Immunoreactive components of Opisthorchis viverrini and Clonorchis sinensis were analysed by enzyme-linked immunosorbent assay (ELISA), radioimmunoprecipitation and immunoblotting. Somatic extracts from these two liver flukes as well as from other related parasites, together with the metabolic products, were tested for their reactivities with sera from patients with opisthorchiasis and clonorchiasis. A significant cross-reactivity in the ELISA was noted between Opisthorchis and Clonorchis. Immunoblotting and radioimmunoprecipitation analyses showed that the 89-kD protein which was previously shown to be a predominant metabolic product of O. viverrini reacted with sera from both groups of patients. However, an antigen with a molecular weight of 16 kD, apparently a predominant somatic component, appeared to be specific for O. viverrini.