THE infectious agents causing scrapie and other transmissible spongiform encephalopathies have been postulated to consist solely of the protease-resistant form of prion protein (PrPSc)(1-6). One unprecedented requirement of the protein-only model is that the ’inheritance’ of pathogen strain differences must be mediated by stable variations in PrPSc structure(2,7,8), rather than mutations in an agent-specific nucleic acid(9). Strain differences in PrPSc structure have been described for the hyper (HY) and drowsy (DY) strains of hamster transmissible mink encephalopathy (TME)(7,8), a a scrapie-like disease originating in mink. Although HY and DY PrPSc are both post-translationally derived from the precursor prion protein (PrPc) they are cleaved at different amino-terminal sites by proteinase K (ref. 8). Here we investigate whether this strain-specific property of PrPSc is transmitted to PrPc during formation of new PrPSc. PrPSc from the HY and DY TME strains converted the protease-sensitive PrPc into two distinct sets of protease-resistant PrP products in a cell-free system. These data provide evidence that self-propagation of PrPSc polymers with distinct three-dimensional structures could be the molecular basis of scrapie strains.