We previously reported an increase in matrix metalloproteinase-9 (MMP-9) levels in the olfactory bulb immediately after nerve transection; however, its role remains unknown. In this study, we determined the source of MMP-9 by monitoring the infiltration of inflammatory leukocytes in the olfactory bulb after nerve transection. We used myeloperoxidase to identify neutrophils and CD68 to identify macrophages at days 1, 7, and 10. MMP-9 colocalized with neutrophils at all three time points but was not contained in macrophages. This is the first study to demonstrate that MMP-9 is associated with early inflammatory response after olfactory injury, and provides insight into mechanisms underlying olfactory injury and recovery processes.