Interleukin-4 (IL-4) and IL-5 are cytokines with important roles in IgE production and eosinophilia. Interleukin-4 is essential for IgE production, and IL-5 is the major factor involved in the production and activation of eosinophils. These two phenomena commonly occur together in parasitic infestation and allergic disease. Both cytokines are produced by T helper 2 (Th2) and Th0 cells but not by Th1 cells, and in a number of experimental systems IL-4 is required for the production of IL-5. This article presents evidence that IL-4 and IL-5 are not always co-ordinately produced. There is evidence for selective production of either IL-4 or IL-5 in response to immune stimulation by different adjuvants. Dissociation of production of these two cytokines has also been reported in several pathological situations. An example is intrinsic or non-atopic asthma, with eosinophilic bronchitis but without elevated IgE production, where there is evidence for excessive production of IL-5 but not IL-4. Different microenvironmental factors may favour production of either IL-4 or IL-5. For example, IL-2 stimulates the production of IL-5 but not IL-4. Therefore the Th2 model does not account for all immune responses involving IL-4 or IL-5. Responses characterized by IL-4 without IL-5, and IL-5 without IL-4, can also occur.