Abnormal isoform of prion proteins (PrPSc), which are infectious agents associated with prion diseases, retain infectivity after undergoing routine sterilization processes. A sensitive method to detect the infectivity is a bioassay, and it has been used for assessing prion inactivation. However, the result is obtained after several hundred days. Here, protein misfolding cyclic amplification (PMCA) in which PrPSc can be amplified in vitro was applied for assessing prion inactivation by dry heating and autoclaving. Scrapie-infected hamster brains were inactivated under various conditions, and residual infectivity and PrPSc were detected by the bioassay and PMCA, respectively. The PMCA results were in good agreement with those of the bioassay. In samples autoclaved at temperatures below 150 degrees C, while infected mice died in the bioassay, protease-resistant PrP (PrPres) signals were detected in the second or third round of PMCA. Three rounds of PMCA require only 6 days, which means that the PMCA method is much faster than the bioassay. (c) 2006 Elsevier Inc. All rights reserved.