The critical initial event in the pathophysiology of transmissible spongiform encephalopathies (TSEs) appears to be the conversion of the cellular prion protein (PrP(C)) into the abnormal isoform PrP(Sc). This isoform forms high-molecular-weight protease K (PK) resistant aggregates that accumulate in the central nervous system of affected individuals. We have selected nuclease-resistant 2'-amino-2'-deoxypyrimidine-modified RNA aptamers which recognize a peptide comprising amino acid residues 90-129 of the human prion protein with high specificity. This domain of prion proteins is thought to be functionally important for the conversion of PrP(C) into its pathogenic isoform PrP(Sc) and is highly homologous among prion proteins of various species including mouse, hamster, and man. Consequently, aptamer DP7 binds to the full-length human, mouse, and hamster prion protein. At low concentrations in the growth medium of persistently prion-infected neuroblastoma cells, aptamer DP7 significantly reduced the relative proportion of de novo synthesized PK-resistant PrP(Sc) within only 16 h. These findings may open the door towards a rational development of a new class of drugs for the therapy or prophylaxis of prion diseases.