The "protein-only" mechanism by which infectious agents of prion diseases such as Creutzfeldt-Jakob disease and bovine spongiform encephalopathy replicate remains undetermined. The identification of several distinct classes of prion inhibitors has created an opportunity to investigate the mechanism of prion formation using pharmacological tools. These new inhibitors include substituted tricyclic derivatives, tetrapyrrole compounds, cysteine protease inhibitors, branched polyamines, and specific antibodies. Each inhibitor class contains at least one active compound that inhibits prion propagation in cell culture at sub-micromolar concentrations and several structurally related, inactive compounds. Work with branched polyamines and specific antibodies has already provided insight into the kinetics and cell biology of endogenous prion clearance mechanisms. Other anti-prion compounds do not appear to bind directly to the prion protein. Detailed investigation of the mechanism of drug action of these compounds may lead to the identification of novel prion propagation factors.