SUMMARY

A single injection of 1 mg dextran sulphate 500 per mouse considerably prolonged incubation times and produced survivors, even when given 3 days after intravenous or intraperitoneal scrapie infection. Since this compound could be detected in mononuclear phagocytes ofspleen and lymph nodes for up to 7 months, it is suggested that it impairs a particular step in the infectious process in these cells of the lympho-reticular system. Blockage of the reticuloendothelial system by trypan blue and silica did not alter the pathogenesis. The scrapie agent causes a slow progressive, always lethal disease of the central nervous system (CNS) in sheep and goats. Following propagation ofthe scrapie agent in mice (Chandler, 1961), quantitative data have led to a reasonable understanding of the pathogenesis of scrapie. It is a widely accepted view that after extraneural infection an early, probably obligatory replication phase occurs in one or more as yet unidentified cell types of the lymphoreticular system (LRS). This concept is based on the following observations: (i)an early rise in titre in the LRS long before the appearance of the agent in the CNS (Eklund et al., 1967; Kimberlin & Walker, 1979), (ii) prolonged incubation periods in spleenless (Fraser & Dickinson, 1970, 1978