Substantial evidence indicates a close relationship between eosinophils and basophils. We examined whether interleukin 5 (IL-5), known to be eosinophilopoietic and capable of selectively regulating eosinophil functions, has an affect on basophil functions. IL-5 enhanced basophil histamine release evoked by anti-IgE, formyl-methionyl-leucyl-phenylalanine, or ionophore A23187 at picomolar concentrations. Direct action of IL-5 on human basophils was confirmed using highly purified basophil populations. These observations reveal the novel fact that IL-5 is able to modulate basophil functions as well as eosinophil functions, and suggest that normal human basophils possess functional receptors for IL-5.