In animals infected with a transmissible spongiform encephalopathy, or prion disease, conformational isomers (known as PrPSc proteins) of the wild-type, host-encoded cellular prion protein (PrPc) accumulate. The infectious agents, prions, are composed mainly of these conformational isomers, with distinct prion isolates or strains being associated with different PrPSc conformations and patterns of glycosylation. Here we show that two different human PrPSc types, seen in clinically distinct subtypes of classical Creutzfeldt-Jakob disease, can be interconverted in vitro by altering their metal-ion occupancy. The dependence of PrPSc conformation on the binding of copper and zinc represents a new mechanism for post-translational modification of PrP and for the generation of multiple prion strains, with widespread implications for both the molecular classification and the pathogenesis of prion diseases in humans and animals.