The distribution of glucuronic acid-and-sulfate-containing carbohydrate (GSC) epitope recognized by two monoclonal antibodies, HNK-1 and 4F4, was studied by immunocytochemistry in adult mouse brain. Both antibodies recognized proteins ranging in molecular weight from 60 to above 250 kDa in Western blot but no glycolipid was recognized in the adult brain. With both light and electron microscopic study, two patterns of staining are observed: diffuse neuropil staining, and individual neuronal somata staining. The diffuse neuropil staining is concentrated in discrete anatomically defined areas. At the EM level, this immunoreactivity is associated with numerous dendrites or astrocytic processes. At cell somata, most of neurons are stained only at Golgi apparatus (type 2); however, a distinct population of cells showed membranous staining (type 1) as well. Type 1 membranous immunoreactivity is observed only in membrane adjacent to astrocytic processes. In the cerebral cortex, type 1 neurons are found in layers III and V-VIa of somatosensory cortex, but only in layers V-VIa in most other cortical fields. Other areas containing type 1 neurons include the globus pallidus, the thalamic reticular nucleus, the hippocampus, the deep cerebellar nuclei, and a majority of the primary sensory and motor nuclei in the brainstem. The subpopulation of type 1 neurons show an overlap in distribution and morphology with some GABA-containing cells.