Eosinophilia can be a specific cellular response, but the mechanisms of its control are not yet fully understood. Interleukin 5 (IL-5) is a lineage-specific eosinophil differentiation and activation factor. It can be detected in the serum of mice with eosinophilia, and antibody to IL-5 blocks the development of parasite-induced eosinophilia. The role of other cytokines in the production of eosinophil precursors is not clear, and may be in vitro artifacts. The biological significance of the B cell growth factor (BCGF) activity of murine IL-5 remains a puzzle. Although eosinophilia is often associated with high levels of IgE antibody, IL-5 appears not to be involved in this response, where interleukin 4 appears to be the main controlling factor. Understanding this activity has been made more difficult, since human IL-5 has no activity in analogous human BCGF assays. A better understanding of the role of these activities will probably come from studies in vivo rather than from cell culture.