Aims: This paper describes a procedure for evaluating the presence and the stability of the proteinase K-resistant form of the prion protein (PrPres) in slaughterhouse wastewater. Methods and Results: Wastewater samples were spiked with either scrapie or bovine spongiform encephalopathy agents and PrPres was concentrated and detected by western blotting. The detection limit was estimated to be 2-4 mu g of either scrapie or BSE-infected brain tissue in 15 ml of sewage. Wastewater samples from three abattoirs were analysed, two of which had processed BSE-infected animals. No PrPres was detected. The effect of sewage on the inoculum and the persistence of transmissible spongiform encephalopathy agents in wastewater were also considered. Conclusions: The results of the assay suggest that wastewaters from abattoirs where one positive BSE case has been identified would contain titres lower than 0.6-26 x 10(-4) cattle oral ID50 per litre resulting from specified risk material tissue contamination. Moreover, the effect of abattoir wastewaters is to reduce the persistence of PrPres. Significance and Impact of the Study: The assay may be a useful tool for risk assessment studies and for reducing the potential risk of contamination with BSE via sewage sludge fertilizer procedures.