Transmissible spongiform encephalopathies (TSE) as well as the properties of the major component of the infectious agent-prion, and the most important human and animal prion diseases are characterized. Considering the recent biochemical and molecular biological data, possible explanations of natural resistance, species barrier and lack of the immune response to the unconventional infectious particles are presented. Finally the importance of immunoblotting and immunostaining as the most specific confirmation of TSE diagnosis is underlined. (Ref. 11.)