The main challenge in the diagnosis of prion disease in animals is the lack of a reliable preclinical assay. Current assays using plasma samples through amplification or extraction of proteinase K-resistant prion protein (PrPSC) face a problem of high false-results rate, because of the low concentrations of PrPSC in plasma samples. In this report, we described the development of radioimmunoassay (RIA) of cellular prion protein (PrPC) by using 125I-labeled residue 143-153 of prion protein (125I-PrP143-153). This assay could detect prion protein (PrP) fragments in brain homogenate and plasma sample from healthy sheep. Using RIA, the concentration of PrP143-153 was determined as 0.29 μg/g total proteins in a brain homogenate, and 99 pg/ml in a plasma sample. The detection limit of this assay was 31 pg/ml, and the intra- and inter-assay variabilities were <10%. This assay may have a potential to be used for diagnosis of scrapie in sheep and chronic wasting disease (CWD) in cervids. The animals with prion disease are expected to have lower plasma concentrations of PrP143-153 than normal animals. This hypothesis will be tested in future studies.