To identify cell surface molecules that define position in the mammalian nervous system, we previously characterized the binding of two monoclonal antibodies, ROCA1 and ROCA2, to adult rat sympathetic ganglia and intercostal nerves. The binding of ROCA1 is highest in rostral ganglia and nerves and declines in a graded manner in the caudal segments. ROCA2 labels the same cells in ganglia and nerves as ROCA1, but not in a position-selective manner. We now show by immunoblot analysis that ROCA1 recognizes two antigens in membrane/cytoskeletal fractions of peripheral nerves and ganglia: (1) a Triton X-100-insoluble, 60 kDa protein and (2) a Triton x-100- insoluble, 26 kDa protein. The 60 kDa protein is expressed at higher levels in rostral than in caudal intercostal nerves, and is identified as the intermediate filament protein peripherin. In contrast, it is the ROCA1 epitope on the 26 kDa protein, and not the protein itself, that is preferentially visualized immunohistochemically in rostral nerves and ganglia. We suggest that the ROCA1 epitope on the 26 kDa protein is masked in sections of caudal nerves and ganglia. Amino acid sequence data obtained from the affinity-purified 26 kDa protein indicate significant homology with human CD9, a cell surface protein implicated in intercellular signaling in hematopoietic cells. These results suggest that intermediate filament gene expression and epitope masking on the cell surface may be involved in functions related to position in the nervous system.