Following peripheral exposure to transmissible spongiform encephalopathies (TSEs), infectivity usually accumulates in lymphoid tissues before neuroinvasion. The host prion protein (PrP c) is critical for TSE agent replication and accumulates as an abnormal, detergent insoluble, relatively proteinase-resistant isoform (PrP Sc) in diseased tissues 1, 2. Early PrP Sc accumulation takes place on follicular dendritic cells (FDCs) within germinal centers in lymphoid tissues of patients with variant Creutzfeldt–Jakob disease 3 (vCJD), sheep with natural scrapie 4 or rodents following experimental peripheral infection with scrapie 5, 6, 7. In mouse scrapie models, the absence of FDCs blocks scrapie replication and PrP Sc accumulation in the spleen, and neuroinvasion is significantly impaired 6, 7, 8, 9. The mechanisms by which the TSE agent initially localizes to lymphoid follicles and interacts with FDCs are unknown. Antigens are trapped and retained on the surface of FDCs through interactions between complement and cellular complement receptors 10, 11. Here we show that in mice, both temporary depletion of complement component C3 or genetic deficiency of C1q significantly delays the onset of disease following peripheral infection, and reduces the early accumulation of PrP Sc in the spleen. Thus, in the early stages of infection, C3 and perhaps C1q contribute to the localization of TSE infectivity in lymphoid tissue and may be therapeutic targets.