INTERLEUKIN-5 (IL-5) specifically induces the differentiation of eosinophils, which are important in host defence and the patho genesis of allergies and asthma(1,2). Structurally, IL-5 is a unique member of the short-chain helical-bundle subfamily of cytokines whose canonical motif contains four helices (A-D) arranged in an up-up-down-down topology(3,4). In contrast to other subfamily members, which fold unimolecularly into a single helical bundle(5-8), IL-5 forms a pair of helical bundles by the interdigitation of two identical monomers that contribute a D helix to the other’s A-C helices(3). We predicted that the lack of bioactivity by an IL-5 monomer(9) was due to a short loop between helices C and D which physically prevents unimolecular folding of helix D into a functionally obligate structural motif. Here we report that, by lengthening this loop, we have engineered an insertional mutant of IL-5 that was expressed as a monomer with biological activity similar to that of native IL-5. These studies demonstrate that all of the structural features necessary for IL-5 to function are contained within a single helical bundle.