Brain frozen sections from the first group of three scrapie-infected mice and three age-matched controls were obtained. Coronal sections (20 μm thickness) were prepared using a cryostat and utilized for staining. The sections were incubated with 0.2 nM [125I]IMPY in PBS at room temperature for 1 h. The sections were then dipped in saturated Li2CO3 in 40% EtOH (two 3-min washes) and washed with 40% EtOH (two 2-min washes), followed by rinsing with water twice for 1 min. For in vitro competition of prion deposit labeling with [125I]IMPY, the brain sections of scrapie-infected mice were incubated with 0.2 nM [125I] IMPY in the presence of 0, 200 or 500 nM of nonradioactive IMPY (in 10% EtOH) at room temperature for 1 h. The labeled sections were then processed as described above. After drying, the labeled sections were exposed with BioMax MR film for 40 h together with an 125I microscale standard (GE Healthcare Biosciences). The films were then developed and analyzed using image analysis system Beta Vision (Biospace measures).