PREVIOUS work with scrapie led us to conclude1 that the transmissible agent was unlikely to be a conventional virus. We have presented evidence2 that the transmissible agent may be, or may be associated with, a small basic protein (polypeptide) of the kind involved in the production of experimental allergic encephalomyelitis, and we have recorded the possible occurrence of the agent in tissues of normal animals3–5. These observations have neither been confirmed nor disproved in other laboratories and various other hypotheses on the nature of scrapie and its transmissible agent remain unproved6–8. The “slow virus” hypothesis still favoured by some9 is now at variance with the impressive experimental evidence from ultraviolet irradiation studies10–13 that the transmissible agent may not contain nucleic acid.