Olfactory ensheathing cells (OECs) are Schwann cell-like glial cells of the olfactory system that promote neural regeneration after transplantation into the injured central nervous system. Compared to the closely related Schwann cells, however, the biological characterization of OECs has remained fragmentary. This is due to the fact that the expression of OEC-specific markers is subject to complex regulation and that intricate ultrastructural analysis is essential to determine their localization. The p75 neurotrophin receptor (p75(NTR)) as the prototype OEC marker, for example, is only expressed by a minor population of neonatal rat OECs in situ. The major population carries O4-positive axonal fragments on their surface after dissociation and up-regulates p75(NTR) during culturing (Wewetzer et al. in Glia 49:577-587, 2005). In the present study, we investigated whether the cell surface determinant 27C7, defined by a monoclonal antibody to Schwann cells, is also expressed by neonatal rat OECs in situ and in vitro. Primary cell suspensions of the olfactory bulb displayed 27C7 expression of both p75(NTR)-negative and p75(NTR)-positive OECs, while immature oligodendrocytes and astrocytes were devoid of any 27C7 labeling. This together with the finding that the intrafascicular OECs of the olfactory nerves in the mucosa expressed 27C7 but not p75(NTR), suggests that 27C7 was expressed by the entire OEC population in situ. Maintenance of OECs in the absence of olfactory neurons in organotypic slice culture up-regulated p75(NTR) but did not alter 27C7 expression. It is concluded that 27C7 unlike p75(NTR) is constitutively expressed by OECs and may, therefore, be a useful marker for characterization of neonatal OECs in situ and in vitro.