The characteristics of the human olfactory mucosa were studied immunohistologically. Regular, zonal distribution of the supporting cells, multilayered olfactory receptor cells and basal cells was commonly found in the olfactory mucosa of the human fetus. In contrast, most of the olfactory mucosa in the adult varied to some extent. In the relatively thick, slightly degenerated olfactory mucosa, olfactory marker protein positive receptor cells were arranged irregularly. The most common evidence for variation was the decrease or disappearance of the olfactory receptor cells. Serous-type lactorferrin-containing glandular acini were characteristically found beneath degenerated epithelium. Islands of respiratory epithelium were also seen. The ductules of the Bowman's glands were distended and the openings of these ductules were wide. There was invagination or epithelial cell processes into the glandular lumina. These findings suggest that the epithelial cells of Bowman's glands play an important role in the regeneration of the human olfactory mucosa.