Creutzfeldt–Jakob disease is caused by a slow infectious pathogen, or prion. We found that purified fractions from the brains of two patients with Creutzfeldt–Jakob disease contained protease-resistant proteins ranging in apparent molecular weight from 10,000 to 50,000. These proteins reacted with antibodies raised against the scrapie prion protein PrP 27–30. Rod-shaped particles were found in the brain tissue of the patients that were similar to those isolated from rodents with either scrapie or experimental Creutzfeldt–Jakob disease. After being stained with Congo red dye, the protein polymers from patients with Creutzfeldt–Jakob disease exhibited green birefringence when examined under polarized light. Our findings suggest that the amyloid plaques found in the brains of patients with Creutzfeldt–Jakob disease may be composed of paracrystalline arrays of prions similar to those in prion diseases in laboratory animals. (N Engl J Med 1985; 312:73–8.)