Immunity to Taiwan Taenia infection in pigs can be stimulated using homologous or heterologous non-viable Taenia oncospheres. This study was designed to determine whether homologous non-viable oncospheres could stimulate immunity to Hymenolepis infection in rodents. Hatched oncospheres were prepared from eggs of Hymenolepis diminuta, Hymenolepis nana, and Hymenolepis microstoma and kept at -70 degrees C for more than 1 month. A mixture of 500 non-viable oncospheres of each tapeworm and complete Freund's adjuvant was injected subcutaneously in four groups of Sprague-Dawley rats or ICR mice one to four times at an interval of 1 week; controls were not immunized. After immunization, each rodent was orally inoculated with three fresh active cysticercoids of H. diminuta or H. microstoma or 500 fresh eggs of H. nana. The animals were then necropsied for adult tapeworms. No rats or mice immunized with non-viable oncospheres of H. diminuta or H. nana were infected by the challenge inoculation. However, 28 of 34 mice immunized with non-viable H. microstoma oncospheres were infected after inoculation with cysticercoids. This study demonstrated complete protection against infection by homologous parasites in rats or mice immunized with non-viable oncospheres of H. diminuta and H. nana, respectively. Repeated immunization may not be required if resistance is stimulated in rodent hosts.