Two monoclonal antibodies that specifically detect human prion protein (PrP) were developed. The epitope of both antibodies was mapped using fusion proteins of glutathione-S-transferase and PrP peptides to the C-terminal region encompassing the polymorphic 219 residue. The antibodies recognized human PrP with 219Glu but not that with 219Lys. The unique property of the antibodies was utilized to determine the allelic origin of abnormal PrP deposited in the brain of a patient with Gerstmann-Straussler syndrome (GSS) with 102Leu/219Lys encoded by the same allele. Abnormal PrP was exclusively of mutant allelic origin, suggesting that 219Lys may be permissive to the formation of abnormal PrP in GSS. The antibodies may help to explore the relationship of 219Glu/Lys polymorphism to the pathogenesis of human prion diseases.