Olfactory marker protein (OMP) is a 19-kD acidic protein found throughout the cytoplasm of mature olfactory receptor neurons (ORNs). Its function remains unknown. Following olfactory bulbectomy, the proportion of ORNs mature enough to express OMP declines greatly. However, in the few remaining mature ORNs, it has been observed that the intensity of OMP immunoreactivity (IR) appears to increase over that of ORNs on the unoperated side. We have now investigated this phenomenon quantitatively in rats subjected to unilateral olfactory bulbectomy. Results show that at all postbulbectomy survival periods examined quantitatively (3 days to 6 months), a significant decrease (19-37%) occurs in the transmission of incident light through OMP(+)-ORNs in bulbectomized versus unoperated olfactory epithelium (OE). Further, we also observed a consistent side-to-side difference in OMP IR in control unoperated animals. Possible explanations for these observations and their relation to the still unknown function of OMP are discussed. To test the possibility that OMP might serve a mitogenic role in the OE, recombinant OMP was added to organotypic explant cultures of fetal olfactory mucosa. Addition of OMP resulted in a dose-dependent increase in the density of bromodeoxyuridine-positive cells in the cultures, with a 50% increase occurring at the plateau OMP concentration of 25 pM.