Prions, the aetiological agents of transmissible spongiform encephalopathies, exhibit extreme resistance to degradation. Soil can retain prion infectivity in the environment for years. Reactive soil components may, however, contribute to the inactivation of prions in soil. Members of the birnessite family of manganese oxides (MnO(2)) rank among the strongest natural oxidants in soils. Here, we report the abiotic degradation of pathogenic prion protein (PrP(TSE)) by a synthetic analogue of naturally occurring birnessite minerals. Aqueous MnO(2) suspensions degraded the PrP(TSE) as evidenced by decreased immunoreactivity and diminished ability to seed protein misfolding cyclic amplification reactions. Birnessite-mediated PrP(TSE) degradation increased as a solution's pH decreased, consistent with the pH-dependence of the redox potential of MnO(2). Exposure to 5.6 mg MnO(2) ml(-1) (PrP(TSE) : MnO(2)=1 : 110) decreased PrP(TSE) levels by > or = 4 orders of magnitude. Manganese oxides may contribute to prion degradation in soil environments rich in these minerals.