On the basis of the structural homologies between ST1859 (1[(2-hydroxy-1-naphtyl)methyl]-2-naphthol) and the anti-prion agents and its anti-amyloidogenic activity, we tested whether this molecule altered the biochemical properties of aggregates formed in vitro by synthetic prion peptides and affected prion infectivity in experimental scrapie. Co-incubation of ST1859 with the peptides PrP 106-126 and PrP 82-146 reduced their fibrillogenic capacity and their resistance to digestion with protease K. Hamsters inoculated with the ST1859-treated homogenate showed a significant delay in the onset of clinical signs of disease and longer survival. Survival was also significantly longer in infected hamsters treated peripherally with ST1859 for the whole post-inoculation period until the onset of clinical symptoms. Similar results were found with the analogue ST1745. Our data indicate that ST1859 reduces prion infectivity and can exert a therapeutic effect in experimental scrapie.