Interleukin-5 (IL-5) is a hormone-like protein which plays a key role in the regulation of immune responses to helminth parasites and to allergens. This cytokine regulates inducible white blood cell formation in response to infection by helminth parasites or to allergen exposure and is principally responsible for controlling the eosinophilic inflammation characteristic of asthma and allergy. This inflammatory response is primarily orchestrated by the inducible production of IL-5 by T lymphocytes which results in enhanced production of eosinophils in the bone marrow. IL-5 then cooperates with other cytokines like eotaxin which promote the migration of the eosinophils to the affected site. Our group has had a long term interest in defining the biological role, regulation and signalling mechanism for IL-5. It is expected that these fundamental studies will provide opportunities for the development of novel therapeutic approaches for the treatment of allergic disease and also information relevant to understanding some leukemias.