Previous histochemical studies have demonstrated highly sulfated glycosaminoglycans (GAGs) localized to the amyloid plaques in the brains of humans and animals with prion diseases (Snow et al., Acta Neuropathol 77: 337, 1989). However, the identity of the specific class of proteoglycan/GAG present was not known. The current investigation used immunocytochemical techniques to identify and localize heparan sulfate proteoglycans (HSPGs) in human cases of Gerstmann-Straussler syndrome and Creutzfeldt-Jakob disease, as well as in experimental scrapie of hamsters. Polyclonal and monoclonal antibodies to either the protein core or GAG moiety of the basement membrane-derived HSPG were utilized. The results demonstrate that both the protein core and the GAG chains of HSPGs are immunolocalized to the prion protein amyloid plaques in each of these diseases. HSPG immunostaining was also evident in these tissues in blood vessels, choroid plexus, myelinated axons, and in the cytoplasm of certain neuronal and astrocytic populations, particularly those in close proximity to the amyloid plaques. Additionally, in Creutzfeldt-Jakob disease and Gerstmann-Straussler syndrome cerebellum, positive immunostaining for heparan sulfate GAGs was observed within the cell bodies of Purkinje cells. The specific accumulation of HSPGs in the amyloid deposits of both the prion diseases and Alzheimer's disease (Snow et al., Am J Pathol 133: 456, 1988), suggests that a common mechanism involving HSPGs may occur in the pathogenesis of amyloidosis in each of these diseases.