The time course localisation of parasite antigens and related host pathology were studied in hamsters infected with 100 metacercariae of Opisthorchis viverrini for up to 6 months. Parasite antigens, as detected by immunofluorescence and/or immunoperoxidase-staining, were first observed in the flukes and the biliary epithelium of the intrahepatic and extrahepatic bile ducts as early as day 3 p.i. Antigens increased as the parasite matured, both in tissues in direct contact with the flukes and those surrounding the infection. Opisthorchis antigens were also observed in the first order bile ducts (small bile ducts) of the liver, which are not normally inhabited by flukes. In addition, they were found in damaged liver cells, Kupffer cells, macrophages, and within epithelioid and giant cells in the egg granuloma. The presence of the antigens was associated with heavy inflammatory cell infiltration, particularly with mononuclear cells. The results strongly support the role of fluke-associated antigens and local parasite-specific immune responses in the pathogenesis of opisthorchiasis.