Gelatinase A is an enzyme capable of cleaving soluble beta-amyloid protein (beta AP), and may function as an alpha-secretase to produce secretory forms of amyloid precursor protein. We examined gelatinase A immunoreactivity in the brains and posterior roots of neurologically normal, lacunar stroke, Alzheimer disease (AD), amyotrophic lateral sclerosis, progressive supranuclear palsy and myasthenia gravis cases. The gelatinase A antibody stained only microglial cells in the white matter in all the brain tissues. In AD brain, the reactive microglia located in the center of classical senile plaques, as well as in other microglial cells in the gray matter, showed no immunoreactivity. Gelatinase A in white matter microglial cells may play a role in preventing local deposition of beta AP. In the posterior root, Schwann cells had positive immunoreactivity. As with other metalloproteases, gelatinase A in Schwann cells may play an antiproliferative role.