Opisthorchiasis is the major public health problem in north-eastern Thailand where a large proportion of population are infected. Athough diagnosis is currently based on the detection of eggs in stool, and on some occasions also in duodenal fluid or bile, serological diagnosis is still an essential tool for the case of light infection and biliary obstruction. The development of sensitive and specific serodiagnostic assays requires a thorough understanding the host-parasite relationship which, in turn, depends on information regarding the parasite antigens and host immune response to these antigenic stimulation. Therefore, in this study Opisthorchis viverrini antigens and humoral immune response in patients with opisthorchiasis were characterized. Using ELISA, IgG, IgA and IgE antibodies reactive with various O. Viverrini antigens including, somatic, excretory and secretory (ES), and surface tegument, in the serum of these patients were significantly higher than those of normal healthy controls. In addition to specific antibodies, there was an increase of total IgG, IgA and particularly obvious for the IgE levels. Serum IgG antibody was present in all patients and was found to correlate with the severity of infections as judged from the quantity of eggs in the stools. There was also a significant elevation of antibody in the bile and serum of O. Viverrini infected patients who also had biliary obstruction. Although in both specimens, IgG, IgA and IgE antibody isotypes could be detected, such responses were more vigorous in the serum. Analysis of paired samples from individual patients showed that while in the serum, the IgG response predominated, in the bile it was the IgA. The high level of IgA antibody in the bile together with its presence in association with the secretory component suggested a selective transport and/or local production of IgA antibody by the hepatobiliary system of these patients. Immunological identification of antigens was determined by a combination of immunoprecipitation, SDS-PAGE and autoradiography using infected human sera and also sera from animals and patients with other parasitic infections. Both adult somatic extract and surface tegument gave similar SDS-PAGE patterns and more highly complex with molecular weight varying from > 116 to 12 kD. On the other hand, the ES products and metacercarial extract consisted of components with molecular weight of 89 to 16 and 89 to 35 kD respectively. Comparing with results using sera from normal control, all of these antigens reacted strongly and specifically with infected human serum, particularly those with molecular weight of 89 kD, which reacted consistently with all infected individuals. This 89-kD protein failed to give significant reaction with sera from patients with other parasitic infections including paragonimiasis, schistosomiasis, filariasis, gnathostomiasis, toxoplasmosis and malaria. Moreover, it failed to react with sera from animals experimentally infected with Gnathostoma spinigerum, Angiostrongylus cantonensis, Schistosoma mansoni, S. japonicum, S. mekongi, S. hematobium and Paragonimus heterotremus. An immunofluorescence study showed this 89-kD component to be associated with parasite eggs, lining of the uterus and cecum an secretion therein. From these results it appears that the 89-kD protein of O. Viverrini is a potent immunogen and has potential as antigen for immunodiagnosis of opisthorchiasis.