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The cells expressing immune response associated antigen (Ia) were investigated in the nervous system of the twitcher mouse (an authentic murine model of globoid cell leukodystrophy in humans). With immunocytochemistry using a monoclonal antibody against Mac-I antigen, many Mac-I immunoreactive cells (Mac-I positive cells) were detected in the central as well as the peripheral nervous systems (CNS and PNS). In the CNS, Mac-I positive cells in the gray matter showed cellular morphology of ramified microglia with delicate cellular processes, while in the white matter Mac-I positive cells were more plump in shape. Ia expressing cells (Ia positive cells) were also largely confined to the white matter. About 10% of the Mac-I positive cells were Ia positive. The Ia and Mac-I positive cells were slender and spindle shaped, and morphologically similar to Ia positive cells in the peripheral nerves while the cells expressing Mac-I only were more plump in shape. With immunolectron microscopy, however, both slender Ia positive and plump Ia negative and Mac-I positive cells revealed electron lucent cytoplasmic vacuoles containing characteristic tubular inclusions of globoid cell leukodystrophy. The results suggest that Ia positive cells are a subset of macrophages in the CNS. Whether Ia expression was induced to “endogenous” microglia or whether Ia expressing cells were exogenous cells infiltrated in the CNS in response to pathological lesions is yet to be determined.