Neurodegeneration caused by the transmissible spongiform encephalopathies is associated with the conversion of a normal host protein, PrP-sen, into an abnormal aggregated protease-resistant form, PrP-res. In scrapie-infected mouse neuroblastoma cells, mouse PrP-sen is converted into PrP-res but recombinant hamster PrP-sen expressed in these cells is not. In the present studies, recombinant hamster/mouse PrP-sen molecules were expressed in these scrapie-infected cells to define specific PrP amino acid residues critical for the conversion to PrP-res. The results showed that homology to the region of mouse PrP-sen from amino acid residues 112 to 138 was required for conversion of recombinant PrP-sen to PrP-res in scrapie-infected mouse cells. Furthermore, a single hamster-specific PrP amino acid at residue 138 could inhibit the conversion of the recombinant PrP-sen into PrP-res. The data are consistent with studies in humans which show that specific amino acid residue changes within PrP can influence disease pathogenesis and transmission of transmissible spongiform encephalopathies across species barriers.