Olfactory neuroblastoma (OLF NB) is an uncommon neurogenic tumor arising in the nasal cavity. To identify the immunohistochemical reactivity of OLF NB, we examined specimens from ten patients for the presence of neuron-specific enolase (NSE), S-100 protein (S-100) and glial fibrillary acidic protein (GFAP). Formalin-fixed paraffin sections were treated with rabbit antisera using the peroxidase-antiperoxidase (PAP) and avidin-biotin immunofluorescence methods. Results were compared with those observed in four neuroblastomas (two central and two peripheral sites), 17 medulloblastomas, four pheochromocytomas and five paragangliomas. All OLF NB were positive for NSE and S-100. NSE was uniformly distributed throughout clusters of tumor cells within tumor nodules. The S-100 reactivity was distributed in neoplastic cell nuclei and long cytoplasmic processes of cells which interconnected to form a network at the periphery of tumor cell nests. GFAP was observed only in astrocyte-like cells in one tumor. Although the S-100-positive cells in OLF NB resembled S-100-positive sustentacular cells of paraganglioma and pheochromocytoma, the network pattern was distinctive in OLF NB. The immunodiagnostic pattern of OLF NB includes the following: (a) NSE present in irregular clusters of tumor cells, (b) S-100 present in peripheral cells which form a marginal network, (c) and GFAP rarely present.