Staining patterns for monoclonal antibodies produced against the rabbit olfactory bulb were studied from embryonic day 14 up to 30 days after birth in the rabbit olfactory receptor neurons. One monoclonal antibody, 112D5, stained all of the receptor neurons in the olfactory epithelium and vomeronasal organ during development. The other, 114G12, showed a unique gradual expression in the olfactory receptor neurons. 114G12-Positive cells first appeared in the epithelium of the embryonic day-17 fetus. At embryonic day 25 or 26, 114G12-positive cells were situated in the superficial receptor cell layer. The arrangement in the positive and negative receptor neurons was 'superficial-positive' and 'deep-negative'. Thereafter, a gradual increase in the number of 'superficial-positive' cells was accompanied by a decrease in the 'deep-negative' cells. These changes continued until postnatal day 30. The negativity of staining for monoclonal antibody 114G12 in the deep compartment was retained in the adult rabbit. The supporting cells and basal cells were monoclonal antibody 114G12-negative throughout development. This unique developmental pattern suggests that monoclonal antibody 114G12 is a useful probe for studies on neurogenesis in adult animal. In contrast to the olfactory epithelium, the vomeronasal organ was monoclonal antibody 114G12-negative throughout development. Thus monoclonal antibody 114G12 makes a molecular distinction between the olfactory receptor neurons and vomeronasal system.