Antigens of Opisthorchis viverrini were identified and characterized by enzyme-linked immunosorbent assay, polyacrylamide gel electrophoresis following radioimmuno-precipitation, and indirect immunofluorescence. The immunoreactive protein with a relative molecular weight (Mr) of 89 kD was the predominating component of the parasite metabolic products. An immunofluorescence study showed it to be associated with parasite eggs, linings of the reproductive system and secretions therein. Protein of the surface tegument had Mr of greater than 116, 108, 64, 38, 34, 20 and 16-17 kD. The 16-17 kD surface molecule was the predominant protein, representing approximately 50% of the total protein in crude aqueous extracts of adult worms. However, it was poorly immunogenic when compared with the 89 kD molecule. Together with data presented previously, it appears that in addition to the 89 kD protein, several tegumental molecules are also specific for O. viverrini and have immuno-diagnostic potential.