Prions are unusual infectious pathogens causing scrapie of sheep and goats as well as Creutzfeldt-Jakob disease of humans. Biochemical and genetic studies contend that the scrapie isoform of the prion protein (PrPSc) is a major component of the prion. Limited proteinase K digestion of PrPSc produced a protein of 27-30 kDa. After dispersion of brain microsomes isolated from scrapie-infected hamsters into detergent-lipid-protein complexes, copurification of PrPSc and scrapie infectivity was obtained with scrapie prion protein of 27-30 kDa monoclonal antibody-affinity columns. PrPSc was enriched approximately equal to 5700-fold with respect to total brain protein, whereas scrapie prion infectivity was enriched approximately equal to 4000-fold. The ratio of prion titer to PrPSc remained constant throughout purification. Heterologous monoclonal antibody columns failed to bind either PrPSc or scrapie infectivity. Polyclonal rabbit prion protein antiserum raised against NaDodSO4/PAGE-purified scrapie prion protein of 27-30 kDa reduced scrapie infectivity dispersed into detergent-lipid-protein complexes by a factor of 100. These results represent direct immunologic and chromatographic demonstrations of a relationship between PrPSc and prion infectivity as well as providing additional support for the contention that PrPSc is a major component of the infectious scrapie particle. That PrPSc is a host-encoded protein is an important feature distinguishing prions from viruses.