PrP 27-30, a unique protease-resistant protein associated with scrapie infectivity, derives from the proteolytic cleavage of a larger precursor encoded by a host gene. To identify sites of PrP biosynthesis, in situ hybridization was done using cloned PrP cDNA as a probe. In rodent brain, PrP mRNA was expressed in neurons, ependymal cells, choroid plexus epithelium, astrocytes, pericytes, endothelial cells and meninges of both scrapie-infected and uninfected animals. PrP mRNA was also detected in vitro in isolated brain microglia cells. Pulmonary cells and heart muscle cells contained high levels of this mRNA. Hybridization was not detected in spleen, confirming earlier RNA blot experiments indicating extremely low levels of PrP mRNA in this tissue. Results indicate that PrP mRNA is a normal component in a variety of non-neuronal tissues and may explain the origin of the amyloid plaques present in the subependymal region of scrapie-infected brain.