Using a clonal culture system, we investigated the hemopoietic effects of purified recombinant IL-5 obtained from conditioned media of transfected Xenopus oocytes. IL-5 alone acted on untreated bone marrow cells and supported the formation of a small number of colonies, all of which were predominantly eosinophilic. However, it did not support colony formation by spleen cells from 5-FU-treated mice, in which only primitive stem cells had survived, while IL-3 and G-CSF did. Eosinophil-containing colonies were formed from these cells in the presence of IL-5 and G-CSF together. In contrast, G-CSF alone did not support any eosinophil colonies. The eosinophilopoietic effect of IL-5 was dose-dependent, and was neutralized specifically by anti-IL-5 antibody. To exclude the possibility of interactions with accessory cells in the same culture dish, we replated a small number (200 cells/dish) of enriched hemopoietic progenitors, obtained from blast cell colonies, which were formed by cultivation of spleen cells from 5-FU-treated mice in the presence of IL-3 or G-CSF. From these replated blast cells, eosinophil colonies were induced in dishes containing IL-5 but not in those containing G-CSF alone. From these findings, it was concluded that IL-5 did not act on primitive hemopoietic cells, but on blast cells induced by IL-3 or G-CSF. IL-5 specifically facilitated the terminal differentiation and proliferation of eosinophils. In this respect, the role of IL-5 in eosinophilopoiesis seems to be analogous to erythropoietin, which promotes the terminal differentiation and amplification of erythroid cells. Moreover, IL-5 maintained the viability of mature eosinophils obtained from peritoneal exudate cells of the mice infected with parasites, indicating mature functional eosinophils carried IL-5 receptors. The synergistic effects of IL-5 and colony-stimulating factors on the expansion of eosinophils is supposed to contribute to the urgent mobilization of eosinophils at the time of helminthic infections and allergic responses.