It has been reported that macrophages degrade infectious forms of prion protein (PrPSc). In order to investigate the mechanisms underlying PrPSc degradation in macrophages, the effects of lysosomal and proteasomal inhibitors on macrophage cell lines which were incubated with scrapie-affected brain homogenate were studied. PrPSc degradation was inhibited in the presence of both proteasomal and lysosomal inhibitors. Indirect fluorescence assays to determine the cellular localization of PrPSc were undertaken. PrPSc colocalized with the lysosomal membrane protein Lamp-1 and ubiquitin, a protein that is related to the proteasome. The present data indicate that macrophages might degrade PrPSc via the lysosomal and proteasomal pathways.