The presence of ganglio-N-tetraosylceramide (asialo GM1)-positive cells in mouse cochlea was tested for using an immunohistochemical method with an anti-asialo GM1 antibody which reacts with natural killer cells and macrophages in mouse. Only a few asialo GM1-positive cells were present on the surface of the scala tympani of the basal turn near the collecting venules. Transmission electron microscopic study showed the internal structure of these cells to be that of macrophages. These findings suggest that asialo GM1-positive cells function as resident type macrophages against invasion by pathogens of the mouse cochlea.