Ultrastructural pathological changes in the gut-associated lymphoid tissues of sacculus rotundus(SR) of rabbits infected with rabbit haemorrhagic disease virus (RHDV) were first observed. There were nu-merous holes at the luminal and basement membrane surfaces of the dome epithelium(DE), consistently ac-companied by necrosis of lymphocytes and M-cells, and pronounced depletion of lymphocytes in the domes andfollicles, decrease of DE complex with formation of pseudomembranous structure on the surface of the domeepithelium. A specific finding in lymphocytes and macrophages was that severe destruction detraction of themembrane of rough endoplasmic reticulum(RER) was accompanied by conspicious increase of solitary, ribo-some-like particles in the cytoplasm, with appearances of intranuclear particles and intranuclear inclusions. Itwas found that there were many round and dense virion-like particles, with 26 nm in diameter, in the nucleiand cytoplasm of lymphoctes, plasma cells, macrophages and fibroblasts, or in degenerated cells and cellulardebris. At the same time, another round virion-like particles about 34 nm in diameter were also seen in the cy-toplasm of some cells and interstitium. The results indicated that the appearances of the ribosome-like parti-cles, virion-like particles and inclusion bodies were related to the replication and assembly of RHDV. Thepresent observations suggested that DE of sacculus rotundus could be a open pathway and a transporting routefor the entry of antigens into hosts. While the antigen is profoundly deleterious, DE may be as a closed portalor a barrier preventing the foreign antigenic materials from invading.