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Summary: Scrapie diagnosis is based on the demonstration of disease-associated prion protein (PrPSc) in brain or, in the live animal, in readily accessible peripheral lymphoid tissue. Lymphatic tissues present at the rectoanal line were readily obtained from sheep without the need for anaesthesia. The presence of PrPSc in such tissue was investigated in sheep infected orally with scrapie-infected brain material. The methods used consisted of immunohistochemistry and histoblotting on biopsy and post-mortem material. PrPSc was detected in animals with PrP genotypes associated with high susceptibility to scrapie from 10 months after infection, i.e., from about the time of appearance of early clinical signs. In the rectal mucosa, PrPSc was found in lymphoid follicles and in cells scattered in the lamina propria, often near and sometimes in the crypt epithelium. By Western blotting, PrPSc was detected in rectal biopsy samples of sheep with the PrP genotype VRQ/VRQ, after electrophoresis of material equivalent to 8mg of tissue. This study indicated that rectal biopsy samples should prove useful for the diagnosis of scrapie in sheep. [Copyright &y& Elsevier]