The appearance of a new variant of CJD (veJD) in young patients has caused considerable public concern and there is evidence that this novel disease is caused by the same agent as BSE. BSE is a prion disease that became epidemic in the UK, with a peak incidence in January 1993. New test systems should aim to identify BSE-infected cattle early in the incubation period. We compared the established histological and immunohistochemical methods and the Western blot method used by Prionics with the PET blot method that detects prion Prpsc deposits in formalinfixed and paraffin-embedded tissue. Investigating the obex region with the PET blot, all BSE cases were detectable and no false positive cases occurred. From the Swiss culling program, five clinically healthy cattle out of 1761 were identified as incubating BSE. With the PET blot method four of them showed the same Prpsc deposition pattern that was seen in clinical BSE, though less conspicuous. In one of the five cases, Prpsc was restricted to two brain stem nuclei, a pattern that was reported to be the first manifestation of Prpsc deposits in the brain after peripheral infection and one that occurs after half of the incubation time. In this case, histology and Western blot were negative.