Partial, unilateral olfactory nerve section was performed in mice, and the behavior of the olfactory marker protein (OMP) studied, after this experimental manipulation, with the peroxidase-antiperoxidase method. The protein, which in normal mice is present only in mature olfactory sensory neurons, after unilateral lesion of the fila olfactoria was observed in mitral and tufted cells of both olfactory bulbs. Positive elements, present at 5 days postperative, increased in number up to 30 days and some could still be detected at 60 days. The functional implications of this finding are briefly discussed.