In most sheep infected with a transmissible spongiform encephalopathy (tse) the disease-associated prion protein (PrPd) accumulates in tissues of the lymphoreticular system, suggesting that it might be detected in biopsy specimens. A procedure has been developed to obtain biopsy specimens of rectal mucosa in which PrPd has been detected by immunohistochemistry in preclinically infected sheep of all susceptible PrP genotypes. It is probable that PrPd increases with the age of sheep or period of incubation. PrPd was detectable approximately halfway through the incubation period, with sheep of some PrP genotypes showing positive results earlier than others. For a preclinical diagnosis, the risk of a false negative result was approximately 9 per cent for samples containing 10 follicles, a figure that was reached in 87 per cent of the biopsies. The rectal biopsies had the same sensitivity and time of onset of PrPd accumulation as biopsies of the palatine tonsil, but provided larger numbers of follicles. The procedure is simple and quick, does not require dedicated specific instruments, sedation or general anaesthesia, and can be performed repeatedly on the same sheep without detrimental effects to either the animal or the number of follicles obtained.