Prion diseases are fatal neurodegenerative disorders caused by proteina-ceous infectious pathogens termed prions (PrP Sc ). To date, there is no pro-phylaxis or therapy available for these transmissible encephalopathies. Passive immunization with monclonal antibodies recognizing the normal host-encoded prion protein (PrP C ) has been reported to abolish PrP Sc infec-tivity and to delay onset of disease.