Prion neuroinvasion is accompanied by maximal activation of microglia, the significance of which for pathogenesis is unknown. Here, we used bone marrow (BM) cells expressing GFP (green fluorescent protein) to study the turnover of microglia in mouse scrapie. We found that >or=50% of all brain microglia were replaced by BM-derived cells before clinical disease onset. In terminally sick mice, microglia density increased threefold to fourfold. Hence BM-derived microglia rapidly and efficaciously colonize the brain in scrapie. Whereas reconstitution of wild-type mice with prion protein-deficient (Prnp(o/o)) BM did not alter scrapie pathogenesis, Prnp(o/o) mice transplanted with wild-type BM cells were resistant to peripherally administered prions despite high levels of infectivity in the spleen. Cerebellar homogenates from prion-inoculated Prnp(o/o) mice reconstituted with >10% of wild-type microglia failed to infect transgenic mice overexpressing the cellular prion protein. Hence, in contrast to previous reports, microglia are not competent for efficient prion transport and replication in vivo.