Chondroitin 4-sulfate proteoglycan (C4S-PG) was localized both in rat and human central nervous system (CNS) by monoclonal and polyclonal antisera recognizing the 4-sulfate disaccharide (C4S). In the rat the whole CNS was studied in serial coronal sections. A positive extracellular meshwork was observed both in white and grey matters. In the white matter (WM) C4S-PG formed a network around myelinated axons, sparing myelin sheaths and axoplasms. The neuropil of the grey matter (GM) showed a positive meshwork constituted by delicate intermingling filaments. The cytoplasms of neuronal, glial and endothelial cells were negative. Stronger straining than in the neuropil was observed around the soma and the proximal part of the cell processes of some neurons located in the cortex, in the deep cerebellar nuclei and in some other CNS nuclei. A similar pattern was also observed in human CNS, the only difference being a smaller amount of cortical neurons surrounded by a rim of C4S-PG. This study shows that a PG bearing C4S disaccharide is located extracellularly in the rodent and human CNS and that C4S disaccharides can be present in different types of CNS proteoglycans (PGs).