Interleukin 5 (IL-5) is a cytokine mainly produced by activated T cells upon stimulation with antigen, and by mast cells upon stimulation with allergen/IgE complex or calcium ionophores [1,2]. The study of murine (m) IL-5 originated in the search for T-cell-replacing factor (TRF), which induces 2,4-dinitrophenyl (DNP)-primed B cells to differentiate into anti-DNP IgG-producing cells [3], as well as the proliferation of BCL₁ B-cell tumor cells into IgM-secreting cells (B-cell differentiation factor-μ, BCDFμ) [4,5]. IL-5 also regulates the proliferation of activated B cells (B-cell growth factor II, BCGFII) [6], and enhances IgA synthesis (IgA-enhancing factor, IgA-EF) [7–10] and functions of eosinophils and basophils (eosinophil-differentiation factor, EDF) [11]. Thus IL-5 is a synonym of TRF, BCDFμ, BCGFII, IgA-EF and EDF. A number of mIL-5-dependent mouse B-cell lines have now been isolated and provide a convenient biological assay for IL-5 [12]. In humans, human (h) IL-5 inducs the production of eosinophils from bone marrow progenitors and works for survival and priming of eosinophils in vitro [11]. hIL-5 expression was observed in many diseases with eosinophilia, suggesting that IL-5 plays important roles in promoting the production and function of eosinophils in vivo.