Technical Abstract: Sheep scrapie is the prototype of a heterogeneous group of transmissible spongiform encephalopathies (TSEs) also occurring in humans, cattle, cats, mik, and cervids. TSEs are characterized by deposition of prion proteins (PrP-Scrapie or PrP-Sc) in the central nervous system of affected individuals. PrP-Sc and infectivity have been detected in the lymphoreticular system of sheep well before symptoms occur and detection of PrP-Sc in lymphoid tissue has been proposed as a preclinical diagnostic test for scrapie. In this paper, we report a noninvasive diagnostic assay based on biopsy of lymphoid nodules in the nictitating membrane of sheep and detection of PrP-Sc in situ using monoclonal antibody F89/160.1.5 and a sensitive immunohistochemical technique. We detected PrP-Sc in clinically normal, high risk sheep aged 1,2, and 3 years. This test provides a practical method for early detection of scrapie infected sheep.