Mice deficient in interleukin-5 (IL-5^−/− mice) were generated by gene targeting in embryonal stem cells. Contrary to previous studies, no obligatory role for IL-5 was demonstrated in the regulation of conventional B (B-2) cells, in normal T cell–dependent antibody responses or in cytotoxic T cell development. However, CD5⁺ B cells (B-1 cells) in the peritoneal cavity were reduced by 50%–80% in 2-week-old IL-5^−/− mice, returning to normal by 6–8 weeks of age. The IL-5^−/− mice did not develop blood and tissue eosinophilia when infected with the helminth Mesocestoides corti, but basal levels of eosinophils with normal morpholgy were produced in the absence of IL-5. IL-5 deficiency did not affect the worm burden of infected mice, indicating that increased eosinophils do not play a significant role in the host defence in this parasite model.