To investigate structural changes in cell membranes of the Alzheimer-type dementia (ATD) brain, we immunostained for GM1 ganglioside which is a major component of the cell membrane. Our results have shown that astrocytic membranes and senile plaques (SPs) have the same immunoreactivity against the monoclonal anti-ganglioside GM1 antibody. Moreover, the astrocytic processes within the SPs were altered and their abnormal membranes seemed to contribute to the formation of SPs.