Abstract: Methods for identifying and characterizing inhibitors of protein filament formation are provided. These methods are particularly useful for identifying those agents which inhibit or prevent protein filament formation within neurons of mammalian subjects, particularly human subjects, such as the formation of tau filaments in Alzheimer's patients, and ?-synuclein filaments in Parkinson's patients. According to the methods, protein monomers which are associated with formation of intra- or extra-cellular aggregates are combined under physiological conditions with a fibrillization inducer and the formation of protein aggregates is assessed in the absence and the presence of a test agent. The absence or a reduction in the size or stability of proteinaceous polymeric filaments as compared to a control indicates that the test agent is an inhibitor of proteinaceous polymeric filament formation.