Interleukin-5 is a human basophilopoietin: induction of histamine content and basophilic differentiation of HL-60 cells and of peripheral blood basophil-eosinophil progenitors. Cytokine-induced differentiation of basophils may contribute to various inflammatory processes. We examined the effects of recombinant human interleukin-5 (IL-5) and other human cytokines in vitro on myeloid colony formation in methylcellulose and on alkaline passaged HL-60 basophilic cell differentiation. Myeloid colonies (CFU-C) at day 14, formed in the presence of either IL-3, IL-5, granulocyte-macrophage colony-stimulating factor (GM-CSF), or G-CSF included peripheral blood-derived progenitors of the eosinophil/basophil lineage. IL-5 stimulated a greater proportion of basophil-containing, histamine-positive, eosinophil-type colonies compared with GM-CSF, IL-3, or G-CSF. IL-5 also stimulated dose-dependent increases in histamine content of alkaline-passaged, butyrate cotreated HL-60 cells. The concentration of IL-5 required for half-maximal induction of HL-60 histamine content was similar within twofold to that needed for half-maximal stimulation of the multifactor dependent TF-1 erythroleukemic cell line. Neutralizing rat monoclonal antibodies to human IL-5 were developed and used to demonstrate that each of these IL-5 bioactivities could be specifically blocked. We conclude that in addition to its previously described eosinophil differentiation activity, IL-5 may be considered a basophilopoietin. 