Anti-parasite antibody responses were compared among several strains of mice experimentally infected with the dwarf tapeworm, Hymenolepis nana. The antibody titres were highly variable among the mouse strains in addition to variation in worm fecundity and longevity. The influence of the thymus on both infection and anti-parasite antibody production (especially of IgE isotype) was studied by the use of congenitally athymic (nu/nu) nude and their phenotypically normal (nu/+) CD-1(ICR) mice infected with H. nana. All nude (nu/nu) mice harboured fully mature 70 day old adult tapeworms of the first generation derived from eggs initially given on day 0. In addition, they contained (a) younger second generation adults derived from autoinfection and present in the intestinal lumen, (b) a number of abnormally large (about 1-2 mm in diameter) balloon like, fluid filled cysticercoids in not only the intestinal tissue but also parenteral tissues such as the mesenteric lymph nodes, liver and lung, and (c) normal cysticercoids derived from challenging eggs in the intestinal tissue. Infected nude mice produced no antibodies detectable by PCA (IgE) and double diffusion (IgG) tests. In contrast, normal (nu/+) mice and nude mice reconstituted with thymocytes expelled almost all luminal adults of the primary infection by day 70 and produced antibodies to extracts of adult H. nana. Neither autoinfection nor reinfection following egg challenge occurred in any of these normal (nu/+) and reconstituted nude mice. Therefore, acquired immune responses against H. nana (as assessed by resistance not only to the tissue phase measured by the failure of tissue cysticercoid recovery from egg challenge, but also to the lumen phase assessed by the failure of autoinfection adult recovery and 'worm expulsion' of the initially established adults) are all thymus-dependent in mice. The antibody responses examined are also thymus-dependent.