We characterized PrPSc from sheep infected with a fast-acting scrapie inoculum exhibiting host PRNP (codon 136) genotype dependence on incubation time, using the stability assay to provide molecular evidence for two separate isolates of scrapie and to compare their properties to those of a representative scrapie isolate (NADC 13-7). This evidence concerns the gene PRNP and scrapie.