Experiments in vitro have suggested that IL5 is a late-acting factor in eosionophil production, and that other factors such as IL3, G-CSF and GM-CSF are required for the production of committed eosinophil progenitors. Furthermore, work in vitro indicates that in addition to IL5, both IL3 and GM-CSF are capable of stimulating eosinophil differentiation. Thus, there would appear to be both considerable redundancy in cytokine actions in eosinophilia as well as a complex network of cytokine activities to induce eosinophilia. Experiments in vivo, however, suggest a less complicated control mechanism, dominated by IL5.