The immunohistochemical characteristics of the human olfactory system were (OMP). OMP was detected in the olfactory receptor neurons and processes extending from the olfactory neuroepithelium to the olfactory bulb. The olfactory receptor cells located close to the epithelial surface also contained OMP. In severely degenerate regions, only a few OMP-containing cells were observed. Differences in OMP-staining intensity were noted among the olfactory receptor cells. The thick neuroepithelium. Proliferating olfactory neuroepithelium contained OMP reactive and nonreactive olfactory receptor cells. The presence of OMP reactive and nonreactive olfactory neurons indicates the coexistence of two functionally different phases of olfactory neurons. These findings suggest that continuous cell turnover is occurring in human olfactory neuroepithelium.