Prion diseases are associated with the conversion of the normal cellular prion protein, PrPC, to the abnormal disease-associated protein, PrPSc. This conversion can be mimicked in vitro using PrPSc isolated from the brains of scrapie-infected animals to induce conversion of recombinant PrPC into a proteinase K-resistant isoform, PrPres. Traditionally, the 'cell-free' conversion assay has used, as substrate, recombinant PrPC purified from mammalian tissue culture cells or, more recently, from baculovirus-infected insect cells. The cell-free conversion assay has been modified by replacing the tissue culture-derived PrPC with recombinant PrPC purified from bacteria. Bacterial expression and chromatographic purification give high yields of recombinant radiolabelled untagged protein, eliminates artefacts that may be due to cellular factors or antibody fragments normally present in labelled PrP preparations and allows accurate and rapid variation of protein sequence using standard molecular biological techniques. In addition, these cell-free conversion assays were carried out under more physiological conditions, giving more relevance to the assay as a model for conversion. To validate its use in this assay, this bacterial recombinant PrP has been shown to have the conversion properties of mammalian PrPC: (i) it converts to a proteinase K-resistant isoform in the presence of PrPSc; (ii) the efficiency of this conversion by PrPSc of different strains and species parallels that found in vivo; and (iii) its cell-free conversion is inhibited by Congo Red analogues in a structure-dependent manner similar to that seen in in vivo and in vitro cell assays.