cDNA encoding IL-4, IL-5 and IL-6 have been isolated and the availability of recombinant proteins allowed the demonstration of the pleiotropic effects of these cytokines. IL-4, IL-5 and IL-6 bind to high affinity receptors (Kd = 1-6 x 10(-10) M) expressed in low numbers on cells. The IL-4R is expressed on virtually every cell type whereas the IL-6R and IL-5R are less widely distributed. Low affinity IL-6 and IL-5 binding sites have also been detected but the biological activity of these different cytokines appears to correlate with the presence of high affinity binding sites. Biochemical characterization of the IL-4R, IL-5R and IL-6R by crosslinking of the labelled ligands, immunoprecipitation or purification experiments suggests that these receptors are composed of several proteins. IL-4 binds to three species (130, 75 and 65 kDa) and cDNAs encoding the 130 kDa protein have been isolated. IL-6 binds to a 80 kDa protein for which a cDNA has been isolated. Furthermore the IL-6 80 kDa protein complex binds to a 130 kDa transducer for which a cDNA was recently isolated. The IL-5R consists of three proteins of 105, 75 and 60 kDa but corresponding cDNAs have not yet been isolated. The IL-4, IL-6R 80 kDA protein and the IL-6 transducer protein display significant homology in their extracellular domains. These proteins belong to the new cytokine receptor family (hematopoietin receptors superfamily) which also includes the G-CSFR, GM-CSFR, IL-3, IL-7R and IL-2R 70 kDa chain.