surface membrane of telencephalic neurons.