Information regarding the development of acquired immunity in opisthorchiasis is particularly relevant if one is attempting eradication of the disease, e.g. by anthelmintic chemotherapy. Clinical observations in patients with opisthorchiasis suggest that if acquired immunity develops at all, it is of low degree. Using hamsters as experimental model, it was recently demonstrated that prior infection with Opisthorchis viverrini failed to confer protection against reinfection, regardless of the size of the challenging dose. These observations are not entirely unexpected because O. viverrini does not normally invade tissues during development in mammalian hosts. Therefore, the chance of stimulating the host immune system is minimal in this infection. In the present study, attempts were made to potentiate the immunity of hamsters with crude somatic extract of O. viverrini, with or without previous infection with 5 metacercariae. The animals were treated by an intraperitoneal injection of 200 micrograms of antigen in complete Freund's adjuvant (CFA) and were challenged with 25 metacercariae (Mc) 3 weeks later. As was to be expected, immunization of animals that had never been infected before did not provoke any immune response against a subsequent challenge infection. On the other hand, immunization of animals that had been infected previously with 5 Mc appeared to have developed some degree of immune response against a subsequent challenge with 25 Mc. Compared with unimmunized infected controls which received only CFA and the 25 Mc challenge, there was about 30% reduction in the worm burden in the group immunized with somatic extract. It is anticipated that with a better method of immunization, a higher degree of protection may be achieved.