Immunohistochemical staining of brain tissues from patients with Alzheimer-type dementia (ATD) with an anti-GD1a ganglioside monoclonal antibody is described. This monoclonal antibody labeled some myelinated nerve fibers in brain tissue from a non-demented control subject, in which the staining was distributed preferentially in the cerebral white matter. In brain tissue from ATD patients, some senile plaques (SPs) were also immunostained, with the strongest staining in the hippocampal subiculum, where most of the SPs appeared as clusters of dots. When the immunohistochemical staining was compared with a methenamine silver stain (MS stain), these immunopositive dots were found to be argyrophilic dystrophic (or degenerating) neurites. No amyloid deposits, neurofibrillary tangles (NFTs) or neuropil threads were immunostained. In this study, we used sections cut from formaldehyde-fixed brain samples with a cryostat and pretreatment of the sections with chloroform was essential to obtain positive immunostaining. Gangliosides have been demonstrated to possess some neurotrophic activity and to be localized on cell surface membranes. The localization of the GD1a ganglioside observed in dystrophic neurites suggests that such neurites accumulate a membranous component. In addition, the accumulation of the GD1a ganglioside in SPs suggests it may contribute to SP formation.