Peripheral blood and bone marrow eosinophilia occurred in nude rats and nude mice following F. hepatica infection, with the magnitude of the response in nude rats greater than that in nude mice. Injection of E/S antigens induced eosinophilia in nude rats and a limited eosinophilia in nude mice. Bone marrow eosinophilia was greatly enhanced in Fasciola-infected nude rats, particularly 14 days after infection and later. In nude mice, bone marrow eosinophilia developed soon after infection and persisted for the duration of the experiment, but was not as marked as in nude rats. Bone marrow eosinophils in antigen-injected animals were also elevated, and again this was more marked in nude rats. The number of colonies formed in agar culture from bone marrow cells of both nude rats and nude mice also increased following infection and remained significantly elevated throughout the experiment. Bone marrow colonies in antigen-injected nude rats increased on day 8, while in injected nude mice, the number of colonies rose rapidly following injection with antigens. Thus, nude rats and nude mice develop T-cell-independent eosinophilia, which appears to originate in the bone marrow. The magnitude of eosinophilia is greater in nude rats and it has yet to be determined whether these effects have any relevance to the ability of rats, but not mice, to develop resistance to reinfection with F. hepatica.