THE infectious agent (or ’prion’) of the transmissible spongiform encephalopathies (TSEs) such as scrapie resembles a virus in that it replicates in vivo and has distinct strains(1), but it was postulated long ago to contain only protein(2-3). More recently, PrPSc, a pathogenic, scrapie-associated form of the host-encoded prion protein (PrP), was identified as a possible primary TSE agent protein(4-6). PrPSc is defined biochemically by its insolubility and resistance to proteases(7) and is derived post-translationally from normal, protease-sensitive PrP (PrPc)(8,9). The conversion seems to involve conformational change rather than covalent modification(10-13) However, the conversion mechanism and the relationship of PrPSc formation to TSE agent replication remain unclear. Here we report the conversion of PrPc to protease-resistant forms similar to PrPSc in a cell-free system composed of substantially purified constituents. This conversion was selective and required the presence of preexisting PrPSc, providing direct evidence that PrPSc derives from specific PrPc-PrPSc interactions.