The presence of olfactory marker protein (OMP) in the olfactory tissue of rats, gerbils, and humans was demonstrated with goat antiserum to rat OMP. In control studies on rat neural tissue, OMP was found to be located in the olfactory receptor cells, nerves, and bulbs. Likewise, staining of OMP was found in the olfactory tissues of gerbils. Olfactory marker protein reactivity was also demonstrated in human olfactory receptor cells, nerves, and bulbs. The reactivity of OMP in human olfactory neurons to goat antiserum has allowed determination of the exact junction of olfactory and respiratory epithelia immunohistochemically.