Of the diverse types of naturally occurring animal toxins, the prion proteins of mammalian meat- and bone-meal have recently emerged as important feed contaminants necessitating statutory control. Prion proteins are harmless animal tissue components with the capacity to transform themselves into agents causing fatal neurological lesions in a wide range of species. The significance of prions has been highlighted following the emergence of bovine spongiform encephalopathy (BSE) as a major disease of cattle in the United Kingdom. The onset of this disease was attributed to the feeding of cattle with meat- and bone-meal prepared from the carcasses of scrapie-infected sheep. The latter disease is also caused by prion proteins, as is the human equivalent - new variant Creutzfeldt-Jakob disease (vCJD). The incidence of vCJD in humans has been linked to the consumption of BSE-infected beef. It is this association that has led to extensive and stringent legislation in the European Union (EU) concerning the use of specified animal products in livestock feeding.