The "protein only" hypothesis postulates that the infectious agent of prion diseases, PrP(Sc), is composed of the prion protein (PrP) converted into an amyloid-specific conformation. However, cell-free conversion of the full-length PrP into the amyloid conformation has not been achieved. In an effort to understand the mechanism of PrP(Sc) formation, we developed a cell-free conversion system using recombinant mouse full-length PrP with an intact disulfide bond (rPrP). We demonstrate that rPrP will convert into the beta-sheet-rich oligomeric form at highly acidic pH (<5.5) and at high concentrations, while at slightly acidic or neutral pH (>5.5) it assembles into the amyloid form. As judged from electron microscopy, the amyloid form had a ribbon-like assembly composed of two non-twisted filaments. In contrast to the formation of the beta-oligomer, the conversion to the amyloid occurred at concentrations close to physiological and displayed key features of an autocatalytic process. Moreover, using a shortened rPrP consisting of 106 residues (rPrP 106, deletions: Delta23-88 and Delta141-176), we showed that the in vitro conversion mimicked a transmission barrier observed in vivo. Furthermore, the amyloid form displayed a remarkable resistance to proteinase K (PK) and produced a PK-resistant core identical with that of PrP(Sc). Fourier transform infrared spectroscopy analyses showed that the beta-sheet-rich core of the amyloid form remained intact upon PK-digestion and accounted for the extremely high thermal stability. Electron and real-time fluorescent microscopy revealed that proteolytic digestion induces either aggregation of the amyloid ribbons into large clumps or further assembly into fibrils composed of several ribbons. Fibrils composed of ribbons were very fragile and had a tendency to fragment into short pieces. Remarkably, the amyloid form treated with PK preserved high seeding activity. Our work supports the protein only hypothesis of prion propagation and demonstrates that formation of the amyloid form that recapitulates key physical properties of PrP(Sc) can be achieved in vitro in the absence of cellular factors or a PrP(Sc) template.