The pathogenic isoform (PrPSc) of the host-encoded normal cellular prion protein (PrPC) is believed to be the infectious agent of transmissible spongiform encephalopathies. Spontaneous conversion of α-helix-rich recombinant PrP into the PrPSc-like β-sheet-rich form or aggregation of cytosolic PrP has been found to be accelerated under reducing conditions. However, the effect of reducing conditions on PrPSc-mediated conversion of PrPC into PrPSc has remained unknown. In this study, the effect of reducing conditions on the binding of bacterial recombinant mouse PrP (MoPrP) with PrPSc and the conversion of MoPrP into proteinase K-resistant PrP (PrPres) using a cell-free conversion assay was investigated. High concentrations of dithiothreitol did not inhibit either the binding or conversion reactions of PrPSc from five prion strains. Indeed, dithiothreitol significantly accelerated mouse-adapted BSE-seeded conversion. These data suggest that conversion of PrPSc derived from a subset of prion strains is accelerated under reducing conditions, as has previously been shown for spontaneous conversion. Furthermore, the five prion strains used could be classified into three groups according to their efficiency at binding and conversion of MoPrP and cysteine-less mutants under both reducing and nonreducing conditions. The resulting classification is similar to that derived from biological and biochemical strain-specific features.