To examine the dividing cells in the olfactory epithelium, an experiment using a novel immunohistochemical technique with bromodeoxyuridine (BrdU) was performed. Tissue specimens were obtained from the olfactory epithelium of guinea pigs at days 7 and 21 after olfactory nerve axotomy. BrdU uptake was detected in the epithelial cell layer directly above the basal cell layer rather than in the basal cells per se. The BrdU-immunoreactive cells were found more numerously at 7 days than at 21 days after axotomy. The basal cells showed no immunoreaction to the anti-BrdU antibody on either day. The cells reacting with the anti-BrdU antibody also showed no reaction to the anti-cytokeratin antibody used to identify the basal cells. These findings indicate that the cells showing mitotic activity have characteristics different from those of basal cells, which has been considered previously to be the precursors of regenerating olfactory receptor cells.